Alternator charging

An alternator in a 1993 Chevy Silverado 1500

Alternator charging is using your vehicle's alternator to quickly charge your house batteries.

With a isolator you would run the truck early to get a fair amount of the bulk charging done and let the solar finish it off the rest of the day.

TLDR

Alternators can generate massive quantities of electricity when the engine is running, which can be used for charging house batteries. Their ability to generate large amounts of current makes alternators particularly good at charging deeply-discharged batteries (especially lead chemistry batteries)

• In many situations, it is possible to use a simple/inexpensive battery combiner or isolator between the house batteries and the vehicle battery. These will allow the house battery to charge when the vehicle is running, but will prevent the vehicle battery from being drained when the engine is off.
  • Some situations may benefit from or require DC-DC chargers in between the vehicle batteries and the house batteries.
• Idling the engine while parked for long periods of time just to recharge the batteries is possible, but generally a bad idea.
  • Pulling a lot of power from the alternator causes it to generate a lot of heat. When the vehicle is moving there's enough airflow to keep the alternator cool, but when parked the heat will build up and can damage the alternator.
  • Long periods of idling is also not great for engines, especially modern emission equipped diesels.
• The amount of amps the alternator can provide depends heavily on the vehicle
  • While many vans have alternators rated at 150+ amps, at least half of that is typically needed just to run the vehicle's driving functions.
  • It's usually recommended to not draw more than 50 amps unless you know for a fact that your vehicle's alternator can handle it.
  • Some vans were offered with second alternators from the factory, which allow for charging your house batteries at hundreds of amps.
  • It's also possible to install aftermarket alternators, or to have the factory alternator replaced with a higher power one.
• Alternator charging alone is unlikely to keep lead-chemistry batteries healthy, so combining with solar is common

Overview

The vehicle's alternator is designed to turn some of the engine's mechanical power into electrical power in order to recharge the vehicle's starter battery after starting the engine, and run electrical accessories, including the vehicle's built-in air conditioner, wipers, fans, headlights, etc.

Dual-battery systems

We can, within limits use this power to charge our batteries or run our electrical loads. This page is about how to do that with a dual-battery system (i.e. starter battery + house or aux battery).

In a dual-battery system (starter battery and house battery) some of the alternator output is used to charge the house batteries when the engine is running. When the engine is not running the house battery is electrically isolated from the starter battery to keep from draining it and leaving you stranded. So the devices that handle the isolating/combining duties are sometimes called battery isolators.

The setup is typically:

starter battery -> fuse -> wire -> isolator -> wire -> house battery

We might call them "isolators" generically but there are three kinds of devices used to charge from alternator:

• combiner - which parallels the starter and battery bank together under certain conditions. Inexpensive, historically most common. Includes manual switches.
• DC-DC charger - includes electronics to adjust voltage and current. More expensive, increasingly common
• actual isolators - which split alternator power and distributes it separately to starter and house batteries (sits between alternator and batteries rather than between starter and house batteries) Uncommon these days, but could be useful in some scenarios where some amount of voltage and some current attenuation are desirable.

when alternator charging works well

• when batteries are deeply discharged
• when combined with solar or some other higher-voltage charging source
• when used with battery chemistries like lithium that do not need staged charging.
• when used with a battery-to-battery boosting charger that can produce correct charging voltage for the battery chemistry

> The bottom line is that current simply flows where it is needed, batteries will take what they need when batteries are combined, and the voltage becomes equal among the new combined bank. Unless your charger, alternator or solar/wind system is pumping out an incorrect voltage for you bank you will not over charge using an ACR. mainesail

limitations

With lead chemistries, alternator charging by combiners is generally only practical for the bulk charging stage due to relatively low alternator voltage and the long time periods required for absorption; DC-DC chargers can help with the voltage can address the voltage issue. Failure to fully charge lead batts regularly will impact battery longevity. For this reason some solar is usually added to the power mix for lead banks. In contrast, Lithium can charge fine from alternator alone, if one drives enough to get sufficent charging.

Combiners are effectively a pass-through:

• they have no way to limit current other than hitting their max rating and failing or tripping. This might mean the battery can pull more current than is good for itself or for the alternator, necessitating the use of a DC-DC charger. This typically is an issue only with oversized banks or small alternators.
• they can't boost voltage like DC-DC the charge acceptance rate (Amps) will decrease as bank voltage rises; this is called the current taper.
• they are not "smart" and have no charging stages -- the house bank will be charged at alternator voltage while the engine is running. Compare your alternator's voltage output to the battery manufacturer's Absorption charging voltage setpoint before going this route.

Vehicles with smart (variable voltage) alternators may not be suitable for charging with normal combiners. See below.

effect on alternator

{WARNING from secessus: idling to charge can cause alternator temperatures to spike, damaging the alternator or its diodes. So don't do that. }

Charging the house batteries from the alternator increases the load on the alternator and can be expected to contribute to somewhat earlier failure. In practice it's usually a non-issue if one avoids overheating or overloading the alternator; alternator failures from aux battery charging are quite rare.

> I have created hundreds of designs and installed around 100 systems, many with isolators or solenoids. In four plus years not one customer has come to me saying that their alternator failed. I do tell them not to sit in a hot parking lot idling their engine to charge their batteries. -- jimindenver

But see this cautionary tale of using a 60A (!) DC-DC charger to charge 200Ah of AGM from a 145A alternator.

If/when the OEM alternator does fail a higher output one can be installed for not much more than it would cost to replace the original.

heat

Be aware of how heat affects the alternator and its health:

• alternators are about 50% efficient; making 400w of power means it's also making 400w of heat
• heat is hard on internal components; excess heat can cause derating or failure
• idling greatly reduces the ability of the alternator shed this heat
  • the alternator's own internal fan is running slower
  • ambient temperatures under the hood are higher; there is no forward motion to introduce cooler air through the car's grille

If the bank is slurping a lot of current and you are stuck in traffic on a hot day it might be a good time to disable alternator charging (see below).

SternWake reports idling while charging causes a sharp increase in alternator temperature. To avoid this, do your alternator charging while driving so airflow over the hot alternator will help cool it. Other measures included additional alternator cooling or pulley size tuning to alternator RPM at idle.

See this sub-article on alternators and heat

charging current patterns

• charging lead chemistries directly from the alternator tends toward
  • high initial inrush currents
  • then a linear taper of current as the bank comes up to alternator voltage((this happens because lead chemistry voltage rises (and acceptance drops) linearly while charging))
• charging lithium batteries directly from alternator tends toward
  • high initial inrush currents when SoC is low, then
  • middling and gradually-tapering currents across the middle 80%, then
  • very low currents as the bank approaches full charge
• charging lead with DC-DC tends to be
  • rather flat at the charger's rating until
  • Absorption voltage is achieved, at which point it begins a linear taper.
• charging lithium with DC-DC tends to be
  • rather flat and dictated by the charger's rating.
  • At the tail end of charging will taper off.

alternator current rating

In general, vehicles with higher-rated alternators (150A, for example) will handle a given load better than vehicles with lower-rated alternators (60A, for example). The rating in Amps will be listed on the window sticker, often on the alternator housing itself, or can be looked up using a VIN decoder for your automaker.

see this related article on assessing how much current you can safely take from the alternator

paralleling different chemistries

It is often said that one cannot parallel batteries of different chemistries. The problem is paralleling batteries with different resting voltages when the batteries are not being charged. The isolator setup is specifically designed to isolate the batteries when there is no charging.((DC-DC chargers and diode isolators keep them isolated 100% of the time, even when charging is present))

There may be an issue of the alternator having improper voltage for charging one or the other but that has nothing to do with the isolator.

alternator voltage

Traditional alternators typically try to hold a consistent voltage. The exact voltage being held might vary but it's relatively stable:

>> most alternators with internal regulators are designed with a voltage turn-down with temperature. Because the battery is often under the hood, it gets hot from the engine and needs a lower charging voltage to remain healthy. Typically a 14.4V regulator target will drop to a 13.8V regulator target by the time the alternator gets to 140F. -- MechEngrSGH

While this is not intended to be protection from overheating due to high current production, lowering the output voltage will indirectly reduce current when charging by relay/isolator. Example: a setup with accepting 85A @ 14.4v would only accept 65A @ 13.8v.

smart alternators

Smart alternators on the other hand talk to the vehicle's ECU (computer) and can vary output voltage wildly moment by moment depending on present conditions. It might unload the alternator during heavy acceleration to reduce parasitic losses, or run the alternator at high voltage just after starting to speed up the recovery of used energy.

