What does 13.3V on a 12V LiFePO4 battery tell you about its charge level?

Direct answer: On its own, not much. A 12.8V LiFePO4 battery spends most of its usable range within a few tenths of a volt, so 13.3V could fit a wide range of charge depending on whether the battery is resting, charging or under load, and on temperature. Use a battery monitor with a shunt or the battery’s own state-of-charge reading for decisions, and use voltage mainly to spot the ends of the range: near full on charge and near the low cut-off.

The numbers you can rely on. Redway’s 12V 100Ah, 12V 150Ah and 12V 200Ah pages all list 12.8V nominal, 14.6V maximum charge and 10V discharge cut-off. Those are the window limits. Between them, the voltage curve of LiFePO4 is flat over much of the range, which is why a small voltage change can mean a large change in charge.

Rest, load and charge. Voltage read while a charger or solar controller is active will be higher than resting voltage; voltage read while an inverter is running will be lower. For a meaningful voltage reading, disconnect loads and chargers and let the battery rest, then measure at the terminals with a reliable meter. Even then, treat the result as a rough guide.

Why we do not print a table. Generic LiFePO4 voltage-to-charge tables circulate widely, but cell makers, temperature and measurement conditions differ, and Redway does not publish one on these product pages. If you need a reference table for a specific model, ask Redway for it in writing and note the rest time and temperature it assumes.

Better tools. A shunt-based battery monitor counts amp-hours in and out and gives a state-of-charge figure that does not depend on the flat curve. It needs setting up with the bank’s capacity and occasional full charges to resync. Some Redway 12V pages list optional Bluetooth or an LCD display; if your battery has one, use its reading. For charger settings see charger settings for 12V 100Ah.

Avoid false ‘dead battery’ calls. A battery that reads 13.3V is not faulty, and a battery that drops quickly near the bottom of its range is behaving as LiFePO4 does. If the battery shuts off under load with voltage apparently left, see why a 12V lithium battery shuts off when an inverter starts.