AGM batteries

the case for AGM

There are situations where AGM is worth spending twice as much compared to flooded lead-acid.

• the ‘dweller installs the battery bank in a location where it is not reachable for normal maintenance
• the ‘dweller needs to mount the battery on its side for some esoteric reason (or is 4×4 trekking so the RV itself can end up in weird angles)
• the nomad is a heavy off-roader and needs the internal plate support AGM provides in violent shaking and vibration
• the ‘dweller wants to run heavier-than-normal loads relative to bank size (microwaves, electric coffeemakers, electric cooktops, etc). Note this means the battery can be drained faster, not that it provides more capacity. Overall charging will take roughly the same amount of time.
• the ‘dweller has has an actual (not imagined) chemical hypersensitivity
• the advanced ‘dweller is running a single battery for both house and starter systems (“shallow cycling”) <– not for beginners!
• the nomad is intentionally running the bank in PSOC and wants to get through Bulk charging as quickly as possible due to limited charging opportunities.

Lithium does things as well or better for lower overall cost-per-kHw, but still costs more upfront.

Keeping your AGM bank happy and healthy

In The Beginning, emergency backup power for industrial purposes was done with Flooded Lead-Acid batteries (FLA) and inverters. It worked well at the time, but there are drawbacks:

• FLA batteries require regular maintenance ("watering")
• FLA batteries have relatively high internal resistance and so cannot dump massive current on demand. This means huge banks of FLA to meet big current demands.
• When sitting still for long periods FLA electrolyte can stratify (separate into layers with different acidic strength). This affects both performance and longevity as plates are damaged by areas of concentrate acid.^[1] In mobile installs like vans stratification is much less of an issue.

AGM addressed all these issues and became the standard for backup-power banks. This is why telcos have lots of AGM to surplus out -- the base of cell towers contain AGM banks for backup, and they are rotated out on a schedule.

Note that sitting on shore power indefinitely then dumping power on demand is the opposite of deep cycle use.

There are use cases where AGM makes a lot of sense, but otherwise AGM is not a natural fit for vandwelling. It's advantages are difficult to realize in a van and the disadvantages are front and center (2x cost per kWh, charging issues discussed see below).

So why do people default to AGM?

• because everybody else does, or did until LiFePO4 became the hot new trend
• concern about maintenance or ventilation

The dark side of AGM's ability to discharge and bulk charge at higher rates is

• AGM requires more current during Bulk charging. Typical minimum charge rates for AGM are 0.15C to 0.2C. Some premium AGM require a minimum of 0.5C, or 50A per 100Ah of capacity. Failure to provide sufficient current regularly leads to a "walk-down effect" where the AGM underperforms until it receives a good kick in the pants.
• Higher-current bulk charging means shorter Bulk duration and therefore extended Absorption duration. Vandwellers struggle to provide adequate normal Absorption duration, and completing an extended duration is even less likely.

The easiest way to provide AGM with what it needs is to fully charge daily, combining

• alternator charging for Big Current in Bulk Stage
• solar charging for sufficient duration in Absorption stage