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:
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.
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.
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)