When the smart alt goes low voltage the normal relationship between the two systems (higher-voltage chassis charging lower-voltage house battery) is disrupted. It can result in rapid ON/OFF cycling of the isolator (voltage sensing types) or the discharge of the house battery into the starter battery (solenoid type triggered by D+).

>> There are several methods of identifying a smart alternator... the most conclusive is the existence of a current sensor on the negative battery cable. It is often a small box with 2 or 3 wires placed right at the battery terminal. In some cases (GM mostly) it may be 6-8" down the cable from the battery. -- MechEngrSGH

solutions

• the usual solution is to use a DC-DC charger with an awareness of smart alternators. The DC-DC is already upconverting chassis voltage to house battery charging voltage, and does not mind unusual source voltages from the alternator.
• Some smart alternators can have the "smarts" disabled or tricked into producing higher voltage; this depends on the alternator/vehicle.
• in extreme situations the OEM alternator might be replaced with a traditional one, or a dual alternator setup (one smart, one traditional) might be engineered

further reading on smart alternators

• Redarc - standard vs smart alternators

combiners

"Split charge relay (SCR)", "split charger", "automatic charge relay (ACR)", "Voltage sensing relay (VSR)", solenoid, relay, etc.

Power from the alternator is shared with the house battery by paralleling the two sets of batteries at certain times. This allows the house battery to charge but does not allow the house battery to pull power from the starter battery when not combined.

constant-duty solenoid

[[https://m.media-amazon.com/images/I/41zFkQ0pUIL._AC_UY218_ML3_.jpg?75 (external image)]]A constant-duty solenoid is an electromechanical device which uses an electromagnet to complete the charging circuit when the engine is running. The basic idea is the relay uses a low-current circuit to activate a higher-current circuit.

Solenoids are generally cylindrical**. Energizing the solenoid will cause a **0.5A - 1A current drop between the alternator and house battery. Exception:

Latching isolators use latches instead of electromagnets to hold the circuit closed, eliminating that vector of power consumption. and therefore heat. SternWake recommends the Blue Sea 9012 although [units in the $20-$50 range] are more common in vans. \\ Solenoids can be used for Self-jump startingself-jumpstarting if the chassis battery has enough juice to engage the solenoid. 

See this video that shows the theory and practice of how these relays work.

Note: some solenoids only have three terminals: 2 big load terminals and 1 small control terminal. This type gets the "ground" through the body of the solenoid. The pic above is of a "three post" solenoid -- the case is grounded to the chassis through the metal feet.

starter relays vs constant duty relays

While they may be externally identical, starter relays and constant duty relays are built differently inside.

The starter relay needs to switch huge currents for brief amounts of time. The switching has to be very fast and powerful to minimize arcing. To achieve this the solenoid will pull several amps to run a powerful electromagnet. The solenoid will not overheat because it is only "on" for a few moments. The control terminals typically have resistance of 3-4 ohms.

The constant duty relay is used for much longer periods of time and is rated for less current. The lower current means the connection doesn't have to be slammed closed as fast with a powerful electromagnet. As a result this relay type typically draws <1A and the control terminals have resistance of 15-30 ohms.

Note that per Cole-Hersee even a CD relay will get hot:

>> The coil circuit (control circuit) in a continuous duty solenoid is usually energized for long periods of time. Under these conditions the coil will generate heat and within less than an hour the solenoid housing will become hot to the touch. This is normal. Always make sure that all wiring is properly sized for the load it is carrying, that the terminals are the correct size and have been securely crimped to the wire, that the terminals have the proper torque to the solenoid studs.

orientation

Cole-Hersee recommends mounting relays with the dimple facing downward:

>> Our research shows that it might be best to mount the Solenoid dimpled end down. Electromechanical Switches can over time build up deposits due to arcing. By orienting your Solenoid as recommended, deposits will have a tendency to fall to the bottom, clear of the contacts, thus prolonging the life of the Solenoid.

voltage-sensing relays

[[https://images-na.ssl-images-amazon.com/images/I/418qcncjT0L._AC_US218_.jpg| (external image)]]voltage sensing relays (VSR, also called Automatic Charging Relays or ACR) are solenoids with a bit of extra logic to know when to connect/disconnect. The VSR does not get trigger voltage from the fuse panel but rather reads the voltages of one or both batteries to know when to switch on. \\ This kind of isolator may have a "combine" override function to enable Self-jump startingself-jumpstarting.

> in its simplest form, all an ACR really does is parallel batteries when charging is present and un-parallel batteries when there is no charging present. It does this automatically with no human forgetfulness.

starter battery priority

Some VSR have a feature where they delay the connection a few seconds until the starter battery has recovered a bit from starting the engine. Often misunderstood as "charging house batteries after the starter battery is fully charged", the typical criterion is chassis voltage of ≥13.4v. Contrary to common belief the starter battery is not fully charged at that point but the current inrush to it has settled down enough that the alternator can do other things.

examples

   - single voltage sensing - this type reads the voltage of only one battery. In the case of an RV it would read the voltage of the starting battery. When it is high enough above resting voltage (ie, being charged by alternator) it connects the starting and house batteries. \\ [secessus says: "IMO the practical benefit (if any) to charging the starter battery "first" is keeping the load on the alternator reasonable. In practice, the isolator generally connects the two within a few seconds."]   \\ Examples: 

• • Sure Power 1314 - Connect >13.2v, disconnect 12.9v, Automatic Self-jump startingself-jumpstarting when aux bank voltage > starter batt voltage.
  • Battery Doctor - connect >13.4v, disconnect <12.8v.
  • ["Smart Battery Isolator"] and many similar models

   - dual voltage sensing - this type reads the voltage from both sides and when either is high enough it connects the batteries.  This may or may not be what an RVer wants \\ Examples: 

• • Victron Cyrix-ct - connect >13.0v, disconnect <12.8v,
  • Sure Power 1315
  • Blue Sea - unless otherwise noted, these ACR combine at 13.0v after 120 seconds or 13.6v after 30 seconds. Disconnect at 12.75v after 30 seconds.

       * M-ACR 7601 - 65A. manual  
       * SI-ACR - 120A. SI is for starter isolation, a way to prevent unintentional Self-jump startingself-jumpstarting. If the SI post is wired to the output of the starter relay the ACR will be disabled whenever the starter itself is engine. SI is used mainly in the marine world to minimize the effect of house bank voltage sag during combined starting;  this protects marine nav, communications, and similar electronics. If the SI post is not used it acts like a normal dVSR. 
       * BlueSea BatteryLink 7611 - 120A. Connect 13.0v, disconnect 12.75v. If "auxiliary battery priority" is triggered the cut-out voltage drops to 12.25v. This can be triggered by D+ so it's only active when the engine is running. It can also be always-on from 12v, in effect acting like an LVD. manual
       * BlueSea ML-ACR 7620 - 500A. This is a latching relay which allows the solenoid to de-energize after changing state. This minimizing parasitic losses and heat. The ML has Starter Isolation as described on the SI-ACR. Allows Self-jump startingself-jumpstarting. Cut in and cut out voltages are slightly different;  see the manual.
       *  BlueSea ML-ACR w/manual controll 7622 - 500A. Version of the ML-ACR above with a manual control knob to override isolation or combination if desired.

• • JayCorp Smart Dual Battery 140A Isolator
  • Precision Circuits BIM. Latching relays in 160A** and **225A versions. . Disconnects after 1 hour max to prevent overcharging: "The BIM does not guarantee 100% battery charge, but prevents harmful battery charge levels. Sig terminal can use used for Self-jump startingself-jumpstarting. \\ Also see Li-BIM below.
• DC-DC isolators (aka b2b isolators) that boost alternator voltage to more appropriate levels for lead chemistries and can do multistage charging. These have their own page.

Note: voltages-sensing (with or without delay) can be added to plain solenoids.

lithium-specific VSR

A "lithium compatible" VSR has a higher voltage setpoint for disconnecting the batteries after charging stops; the charging function is no different.

