Battery monitors

Battery monitors are a necessary component of any residential vehicle with an leisure batteries.

We want battery monitors be analogous to a fuel gauge, but they are not; rather, they are more like a "miles/kilometers until empty" range prediction system that analyzes recent usage data to make an educated guess about how far you might get.

Battery monitors typically show:

• battery voltage (Vbatt), more helpful with lead than lithium (due to Li's very flat voltage curve); and
• Amps or Watts in (charging) / out (discharging)
• a calculation on capacity used/remaining

For the monitor to keep accurate count, power from all charging sources and to all loads must pass through its shunt, a relatively heavy piece of metal that reacts predictably to current. Typically the shunt is placed between the battery bank's negative post and the system's "ground" (negative/return side of the circuit). The monitor display is mounted remotely where it is convenient to view.

Most people will choose a "bidirectional" monitor (counts both charge and discharge amps); read the specs and reviews to make sure.

How battery monitors work

Broadly speaking the monitors work like this:

• you tell the monitor the battery's capacity, usually in Ah and printed right on the battery. Remember that lead batteries are typically only used to about 50% of their rated capacity. You'll either be making that mental adjustment when you configure the meter or when you read it later. Battery capacity will also degrade over time, requiring the capacity setting to be adjusted periodically.^[1]
• the monitor will watch for your system to hit the configured 100% state of charge voltage, at which it assumes your bank is "full". It will also stop at 100% if it counts enough amps going in. Then,
• when your system drops below that voltage it counts Amps/Watts in and out, thereby calculating the percentage of capacity used and percentage remaining. This is an imperfect science with lead batteries because 100% SoC is only reliable after a complete charge, and because different discharge rates will change the effective capacity. Think of the monitor as general guidance for lead batteries rather than gospel. Lithium banks will align more closely with what the monitor shows.

Note: Watching amps trail off at the end of lead Absorption (endAmps) will also tell you when the bank is fully charged; you might manually reset to 100% at that time.

Gotchas

• Monitors aren't magic; they are estimations.
• Lead batteries have a noticeable Peukert effect, varying apparent capacity depending on discharge current. The battery's rating is measured at C/20, or 5A per 100Ah of rated capacity. Heavier loads will reduce apparent capacity, and lighter loads will increase apparent capacity.
• battery capacity "walks down" (reduces) with age and injury.
• SoC estimates tend to "drift" so the monitor needs to be recalibrated from time to time
• Monitors can count inaccurately or fail to count at all when currents are very small, typically <1A
• very high or very low charging currents can cause the reset voltage setpoints to be unreliable

Commercial examples

Bogart Trimetric

The most famous battery monitor is the Bogart Tri-Metric TM-2030 series. This monitor will interface and operate their SX-2030 solar charge controller. The monitor will still provide amp-counting and other metrics when used on it's own.

Inexpensive shunts

For folks who are [dis]charging at 50A/75A, a cheaper shunt is available for 1/5th the price of a Bogart.

Heavier-duty shunts are available (up to at least 350A).

Read the specs and comments for the monitors to ensure they measure current in/out of the battery. Some inexpensive displays only measure in one direction. Monitors that appear to be useful for vandwellers: (do your own reading!)

• DC 0-120V 0-100A Volt Amp Ah Power Capacity Percent Battery Monitor Watt Meter
• DROK DC 0-300V 200A... AH Monitor Panel with Hall Sensor
• AiLi Battery Monitor DC 300V 100A Voltmeter Ammeter Voltage Amp Meter Gauge with Hall Effect Sensor Transformer
• AiLi Battery Monitor Voltmeter Ammeter Voltage Current Meter 8-80V 0-100A Note: on some of the AiLi you can press V+% to toggle the backlight.
• Yeeco Digital DC Multimeter, DC 0-90V 0-100A... Coulomb Time Meter Battery Capacity Tester Monitor...Hall Sensor