Short answer: a shunt-based monitor is the most accurate way to see state of charge for any battery bank; built-in battery Bluetooth is fine for one or two batteries; voltage-only meters are unreliable for lithium because LiFePO4 voltage stays nearly flat.
| If you need… | Look for |
|---|---|
| Accurate % for a multi-battery bank | Shunt monitor rated above your max current (e.g. 500 A shunt for a 2–3 kW inverter) |
| One lithium battery with Bluetooth | Use the battery's own app; add a shunt later if you expand |
| Cheap lead-acid check | Voltage meter (rough estimate only, at rest) |
| Seeing solar + loads at once | Monitor that pairs with your charge controller, or a system display from the same maker |
Install the shunt on the negative side so all loads and chargers pass through it, and set the battery capacity and chemistry in the monitor. Follow the monitor's wiring diagram and fuse its sense lead as instructed. Pair with the runtime calculator to plan.
Safety: These are general estimates, not a design. Follow your equipment manufacturer's instructions and local electrical code, and consult a qualified electrician for anything connected to shore power, a house, or high-current batteries.
LiFePO4 voltage barely changes between ~20% and ~90%, so a voltmeter can't tell you much; a shunt counts amp-hours in and out.
Rated above your maximum continuous current, including the inverter's DC draw (inverter calculator).
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