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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 information to make an educated guess about how far you might get.
Battery monitors typically show:
Broadly speaking the monitors work like this:
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.
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).