Battery Monitor (Shunt)

Voltage alone is a poor guide to how full a battery is, especially LiFePO4 whose voltage stays almost flat from full to nearly empty. A battery monitor solves this by counting current. A shunt — a precision resistor wired into the battery's negative lead — lets the monitor measure exactly how many amps go in (charging) and out (loads), and integrate that over time into a true state-of-charge percentage and time-remaining estimate.

It is the single most useful instrument in a van: it tells you whether the fridge plus laptop will outlast the night, and whether the solar actually replaced what you used. Victron's BMV and SmartShunt are the common reference units, read over Bluetooth. Without one, an all-electric build runs blind.