Reasoning: A traditional VSR might disconnect at ≤12.9v, since that is slightly above resting voltage for a lead-chemistry battery. (When it sees ≤12.9v it knows charging must have stopped and the batts should be disconnected from each other) Fully-charged LiFePO4 rests at higher voltage so the voltage at which the VSR disconnects needs to be higher. If it waited for 12.9v the batteries would get "stuck" together longer than appropriate. In effect the lithium batt would prop up the lead starter batt (and parasitic chassis loads) until the LFP were substantially discharged. So a "lithium compatible" VSR disconnects at something like 13.2v - 13.4v.

Note: a normal VSR can be used with LiFePO4; it may be useful to add a way to disable temporarily it to break the "stuckness". Also see the Gotchas section for related information.

Li-BIM

[[https://m.media-amazon.com/images/I/41d3IImVdJL._AC_UL320_.jpg?100 (external image)]] The Precision Circuits Li-BIM is a lithium-specific dVSR isolator with some differences:

• the VSR circuit disconnects at 13.4v when there is no charging occuring on the alternator side. See section above for more info on why this feature is present.
• the isolator opens the circuit (disconnects) regularly to allow alternator cooling. Charge for 15 minutes, disconnect for 20 minutes, repeat. So the BIM is charging ~43% of total runtime.
• the isolator disconnects when it detects >= 14.4v on the alternator side, to avoid overcharging

The unit supports Self-jump startingself-jumpstarting but a switch must be installed by the user. 160A and 225A models are available. BB suggests the BIM is recommended for lithium banks >= 300Ah.

An interesting teardown of the Li-BIM can be seen in this video.

manual switch

[[https://m.media-amazon.com/images/I/81n-ap+0lPL._AC_UY218_.jpg?125 (external image)]]The simplest and least-featured isolator is a manual switch.

A manual battery switch normally has 4 positions: A, B, A+B, and Off. A would be for the starter battery and used during starting. B would be used for house use when one is not driving. A+B could be used to combine both sets for starting or for charging while driving. This kind of setup is prone to user error. A manual switch has no current or voltage losses.

RV-specific combiners

>> All late model Essex and King Aire’s use Silverleaf systems with the White Rodgers Solenoid. In 2014, the Mountain Aire will also be using Silverleaf. It is a computerized system that controls all aspects of charging. All late model Mountain Aire and Dutch Star Diesel Pushers use the Battery Isolation Manager (BIM). This is an all in one system that is made by Precision Circuits. Class A models before 2010 use the Bidirectional Isolator Relay Delay (BIRD) with a solenoid. The Bay Star Sport has a manual switch to disconnect power, which is located in the overhead above the entry door. This is similar to the ones in the fifth-wheels. All others have a single lighted switch that is in the front overhead to turn off house voltage.

also see DUVAC

BIRD

Some RVs came with "BIRD" controllers that drove the constant duty relay. These controllers from Intellictec combine when either side is >13.xv, depending on model/vintage.

• the delay in the name is a 2.5 minute delay after the chassis voltage reaches the setpoint. "This delay allows a cold engine an opportunity to start and warm up before having the heavy load of a discharged coach battery placed

on it."

• BIRDs with GEN terminals may disable chassis battery charging when the generator is operating.
• BIRDs with AUX terminal will override and close the relay regardless of voltage conditions. This could be useful for Self-jump startingself-jumpstarting.

The functionality of the RV-specific BIRD is largely supplanted by dVSR (see above). In some cases RV manufacturers have moved to the BIM (see above).

BIC

Intellitec also made a Battery Isolation Controller, which drives the relay based on chassis voltage. It also has manual override for self-jumpstarting.

The functionality of the RV-specific BIC is largely supplanted by a VSR (see above).

Silverleaf

Silverleaf is a CAN-based system for controlling various functions, including charging relays. Technical reference (pdf)

proper isolators

solid state isolator: diode-based

(external image)

Note: this type of isolator is no longer common for our uses for several reasons. Nevertheless they might be useful in some setups where bidirectional charging is undesirable.

These are proper "isolators" and never combine the starter and house batteries. The isolator is a Y connection that receives power from the alternator and distributes it separately to the starter battery and house battery. The rating is the maximum input from the alternator, and the two "legs" are assumed to each have half the capacity.

These isolators are electronic devices which use diodes to prevent backflow from either battery. Isolators are generally brick-shaped**. Silicon diode isolators typically have a **0.7v voltage drop ("forward voltage drop") between the alternator and house battery when running near rated capacity. This may be desirable if the house battery is a wants lower-voltage charging like LiFePO4.((caveat related to the note above - we cannot count on a stable 0.7v drop; it will likely be less at lower currents)) The slightly-lower voltage will also reduce charging current somewhat.

It is possible to have the alternator voltage-sense the battery voltage through a diode so it sees the voltage as artificially low and so increases its output voltage. The net effect can be almost no voltage loss.

Notes:

• solid state relays typically can't combine batteries for Self-jump startingself-jumpstarting
• the unidirectional nature of the diode isolator may be desirable to prevent "backflow" of higher voltage from the house bank to the chassis.
• diode-based isolators are typically installed between the alternator and starter battery. This is in contrast to solenoids and VSRs which can be daisy-chained off the starter battery. The batteries are, in effect, always isolated and never electrically combined.
• It would be possible to insert a two-terminal diode isolator between the starter and house batteries
  • starter batt --> LVD or IGN --> relay --> diode isolator -> house battery.
  • Ctek Smartpass (see below) is a commercial packaging of this idea
• some diode-based isolators (Victron, see above) have a feature to slightly tweak alternator voltage upwards to compensate for voltage drop across the isolator
• isolators cannot be disabled the way DC-DC and relays can.
• see info on alternator self-excitation below

three+ pin examples

These mount between the alternator and the target batteries.

• Cole-Hersee 48161, 250A.
• Cole-Hersee 48162, 250A. With exciter.
• Cole-Hersee 48122, 140A. With exciter.
• The Victron Argo-diode, 200A, claims to use Schottky diodes instead of silicon diodes so that "voltage drop is approximately 0.3V and at the rated output approximately 0.45V". With exciter and built-in voltage-sensing diode function. Schottkys are prone to heating up at high current so proper mounting with airflow may be critical.

two-pin examples

[[https://m.media-amazon.com/images/I/314V99TPtsL._AC_UL400_.jpg?125 (external image)]] Instead of mounting between alternator and battery it goes between the power source (battery, alternator, etc) and the house battery like a combiner.

When daisy chained with a relay we get the unidirectionality of an isolator without rewiring, alternator-excitation issues, etc, mentioned elsewhere in this article.

• Cole-Hersee 48051

solid state isolator: FET-based

This type of isolator is similar to the diode-based one above, except that FET components are used instead of diodes, minimizing voltage drop and allowing two-way power if designed to do so. They tend to cost 2x as much as the diode versions.

three+ pin examples

• Victron Argo 200A FET Battery Isolator
• the Ctek SmartPass.((https://www.ctek.com/storage/5CDB4E293A03CBF6442A620EC8E49CFB548CB48E9BF93425638077279A13BBA1/ac5639c5feaa46d38411ebd69f915b5b/pdf/media/65589cee7d4342e0891e2130149be66f/SmartPass%20120S.pdf says it is MOSFET)) is a MOSFET-based isolator with additional features/logic:
  • Connect at >13.1v, disconnect at 12.8v, or 11.8v / 11.4v with smart alternators.
  • Battery overtemp protection at 140F.
  • Starter assist when starter batt voltage ~6v.
  • Will trickle charger starter batt when house battery is charging from other means.
  • See this article about DIYing a similar solution.

two-pin examples

• Perfect Switch, 100A-600A. Presumed to be FET-based since forward-voltage drop is given as 30mV.

misc

[note from secessus: "not sure what's inside these solid state isolators"]

[[https://m.media-amazon.com/images/I/51XoShK5uIL._AC_UY218_.jpg?125 (external image)]] The Magnum Energy ME-SBC is notable for some unusual features:

• configurable connect/disconnect setpoints
• ability to drive a solenoid, which allows for much greater current

Xantrex makes a 15A Digital-Echo Charge isolator.

The Mastervolt Charge Mate Pro 90 is an electronic current-limiting isolator.

self-exciting alternators and isolators

All alternators need to be able to sense battery voltage to regulate their output. This was traditionally done with a separate voltage sense wire.

"1-wire self-exciting" alternators do not have a sense wire and rely on the output wire from the alternator ("B+") to the starter battery. This is normally not an issue since the battery and alternator are wired to each other.

