Enter the resting voltage of a flooded or AGM bank and the calculator reads the state of charge off the maker's table, interpolating between its points. The table underneath shows every point for the bank voltage you chose.
Where the figures come from
The flooded table is Trojan's, from the T-1275 data sheet: 2.122 V per cell at 100% down to 1.918 V at 10%, which is 12.73 V to 11.51 V on a 12 V battery. The AGM table is from Trojan's 31-AGM data sheet: 2.14 V per cell at 100% down to 1.94 V at 0%, which is 12.84 V to 11.64 V. A 24 V bank has 12 cells, so its figures are double.
Makers differ. Lifeline treats its AGMs as full at 13.0 V or more after 4 hours of rest. When you know the battery, check its data sheet before you put a percentage in a report.
Taking the reading
- Disconnect the loads and the chargers, including solar and shore power.
- Let the bank rest: Trojan asks for at least 6 hours, preferably 24.
- Measure across the bank terminals with a DC voltmeter.
A survey rarely allows 24 hours. Note in the report how long the bank had rested, and read anything taken soon after charging as high.
Trojan's maintenance guide says to charge at 70% or below, and that a battery reading below its table may have been left discharged too long or have a bad cell.
Lithium banks
LiFePO4 batteries keep a nearly flat voltage across most of their range, so voltage tells you little about their charge. Read the BMS display or a shunt monitor, and check the BMS is there at all: see lithium batteries without a BMS.
Citing it in a report
"House bank, 2 × 12 V AGM in parallel, 12.31 V after 8 hours at rest, about 55% charged. Recharge fully and capacity test before relying on the bank." Give the reading, the rest time and what the owner should do.