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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; you might manually reset to 100% at that time.
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.
Not everybody needs prosumer-grade monitors. There are simpler models available for less money.
{{ https://m.media-amazon.com/images/I/51-5-oQoDAL._AC_UL320_.jpg]] The most popular inexpensive shunts are probably the round AiLi. They are somewhat cheaper off ebay and can be found for ~$25 shipped off AliExpress if you can wait that long.4)
As with many other Chinese products, they are also available under different names but appear to be functionally identical.
The three buttons access 4 screens, described below. Each screen shows a 10-bar battery gauge State of Charge icon at the top and a small charge/discharge icon on the left. When charging the battery gauge icon is animated (present SoC –> 100%) and when discharging it statically shows present SoC.
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).
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:6)