However when a diode- or FET-based isolator is inserted between them the alternator's B+ can no longer sense the battery voltage; the isolator is one-way by design. The solution is to provide a small amount of current from the starter battery to the isolator's common input terminal where the B+ can sense it. Because this "leak" violates the isolator's one-way principle it should only occur when the vehicle is on.

So a D+ (IGNition) signal is provided to so-equipped isolators on additional "alternator excite" stud, which triggers the leak from starter battery to common INPUT.

[note from secessus: it's not clear if the leak is momentary or if there is a diode on the leak circuit to prevent charging from flowing down this tiny circuit. Some manufacturers suggesting getting the 12v signal from the starter relay so it is momentary.]

Per Littelfuse (maker of Cole-Hersee isolators):

>> Most alternators on new vehicles have an integral electronic voltage regulator that requires the use of the 4-stud battery isolator. The small 4th stud is for connection to a circuit switched by the ignition switch... A 4-stud battery isolator can be used with older pattern alternators (in this case the 4th stud will remain unconnected), but a 3-stud battery isolator cannot be used with the Delcotron-type alternator.

Yet another explanation, this time from Victron:

>> The new Argodiode isolators have a special current limited energize input that will power the B+ when the engine run/stop switch is closed.((ie, "closed" = "circuit is complete/active"))

effect of isolator type on charge rates

Because I=V/R we can compare how different "forward voltage drops" affect charging. Assuming 20mR resistance as a baseline, 14.4v alternator output and 12.1v bank voltage:

• direct charging (0v loss) would be ~115A
• charging with FET-based isolator (0.3v loss) would be ~100A
• charging with Schottky diode isolator like ArgoDiode (0.45v loss) would be ~92.5A
• charging with normal diode isolator (0.7v loss) would be 80A

These are relative numbers, not intended to predict actual charge rates.

.

how to choose

For many use cases a plain constant-duty solenoid triggered by an ignition circuit will augment aux battery charging nicely. It can deliver large amounts of current when battery state of charge is low, and is quite inexpensive. The wiring might cost more than the solenoid.

When access to an ignition circuit is impractical, a voltage sensing relay will do the job, no external trigger required.

In some cases a DC-DC charger is preferable or mandatory:

• when using alternator as the sole form of charging lead chemistries
• when the vehicle has a "smart" alternator
• when using an alternator to charge large aux battery banks that may strain the alternator
• when alternator and aux battery bank voltage are different (12v alternator and 24v bank, for example)

A diode-based isolator may be preferable in niche cases.

See also the effect of alternator charging method on current

See the gotcha section below to see if there are hidden traps in your intended use case.

sizing an isolator

If an isolator is oversized it will cost more for no benefit, will self-consume somewhat more energy to hold the combining circuit closed, and may take more physical space. \\ If an isolator is undersized (less common) it will not be able to carry enough current, resulting in overheating and/or sudden shutdown.

Most AGM will pull about C/3 when deeply discharged (33A for a 100Ah bank) but premium brands may do more. Flooded lead-acid batteries tend to pull less current (C/5, 20A per 100Ah of bank). If your flooded back will only pull ~40A, or your AGM bank 70A then there is little reason to spend more money on a 150-200A isolator.

Lithium in particular has low internal resistance and can pull 1C (100A for an 100Ah bank) or more. In practice, they tend to pull about the same as AGM.

Since lithium does not care much about state of charge, there is little reason to go for maximum force lithium charging. Some Li bank owners use DC-DC isolators which limit themselves to a particular output (20A, 60A, 100A, etc).

Reasonable charging rates can also be easier on the alternator when charging suddenly stops, whether by completion or BMS intervention. Blue Sea makes an alternator field disconnect which shuts down alternator power just before disconnecting the load, but this may be chiefly applicable to marine alternators. Others have discussed installing a small lead-acid battery parallel to the Li bank; in theory this could soften the blow from Li leaving the circuit. Other sources suggest the presence of the starter battery would be sufficient.

flooded lead-acid

FLA batteries can accept up to C/5 in Bulk stage.

Example: a 200Ah FLA battery bank will pull up to 40A in Bulk charging. An isolator rated for constant duty at 40A would be sufficient.((assuming you aren't applying heavy loads like a microwave while driving))

AGM lead-acid

Consumer-grade AGM batteries typically will accept C/5 - C/3.

Example: a 200Ah AGM bank will pull up to 67A in Bulk. A 75A isolator would be sufficient.

Note: high-end AGM like Lifeline, Odyssey, Rolls, etc, can pull massive current when charging. 200A+ would be possible for the example bank and could shorten the life of a stock alternator.

lithium

Lithium has the potential to accept massive amounts of charging, up to 1.0C. All other things being equal, heaviest current will be pulled when battery bank voltage is the lowest.((It's actually the delta (difference) between alternator and battery voltage but we assume that alternator voltage is stable))

There are mitigating factors that tend to reduce current in real world use:

• resistance in the charging path often limits lithium charging current to ~0.5C.
• Because lithium can use about 80% of it's capacity instead of 50% for lead-acid if one is upgrading to a LiFePO4 bank with the same usable Ah the Li bank will be ~0.6 the rated capacity of the lead it replaces.
• Li voltage remains in the thirteens for most of its usable capacity, reducing the voltage delta mentioned above.

sudden disconnection

Note: this is an issue for rigs with secondary alternators dedicated to charging a battery bank. In normal setups the vehicle's starter battery acts as a buffer to cushion sudden disconnects.

Sudden disconnection of a large load when the alternator is making substatial power can damage the alternator and chassis electronics. Sudden disconnection can occur when:

• an isolator shuts off for whatever reason
• a BMS shuts off lithium charging. This can include overvoltage, overcurrent, temperature extremes, etc.

externally-regulated alternators

It's more common in marine setups than vehicles, but external regulators can be used to trick the alternator into outputting specific non-OEM voltages((even "smart" staged charging)). Balmar appears to be the industry leader in external regulation.

Note that while your battery bank might like higher voltages the vehicle chassis may not.

secondary alternators

In RVs with heavy electrical consumption a secondary alternator may be installed for aux power and charging. It runs off the engine and effectively replaces the generator; some systems will auto-start the engine similar to how gens can auto-start. The secondary alt is typically rated for heavier current and/or externally-regulated (see above). It may be run off a smaller pulley that increases alternator RPM at idle for more power and/or cooling.

Challenges include hefty cost, already-cramped space in van engine bays, mechanic unfamiliarity with non-OEM systems, and potentially-increased time running the engine.

gotchas

The average user will likely not notice these effects; some of them rather subtle.

• Solar charging while the isolator circuit is closed (ie, batteries connected) can pass higher-than-normal voltage to the chassis and starter battery. Workaround: see notes on HVD and DC-DC charging below.
• Voltage-sensing relays can be unintentionally triggered or "held closed" by voltage from the solar-charged side in some scenarios. Workaround: address with HVD as below if desired, or with a DC-DC charger, or by adding a switch to disable the VSR.
• In early morning or other times when house battery voltage is lowest, a plain solenoid may unintentionally allow depleted batteries to pull down the starter battery. Workarounds: use a VSR, a DC delay timer, a DC-DC charger, or start the vehicle immediately after inserting the key.
• Solar charging while the engine is running may get "stuck" at alternator voltage. Workaround: higher solar wattage, DC-DC charger, or diode/FET-based isolator, or a switch to disconnect isolator after alternator voltage is reached. The Victron Cyrix-ct isolator could be useful here, as it appears to disconnect >13.8v.
• Alternator charging may bring some battery chemistries (like bare lithium cells with no BMS) to unsuitably high voltages. Workarounds: A high voltage disconnect can restrict alternator charging to lower voltages. DC-DC chargers can also regulate voltage provided to the house battery.
• It is possible for a plain combiner to "mask" the symptoms of a failing starter battery since the house bank would be assisting with the start.

disabling alternator charging

It may be desirable to disable alternator charging on-the-fly when stopped in traffic, on hot days, to avoid charging frozen Li cells, stop charging at a given voltage,((using a High Voltage Disconnect)) or neutralize gotchas, etc. The method of disabling will vary depending on the gear:

• relays and DC-DC that are triggered solely by D+ can be disabled by a switch on the D+ wire
• VSR triggered by voltage can can be disabled by a switch on the thin ground wire on the VSR itself.
• some DC-DC can be derated by providing a 12v signal to the CURRENT LIMIT terminal (Renogy DC1212 series, Leaptrend)
• some DC-DC can be disabled by providing a 12v signal to a remote ON/OFF terminal (Victron Orion-TR), Kisae.
• on units that have a dedicated NEG ("ground", negative return) to the starter battery there may be no easy disable method. The brute force method would be to insert a relay between the starter battery POS and the unit's POS input, and control the relay with a switch.
• bluetooth-enabled DC-DC can often be disabled or derated from the app

A NC thermal switch affixed to the alternator case might be used to automate this shutoff, or a NO switch to trigger a low-output mode on the Renogy or Leaptrend chargers mentioned above.

