Voltage is a terrible fuel gauge

Here's a thing that surprises people: you cannot judge a LiFePO₄ battery's state of charge by voltage.

Lead-acid has a fairly clean voltage curve — a decent guess is possible. Lithium has a famously flat discharge curve. Which means the voltmeter says "fine, fine, fine, fine — empty."

The fix is a shunt-based battery monitor. A shunt is a precision resistor on the main negative cable. Every amp in and out passes through it, and the monitor counts them. That's real accounting, not inference.

What it gives you: - True state of charge as a percentage — the fuel gauge you thought you had - Live current in and out — instantly see what each appliance costs - Time remaining at current draw — the number that changes behavior - Historical data — how deep you actually cycle, so you size the next upgrade with facts

The benefit most people discover within a week: you find your phantom loads. Almost everyone has something drawing power they didn't know about.