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We want battery monitors be “gas gauges” but they are not. The monitor is more like the Range information on the dash that uses historical and present conditions to make an educated guess about how far you might get.
Battery monitors typically show:
Broadly speaking the monitors work like this:
In general, the monitor will watch for your system to hit a configured 100% state of charge voltage and starts subtracting amps (and percentage of capacity) when the system drops below that voltage. 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 rather than gospel.
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.
Another note: one benefit of having a shunt-based monitor over a charge controller monitor is the shunt can discern between [dis]charging vs [dis]charging+loads.
Meters typically need a few settings defined:
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.
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).