A voltage-sensing relay might be used to add voltage-sensing to a relay triggered by D+, or to disconnect at a given setpoint.

wiring the setup

In most setups nomads:

• install a dedicated POS line from the starterbatt or alternator to the charger/relay/isolator
• then use the vehicle chassis/body for the NEG return path back to the alternator/starterbatt

Exceptions:

• vehicles without solid connection between the chassis and alternator (pickup truck beds, travel trailers, fiberglass vehicles) will require a separate NEG return wire to complete the circuit
• the chassis introduces considerable resistance, so high-current setups may require a separate NEG return. This resistance is often beneficial, reducing the current acceptance of LiFePO4 or large lead banks to manageable levels.
• Solar NEG doesn't need to be connected to the chassis because it brings its own NEG/POS to the party and isn't trying to talk (electrically) with the vehicle itself.

sizing the wire

• Assess how much current the charging system will pass. With DC-DC that current will be the output rating of the charger (40A, 50A, etc). With plain relays there will be a bit of arithmetic involved
• use the Blue Sea chart to choose the appropriate wiring gauge. Note: distance includes the entire circuit, even if NEG is bonded to the chassis.

wire size and voltage sag

The Blue Sea chart shows wire sizes for both Critical** and **Non-Critical loads. The difference is in how much voltage sag will be present at high levels of current. Critical in this context refers to electronics with narrow input voltage requirements, or power transmission where every watt counts(. Both critical and noncritical wiring specs in the chart are safe.

All other things being equal, it is generally preferable to size the wiring to the Critical loads criteria. It makes little sense to buy a 60A DC-DC then choke it to 40A with thin wiring (see below).

There are reasons where one might choose Non-Critical sizing:

• fatter wire won't fit through the available space, or fit into the charger
• the DC-DC is voltage sensing the house battery and can adjust for sag
• a relay is being used and the owner wants to tamp down current acceptance
• a relay is being used alongside solar so we are not relying on every Amp and volt from the alternator

effect of voltage sag on direct charging rates

We will use alternator voltage of 14.4v and discharged battery voltage of 12.1v below to illustrate how sag affects charging with a relay.. Current obeys the formula I=V/R, so the greater the voltage sag the lower the current. The actual values here aren't important, only the general pattern:

• a bank with theoretical max charging (massively oversized wiring for ~zero sag): 115A
• critical load wiring (3% sag): 93.5A
• noncritical load wiring (10% sag): 45A

examples

Let's assume a 200A AGM bank that sits 10ft (20ft of wire for the complete circuit) from the starter battery or other connection point. 200Ah of AGM will pull ~60A (0.3C) at 50% depth of discharge.

If we want to make full use of DC-DC charging we would

• plan on a 60A DC-DC charger. Note that due to voltage boosting, 60A of charging to the battery may require ~20% more current (72A) from the battery.
• fuse for 72A + 20% = 86.4A between chassis and charger
• select 2awg from the chart (20', critical, 70A-80A)

A plain isolator can't boost voltage while meeting a charging current target so we only have to worry about 60A. Run critical spec wiring if you want "full blast" and noncritical for reduced current:

• use any isolator rated at least 60A
• fuse for 60A + 20% = 72A between chassis and charger
• select wiring from the chart
  • 4awg (20', critical, 60A), yielding ~60A when first connected. Maximum charging current for 200Ah of consumer-grade AGM.
  • 6awg (20', noncritical (10% voltage sag), 60A), yielding ~30A when first connected. Minimum charging current for 200Ah of AGM.

Over time isolator charging will deliver the same Ah back to the battery bank with either critical or noncritical wiring. The critical wiring will deliver high current that falls off sharply and linearly. The slope is quite steep. Noncritical wiring will deliver moderate current that also falls off linearly although more gradually. The slope is shallower.

A side effect is that if one drives on very short trips the critical wiring may deliver more Ah to the bank in the limited time available. The noncriticial wiring would be gentler on the alternator.

Reminder: it is hard to keep lead batteries healthy by alternator charging alone. In order of decreasing effectiveness:

• add solar to augment alternator charging
• use a DC-DC charger to get at least the correct charging voltage
• if charging by relay only, use Crtical size wire to get the most voltage and current to the lead house battery

alternator hacks

There are ways to get the alternator to pump out more power:

• a higher-output alternator can handle higher continuous output in a given set of conditions
• a different voltage regulator for older vehicles, as demonstrated by SternWake, increases the voltage available for charging but also increases the coach voltage.
• a Sterling "fake load" regulator will cause the alternator to put out more amps and then will DC-DC convert the voltage up to correct charging range.((http://www.sterling-power-usa.com/library/What%20is%20an%20alternator%20to%20battery%20charger.pdf)). This is the opposite direction of how MPPT charging works. Also see b2b chargers.

also see the Alternator Details page

using the coach battery only

A simple possible approach would be to replace the starter battery with a marine or AGM battery.

charging trailer batteries

Some amount of power can be passed along the 7pin harness, usually enough to maintain the trailer battery's voltage and run small loads. For the purposes of this discussion the important wires in the 7-pin are:

• battery hot lead, typically black.
• ground wire, typically white.

The minimum size for these wires is 12ga and some heavier models use 8ga.

current on different sized wires

Given: a 3% maximum voltage drop and a 40' round-trip wiring run from alternator to trailer battery we can provide Float voltage to the trailer battery at these rates:

• 12ga ~5A
• 10ga ~8A
• 8ga ~10A

DC-DC charging

A configurable DC-DC charger might be able to pass high enough voltage to overcome sag. If the 7-pin is the conduit then we are still limited to the currents listed above.

high-voltage boosting

More power (and more appropriate charging voltages) can be passed along separate wiring or down the 7-pin by injecting higher voltages into the harness.((disregard the relay complication; you can use a toggle switch if desired)) This section will address 7pin injection.

The basic idea is alternator -> DC boost to 36v or something -> run down the 7pin charging wire to the trailer -> MPPT charge controller -> battery

Using 12ga wire as an example, 5A @ 13.6v = 68w**. After the same 3% voltage drop and MPPT conversion losses the boosted setup would deliver **166w, and be able to "smart charge" the trailer battery at appropriate voltage.

workaround: heavier wiring

It's also possible to run a separate and heavier cable from the TV to the trailer; this would minimize voltage sag. If a plain isolator is used with heavier wiring the voltage will still be insufficient to fully charge lead batteries.

Heavier cabling + a DC-DC charger could provide correct voltage to the trailer battery.

isolator without a house battery

Q. An isolator is typically used to charge a house battery, so why install an isolator if you have no house battery?

A. because an isolator can bring Big Current into the cabin for other uses, and do so only when the engine is running. Auxiliary power outlets are typically limited to 10A (120-150w).

Examples while driving, not while idling:

• 120vac 300w rice cooker running on 500w MSW inverter, (~330w total after inefficiencies).
• 120vac 300w wall charger for your solar generator running on a 400w PSW inverter.
• if you have a stout enough alternator, a 120vac 700w instant pot running off an inverter.

This setup would be: starter battery -> isolator -> inverter -> 120vac devices

tweaking the current

With a combiner there is no easy way to adjust current with precision; the batteries are effectively paralleled. You can influence the current a bit if you know what's going on.

With relays and isolators charge current is dependent on

• the difference in voltage between alternator and battery bank. Because of this you will observe highest charging current when the bank voltage is lowest, decreasing as bank voltage rises. This is called the current taper.
• the willingness of the batteries to accept current((mainly an issue with lead chemistries in late Absorption, "solar generators", and Li with exceptionally hamstrung BMS))
• the resistance across the entire charging circuit

....and is governed by the formula I=V/R.

You can't do much about the first two, other than do any elective driving in the morning when bank voltage is likely to be lowest.

