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Battery monitors

We want battery monitors be “gas gauges” but they are not. The monitor is more like the Range prediction on the dash that uses historical and present information to make an educated guess about how far you might get.

Battery monitors typically show:

  • battery voltage (Vbatt), more helpful with lead than lithium1); 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 systems's “ground”.2) 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 they work

Broadly speaking the monitors work like this:

  1. 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.3)
  2. you tell the monitor the voltage at which the battery is presumed to be at 100% State of Charge. For lead this will be something like 12.7v, and for LiFePO4 it will be something like 13.8v – check your battery manufacturer's technical info for this value.
  3. the monitor will watch for your system to hit the configured 100% state of charge voltage, at which it assumes your bank is “full”. Then,
  4. 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. The battery manufacturer will specify something like C/200 or C/100 as a sign Absorption is complete.


  • 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.

commercial examples

Bogart Trimetric

images-na.ssl-images-amazon.com_images_i_41vczv0vvml.jpg 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

images-na.ssl-images-amazon.com_images_i_41zuqefexvl._sy90_.jpgFor 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:4)

due to Li's very flat voltage curve
negative/return side of the circuit
every 6 months? 12 months?
do your own reading!
electrical/12v/battery_monitor.txt · Last modified: 2021/07/30 17:41 by frater_secessus