Battery monitors typically show:

• battery voltage (Vbatt), more helpful with lead than lithium; and
• Amps or Watts in (charging) / out (discharging)

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:

• battery capacity - 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.
• You may also need to tell the meter what your battery manufacturer's 100% state of charge (“full”) voltage is

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).