The third issue (excess resistance) might be tweaked by the user.

increasing current

• If the NEG return goes through the chassis (the usual approach), then doublecheck the "ground" connection to the chassis for corrosion and looseness. The ground is a likely suspect if the setup charged at a higher rate in the past but has been falling off recently.
• If the ground is good and current acceptance is still too low you can either run a dedicated NEG return wire of sufficient thickness back to the starter battery**. Or **install a DC-DC charger, whichever is easiest/cheapest
• ensure the alternator is putting out its normal voltage((and is not "smart", or variable voltage)). A dying alternator might make less current in absolute terms, or make reduced voltage that indirectly reduces charge acceptance.
• increase the capacity of the battery bank (100Ah -> 200Ah) or replace the bank with a chemistry that will accept more currrent (AGM or LiFePO4); this is a radical change most will not adopt.
• unless the alternator is the sole/main form of charging one might choose to "live with it", knowing that relay charging current will taper off as bank state of charge rises. The silver lining is current will be highest when you need it most (when the bank is low).

Note that, perhaps counterintuitively, the presence of active solar charging will often decrease alternator charging current because the solar is bumping up the bank voltage (case #1 above).

decreasing current

• If there is a dedicated NEG return wire to the starter battery consider routing the "ground" through the chassis instead; that is the usual approach anyhow. (further reading)
• replace the combiner with a diode isolator (bigger drop) or FET isolator (smaller drop) (further reading)
• replace the combiner with a small DC-DC charger

further reading

• Gnomad Home guide to alternator charging
• Split Charging Guide - a British page. Note the following differences in terminology from American English:
  • "split charging" == alternator charging
  • "leisure battery" == house battery
• [How-To articles] related to alternators. Focused on brutal marine charging conditions and externally-regulated alternators, but the info is excellent.
• Thermal Design Challenges in Automotive Alternator Power Electronics

Some or all of the content on this page was originally sourced from this page on RVWiki

This article is incomplete or needs additional work. You can help edit this wiki to improve it!

How much will my battery charge by alternator?

This should be easy, right?

time charging x charge rate in A = Ah replaced in the bank

We know the time part because we're the ones driving. But the charge rate part can be hard to predict with any certainty; this is especially true for combiner ("split charger") charging.

TLDR

• DC-DC charge rate is usually the rated output, so roughly speaking hours driving x rated output = Ah of charging
• charging with a relay is less predictable because current will vary over time
  • The charge rate will likely start off ≤0.33C (33A per 100Ah of capacity) and taper off as the battery charges.
  • the shape of the taper hinges chiefly on battery chemistry
  • this unpredictablity is less important when solar is added.

what limits the current?

the battery bank

In most cases the battery itself will be the limiting factor; it wants what it wants.

These are common maxxes for batteries discharged to their deepest normal state of charge:

• AGM - 0.33C (33A per 100Ah of capacity)
• Flooded - 0.2C (20A per 100Ah)
• lithium - this one is tricky. In theory (like on a bench as in the Victron and Sterling videos) LiFePO4 can pull 1C (100A per 100Ah). In normal split charger installs it tends to pull about the same as AGM.

Since current is a function of C, bigger banks will pull more current than small ones. At 0.2C a 100Ah bank will pull 20A and a 200Ah bank 40A.

Banks will pull more current at lower states of charge and less current at higher states of charge. This affects combiners more than DC-DC chargers (see below).

factors outside the battery bank

Sometimes other factors will intervene:

• DC-DC chargers have a set output limit (20A, 40A, etc)
• resistance in the wiring and connections can limit current
• if the bank is oversized and current otherwise-uncontrolled the alternator's output can be maxxed out. So don't do this unless you want to buy a new alternator

estimating DC-DC charging

DC-DC charging rates are more predictable because of how they work. A 20A DC-DC will likely pump 20A into the bank most of the time. So 1/2 hour driving x 20A = 10Ah returned to the battery bank. The price of this predictability (and other features) is... price.

If you need predictable charging then DC-DC is likely the answer. There are other scenarios where DC-DC is effectively required.

estimating combiner charging

This is the tough one. We know roughly where the charge rate will start out for discharged batteries (≤0.33C) but the taper complicates things. All the batteries would eventually charge to full given enough time, so shorter charging periods are what interest us. Let's assume a short drive = 30 minutes.

NOTE: this section contains guesswork from observations and theory. You will learn how your particular setup behaves in actual use.

• FLA taper is steep; charging current starts at ~0.2C and quickly falls off within seconds or minutes. Current will likely stabilize around 0.1C by the end of the drive. So we might use an average of 0.12C for our charge rate when estimating.
  • 100Ah x .12C = 12A.
  • 0.5hours of driving x 12A = 6Ah replaced.
• AGM taper is more gradual, so charging currrent stays higher on short drives. It might only taper to 0.15C by the end of the drive. We might use 0.175C as our average rate. 8.75Ah replaced.
• Lithium has an odd taper because the charging voltage curve is not linear.
  • in the broad "flat" portion of the voltage curve (20%-80 state of charge) current will likely stabilize around 0.2C. We could use 0.2C as our average rate. 10Ah replaced.

in the real world

In most installs this variability isn't a major issue:

• combiner charging is commonly paired with solar charging (also highly variable!)
• alternator charging is often a backup or "nice when you can get it" charging source. Exception: there are scenarios that depend on it.

Some or all of the content on this page was originally sourced from this page on RVWiki

---

Alternator details

Also see the alternator charging overview article.

theory of operation

An oversimplified overview will help us understand the challenges and approaches to overcoming them.

• An alternator makes alternating current ("AC", hence alternator) by rotating an electromagnetic - field through stationary wires ("windings")
• the AC is turned into direct current ("DC") when passing through "rectifying" diodes
• voltage output is controlled by current production
• current production is controlled by varying the field current energizing the electromagnetic field

All vehicles monitor("sense") output voltage and adjust alternator output to maintain that voltage. Modern vehicles often monitor alternator heat and limit current to prevent damage. Older vehicles may not.

Setups with external requlators (see below) can carefully control alternator output to achieve specific results.

Many of the lessons learned about bank charging by alternator have been learned from people who live on boats. Most of their power is generated from the alternators while they "cruise" at slow speeds throughout the day. Their use case is extreme:

> On boats alternators are low RPM, in an enclosed space with bad airflow, charging huge batteries and large loads

the challenges

Our banks will see two things when charged from alternator: voltage and current. Bot need to be appropriate for the chassis and the house bank.

Producing power makes heat as a by-product. Excess heat can destroy the alternator. How do we balance the bank's thirst for current while protecting the alternator from itself?

voltage

Alternator voltage output is intimately related to current. In the simplest model the alternator's voltage is controlled by a voltage regulator. The regulator uses current to hit the desired voltage setpoint.

• vehicle voltage stable at 14.0v (for example)
• loads are added, causing system voltage to drop to 13.7v
• regulator tells alternator to make more current, causing voltage to stabilize at 14.0v
• loads are removed, causing system voltage to rise to 14.2v
• regulator tells alternator to make less current, causing voltage to stabilize at 14.0v

>> [the alternator] has no way of knowing how great the load is. It only cares about the voltage at the regulator terminals. if it is below the set point, it will produce all of the power it possibly can at the given RPM's. If it is at the set point, it will reduce the power generated to the level that will not overshoot the set point voltage. The battery is what is supplying the power to the load and the alternator is trying to keep it at the set point voltage. -- hwse

Note: Smart alternators do not have a firm voltage setpoint; they vary the voltage target based on conditions. This is intended to improve overall MPG. Voltage might plummet during full acceleration (to free up engine power) then spike quite high in other conditions. This wide variance complicates bank charging, and typically means a DC-DC charger is required.

regulation

Most alternators have been internally-regulated -- the voltage regulator is built into the alternator case itself.

Externally-regulated alternators are alternators where the regulation function is physically separated from the alternator. There may be a stand-alone external regulator or the engine ECU may assume those duties.

In a camper/RV context an externally-regulated alternator typically means an aftermarket regulator, like Balmar - voltage often provides multistage (smart) charging, temperature-based derating, etc. Read on.

A separate alternator (see below) is not required to use external regulation but that is a common scenario. If one externally regulates the vehicle's sole alternator the chassis could be exposed to higher or lower voltages than it was designed for.

Common external regulators include:

• Balmar ARS-5 (no external voltage sensing)
• Balmar MC-614 (external voltage sensing)
• Xantrex Xar (no external voltage sensing)
• Wakespeed WS500 (CANBUS)

tricking the regulator

Since the alternator only knows what voltage it is receiving at the regulator, an alternator can be tricked into adjusting output voltage by:

• sensing voltage at somewhere other than the starter battery
• placing something inline with the sense wire that changes the voltage. This might be done to increase charging voltage in order to increase current, or might be to compensate for voltage drop through a diode-based isolator. Be careful to keep alternator output voltage in a range acceptable to the vehicle itself.

current

The specs for your alternator will include an amp (A) rating. This might be 80A for a passenger car, 150A for a cargo van, or >200A for a heavy duty vehicle. You can often find the actual rated output of your alternator in your manual, vehicle specs, or using a VIN lookup tool for your brand of car.

This rating is a measure of how much electrical current it can make under optimal conditions, typically:

• while the alternator is cold; and
• being spun at an appropriate RPM

These conditions are not often present in normal use. For this reason a continuous duty rating** (CDR hereafter) is more helpful for assessing how hard we can push the alternator. In the absence of a stated CDR we can assume CDR is roughly **50% of the alternator's output rating**. The continuous duty rating is the limiting factor when charging house batteries or running other long-duration loads. Note that **the CDR does not represent how much power is available for house battery charging; the vehicle still has to run its own loads like ECU, lights, fans, etc.

For the purposes of this discussion we will assume a cargo van with an alternator rated 140A.

how many amps can I pull from the alternator?

(external image) Alternator output in the short term and longer term is affected by several factors

• the rating above
• alternator RPM, which is proportionally related to engine RPM.((the alt pully to crank pulley ratio is typically around 3:1; the alt spins 3x as fast as the crankshaft)) The rated output above cannot be reached at idle((and shouldn't be, due to reduced ventilation because the vehicle is standing still))
• heat, which is the destroyer of alternators

In the absence of further information one might use some crude rules of thumb:

• at idle** discretionary loads **≤25% of rated output.((50% of CDR)). For our 140A alternator this would be 35A. Note: this is about stopping at red lights or in traffic; idling the engine to charge batteries is not recommended.
• while driving loads ≤33% (66% of CDR) For our 140A alternator this would be 47A.
• if alternator output voltage drops below normal, the belt starts to squeal or engine RPM is affected the alternator is overloaded and loads should be reduced or charging disabled.

One could make a more informed decision this way

• acquire the output chart for your alternator
• use a clamp meter to see what current the chassis draws with blower on, lights on, etc at a given engine rpm
• do the math: [output rating at that rpm / 2] to get continuous output rating at that rpm. Subtract chassis loads. The remainder is what you have left over to use.

The optimal arrangement** is a temp-sensing external regulator that runs the alternator at **maximum safe output when required and lower output when temps get too high.

A poor man's version of the temp-sensing regulator involves epoxying a thermistor to the alternator housing. This turns off the charger/relay at a given temperature.

In some scenarios output might be limited by placing a resistor in series with the alternator's field control wire.

If one is completely stumped:

• default to the smallest DC-DC available, typically 15A-20A
• avoid idling the van to charge

effect of alternator charging method on current

The various charging methods have different costs, some have different install locations, levels of voltage/current control, etc.

Example: since charging voltage is a fundamental part of the current formula I=V/R we can have different charging currents, all other things being equal. Let's assume a battery bank at 12.1v, alternator at 14.2, total circuit resistance of 40mR, and a DC-DC configured to charge at 14.4v:

• relay - (14.2-12.10) / 40mR = 53A
• diode-based isolator - (13.5-12.10) / 40mR = 35A
• DC-DC - (14.4v - 12.10) / 40mR = 58A

heat

Heat in the alternator comes from

• the engine block (radiation)
• ambient temperatures
• the alternator itself - alternators are most efficient (produced the least heat per watt) at normal vehicle cruising speeds. At low RPM and high RPM more heat will be generated:
  • rectifying diodes, which increases as current demand increases - some setups place the rectifying diodes outside the case to reduce this heat
  • bearing heat (increased friction from failure, misalignment, overly-tight belt, etc)
  • the coils

Cooling comes from:

• alternator fans, typically two attached to the rotor inside the case, but some have external fans. These fans do not move sufficient air at idle RPM.
• ambient airflow (breezes, forward motion, external fans). When the vehicle is not moving airflow under the hood is greatly decreased.
• case surface area, so large-case alts tend to run cooler
• current reductions, if temperature-sensing regulation is present
• moving hot items outside the case (see rectifying diodes above)

> If you want the alternator to run cool rip out the internal rectifier and build an external rectifier... Otherwise fit large frame alt and run it at a current limited output. Maine Sail

heat-related failure modes

Alternator heat is typically measured on the case, although internal parts will be hotter.

> Temp sensor should be measuring as close to the stator as possible

A common rule of thumb for a safe upper limit on case temps is 100C/212F, although more conservative temperatures might be advisable.

(external image)Critical internals will be hotter than the case itself. For example, during the infamous Victron video both internal and external temps were given for the Balmar setup making 50A @ 2,100rpm. The case was 42C (108F) while the internals were 74C (165F). This alt was on a bench rather than mounted to a hot engine block. See table at right for examples of external/internal temps observed during lab testing.

• rectifier diodes are typically designed for a max of 150C (300F). More modern alternators may have diodes that can survive higher temperatures.
• the varnish insulation on the windings will be degrading by 155C (310F)
• if temperature compensation is present it may begin around 255-260F. The quote above about using 120C may be a designed to cut alternator charging just before the ECU/regulator kicks in.

thermal switches

In a comment on an excellent video about charging lithium from the alternator WorkingOnExploring talks about adding overtemp protection with a thermal switch (a small bimetallic switch):((see this teardown and demo video on youtube))

>> a KSD9700 120C, normally closed thermal switch epoxied to your alternator case. That way, if some abnormal event occurred and the alternator overheated, it could deactivate the [combiner] till the alternator cooled. It would likely cost less than $20 to install this thermal safety.

The idea here is to run the relay's control circuit through the N**ormally-**Closed thermal switch. When the switch hits the defined temperature the circuit opens and power to the combiner is cut off; house bank charging from the alternator is stopped. The switch will typically reset when temperature drops by ~25%; read the datasheet for your switch for specific details.

If one had a DC with a current-limiting function (renogy's 20-40-60A models, for example) or DC-DC in parallel one could use two different-spec switches to achieve staged charging. Using placeholder temps:

• run an aggressive charging setup full blast
• at 90C engage current limiting
• at 120C disable charging completely

Note: 120C = 248F. Other temps like 90C (194F) and 100C (212C) are available and may be gentler on the alternator. Because the alternator is mounted to the block the alternator case may be at the ~same temp as the block. Check case temps when engine has been running to get a feel for how hot the case should be, preferably at the location the diodes are heat-sinked to the alternator case.

manual switches

It is often possible to disable alternator charging when ambient heat, traffic jams, or other factors conspire to overheat the alternator.

known temperature configurations

• Balmar alternators are rated for continuous duty at 85C.
• The Balmar Series 6 normal temp is 82C with max of 108C.
• The Balmar ARS-5 with MC-TS-A temp sensor by default starts derating alternator output at 109C.
• The Balmar MC-612 with temp sensor starts derating at 107C. It can derate ~55%.
• DC Power Solutions alternators are ok to 107C.

idling to charge house batteries

Idling the engine to charge house batteries is a perfect storm of underperformance

• the alternator is spinning too slowly to make the CDR

and heat

• the alternator is spinning too slowly for internal cooling fans to work well
• the alternator is producing more heat than power due to inefficiency
• there is no airflow from forward movement to directly cool the alternator...
• ... or to remove superheated air under the hood

The situation is bad enough with relays, but even worse worse with DC-DC because

• they tend to hold full output for hours on end rather than tapering
• when the alternator struggles and voltage drops the DC-DC will demand more current to make its output target. Hopefully the charger has a minimum chassis voltage for operation and will not drag the chassis down excessively

anecdote

>> We have 500ah of lithium that can be charged at 3C, a theoretical 1,500 ampere charge rate. We have the Nations 280 amp second alternator. We control the Nations alternator with a DIY regulator that allows us to pretty much set the charge level anywhere we please. We'd be pleased to set it to 200+ amps, but that turns out to be impractical. The reality is, if we set the charge rate to 125 amperes, we cannot continuously charge at this rate while standing still unless the outside air temperature is 55 degrees or below**. **When driving at highway speeds (which cools the alternator), we can run a continuous 150 ampere charge rate at virtually any outside air temperature. While we have not tested this further, we suspect that should we desire to 'up the ante' much beyond our current 150 (sometimes 175) amperes, we would again find ourselves temperature limited even while driving. - winston

common modifications

external regulation

External regulation can provide overtemperature protection (see section on Heat above), and can also cause the alternator to behave like a smart charger with multistage charging.

P-type vs N-type regulation

The alternator is controlled by varying the signal on the field control wire. The strength of the electromagnetic field drives current, which drives system voltage.

The field control wire can be on the P**ositive or **Negative side of the field control circuit. Most internally-regulated OEM alternators are P-type with the regulation occurring on the positive side.

Balmar only builds P-type regulators because:

>> With an N-Type design, if the field wire shorts to ground, or if the alternator internally shorts to ground, the alternator will immediately produce full output... with... P-Type regulation... if the field wire shorts to ground or the alternator develops an internal short, the included fuse on the field wire simply blows and the alternator stops all output... When converting these units to be used with a Balmar external regulator, the alternator must also converted to P-Type for safety and compatibility with our regulators

Wakespeed apparently regulates on the P side but offers a N-type harness:

>> The vast majority of the time, a P-type harness will work — although there are some regions like Europe, where externally regulated N-type alternators are more commonplace. In most situations where an alternator is being modified to support external regulation, the technician making the modification has the option to set up the alternator for P-type regulation. This is by far the most commonly chosen field excitation route. Just about all of the aftermarket externally regulated alternators available in the U.S. will be configured to be excited on the positive brush. If you are not familiar with the modification process, we do recommend having a competent alternator technician modify your alternator for you.

Heavy duty / high output

Alternators can be built in heavy duty configuration, which typically connotes:

• higher current rating
• increased internal cooling (ie, more or better fans)
• ability to make more power in idling scenarios, which is useful for police vehicles, ambulances, and other vehicles that need to idle for long periods of time.

HD alternators for any particular vehicle may be bolt-ins with no modifications required. They do cost somewhat more. The labor cost would remain the same so some people choose to upgrade the alternator when/if the OEM one fails.

Nations and similar alternator builders make alternators for specialty uses.

Note that increasing alternator output may require upgrading stock chassis wiring:

>> To run a [high output] alternator, ALL of your engine's factory high current cabling will need to be [sized] to support it, including engine and starting battery grounds, and alternator positive. -- MechEngrSGH

Idle optimized

Some tricks are possible to get higher outputs at lower alternator RPM. Denso uses square windings in some units to pack the windings more tightly.

Smaller alternator pulleys may be used to increase alternator RPM at low engine RPM. Caveat: this can overspeed the alternator on higher-rpm engines.

large case

Alternator cases come in two general sizes: large and small. Most vehicles come stock with small case alternators because they are easier to "package" (fit into the available space). Semis, boats, and other vehicles with fewer space limitations may use large case alternators, allowing for more internal cooling, larger or more separated components, ease of repair, higher constant duty outputs, etc.

One is not better than the other; they are meant for different applications.

The mounting bolt pattern may be the same or it may require a different mount. Pulleys, belts, and/or belt lengths may need to be changed.

aftermarket dual setups

In some setups a second alternator is fitted, often externally regulated (see above) and dedicated to charging the house bank. This allows different voltages (24v or 48v) for charging specialty house banks.

If the house bank is Lithium one must plan for sudden BMS disconnect and the effect this would have on an alternator. In a single-alternator scenario the starter battery would provide buffering against load dump.

Balmar 2nd alt kits for Sprinter, Transit, and ProMaster can be seen in this brochure (pdf).

OEM dual setups

OEM (factory) dual alternator setups may not behave as one expects. Typically they are controlled separately but both tied to chassis voltage.

Ford

On recent models the dual alts are set up so the primary maxxes out before the secondary kicks in:

> In dual generator systems, the PCM keeps the secondary generator in a standby state where it does not generate current unless the primary generator is generating full power and more current is needed to support the vehicle loads. The PCM monitors the output of the primary generator and adjusts the control setpoint of the secondary generator to cause it to provide additional current when needed.

This behavior surprised Mortons on the Move, thwarting their alternator-charging plans.

Known configurations:

• 397A = 220A + 157A
• 332A = 175A + 157A

There may be Stationary Elevated Idle Control (SEIC) ("high idle") available.

RAM

• 380A = 220A + 160A

> There is a process to electronically remove the second alternator from the computer by deleting the sales code and that essentially has the truck setup as if it came with just a single one from the factory.

DUVAC

Some older Class A and C RVs came with DUVAC (DUal Voltage Alternator Control) setups, which use separate voltage-sensing wires for the house bank.

> This Duvac system that Leese Neville came up with was a completely integrated dual battery/isolator setup that was “state-of-the-art” in ’99 but soon was obsoleted by newer better technology.

> ...the primary reason for DUVAC is to allow the alternator to "compensate" for the voltage loss (turned into heat) in the old generation diode-based battery isolators. There is approximately a .7 VDC loss through the battery isolator. The sense wire/terminal allows voltage on the battery side of the isolator to control the alternator voltage.

{As far as I can tell this was a workaround from when many isolators were diode-based so the alternator could not "see" aux battery voltage on the combined circuit -- secessus} See this TSB. (PDF)

sudden load disconnect

The alternator's voltage regulator responds to demand, increasing and descreasing field strength (and therefore output) as required to hold a given voltage setpoint.

• when large loads are suddenly added chassis voltage will drop briefly while the regulator responds.
• when large loads are suddenly removed chassis voltage will spike briefly while the regulator responds. If the chassis cannot absorb this spike the excess power can be dissipated within the alternator itself and cause damage

In theory if the BMS disconnects charging the resulting spike could damage the alternator. Team Karst explains it colloquially:

>> Alternator is chugging along, delivering 50 Amps. Suddenly, the output is disconnected. Since the regulator has a time constant, plus the field current can’t collapse instantly, plus stator windings, being coils, therefore inductive, they hold energy, there becomes a high positive voltage at the B+ port to keep the current flowing for a time. This transient energy may manifest itself in switch arcs, and other undesirable voltage excursions.

This is called a load dump; the load is dumped (demand reduced to 0A) and there is nowhere for power in the alternator to go. As we will see, in normal installs there are places for the power in the alternator to go.

In practice:

• [a wise user will not be bouncing off the BMS in the first place -- Secessus]
• the vast majority of lithium banks charged by alternator do so in combination with the starter battery. The lead battery will help absorb spikes from the alternator. "Batteries have about 1000F ( farad ) [capacitance] per 150Ah of capacity so they act as very substantial capacitors. Hence they have enormous ability to absorb spike energy.",
• modern alternators have avalanche diodes and fast-acting regulators that can adjust within 100ms.
• if BMS disconnect caused alternator damage the Li-Bim recommended by Battle Born would quickly eat alts. It disconnects the bank from alternator every 35 minutes.

The question for the user is: how much charging current are we talking about, and does the alternator already handle that level of sudden disconnect under normal operation? Secessus provides this example:

> my van's radiator cooling fans are rated at 65A and turn on/off frequently. My Li charging current is less than that.

In addition, the speed of the disconnect plays a part:

> there is a massive difference in the voltage spike from a 5 uS relay disconnect and a 10mS mosfet disconnect

exception: dedicated secondary alternators

Secondary alternators dedicated to Li charging will not have a starter battery or other loads inline to absorb spikes. In this case the owner might:

• use a CANBUS or similar setup where the BMS can notify the alternator regulator of imminent shutdown ahead of time to allow orderly reduction of power; or
• use a protection device, like [[https://www.sterling-power-usa.com

/SterlingPower12voltalternatorprotectiondevice.aspx|the Sterling Alternator Protection Device]]; (manual), Balmar APM, etc.

• some maintain a small lead battery in parallel with the Li bank

Note that the Sterling device uses "a small resistive load" (milliAmps) to control the spike.

further reading

• Nigel's Too Hot to Handle article
• Thermal Design Challenges in Automotive Alternator Power Electronics
• [case vs Large case alternators. Know your application!] (youtube)
• WorkingOnExploring series on alternator charging
  • Part 1
  • Part 2
  • Part 3
• transient voltages
  • An Introduction to Transient Voltage Suppressors (TVS)
  • Automotive Load-Dump Circuit-Protection Headaches

Some or all of the content on this page was originally sourced from this page on RVWiki