Battery Voltage to Percentage Calculator

The flooded column is the 12V column of the open-circuit voltage table in U.S. Battery's user manual. Trojan prints the same column, row for row, in its battery maintenance guide; the two differ at only two rows, by 0.03V at 60% and 0.07V at 40%. The AGM column is the 12V column of Concorde's Lifeline technical manual.

Notice the two open rows in the AGM column. Lifeline prints its top row as "greater than 12.78V" and its bottom row as "less than 11.58V" rather than as fixed numbers, so this calculator reports anything above 12.78V as 100% and anything below 11.58V as 0%. The flooded column has closed ends, so readings outside 12.73V and 11.35V sit off the printed table and the calculator warns you instead of quietly extending it.

One caution on the AGM figures. U.S. Battery publishes its own AGM table that runs 0.16V to 0.24V above Lifeline's at every row: where Lifeline puts 100% at 12.78V, U.S. Battery puts it at 13.00V, and where Lifeline puts 40% at 12.06V, U.S. Battery reads that same voltage as 25%. Lifeline prints the reconciliation itself, saying its levels apply to aged batteries and that a new battery reads higher, about 13.0V at full charge. So the calculator answers an AGM reading on Lifeline's table and shows U.S. Battery's answer beside it. A new AGM battery reading 12.90V is not broken and it is not overcharged, it is simply young. Sizing a bank around those numbers is a separate job, and our AGM runtime calculator handles it.

For a 24V bank, divide your reading by two before looking it up; for 48V, divide by four. The calculator does this for you when you set the system voltage, which is why 24.48V on a 24V bank reads 60% rather than falling off the top of the table.

Pick LiFePO4 in the chemistry selector and this page returns no percentage. That is deliberate, and it is the honest answer rather than a gap.

Read across the two makers that do publish numbers and they contradict each other. A battery resting at 13.05V is about 25% charged by LiTime's chart and about 50% charged by Mach 1 Lithium's. At 13.18V the same two charts say 50% and about 70%. Renogy publishes two voltage charts of its own that disagree with each other as well. Picking one and printing it as your state of charge would be picking a favourite, not doing a calculation.

The cause is chemistry rather than carelessness. A LiFePO4 cell holds a nearly constant voltage across most of its usable range, so the whole 12V pack moves only a few hundredths of a volt between comfortably full and getting low. A hobby multimeter reads to 0.01V and drifts by more than that with temperature, which puts the measurement error and the real signal at the same size. Lead-acid works here because its voltage falls steadily as it empties, which is the property the tables above depend on.

What to do instead: fit a shunt-based battery monitor, or read the state of charge from the battery's own app or BMS display, which counts amp-hours in and out rather than inferring them. If you are planning a system rather than checking one, size it by energy and load with our LiFePO4 runtime calculator.

Voltage measured under load is always lower than resting voltage, because current through the battery's internal resistance drops some of it before it reaches the terminals. A flooded 12V battery at 50% sits at 12.10V once it has settled, and the same battery can read well under 12V while it is powering a load. Check the voltage while the battery is working and you will read the bank as emptier than it is.

Charging skews the reading the other way. Straight off a charger, a 12V lead-acid battery carries a surface charge that can put it above its own full-charge row, which is why a fresh-off-the-charger reading of 13V does not mean 13V worth of stored energy. The charge sitting on the plate surface has to disperse before the terminals show what the battery actually holds.

So the manufacturers all set a rest period, and all three of them ask for longer than most people wait. Trojan prints it plainly: "For accurate voltage readings, batteries must remain idle (no charging, no discharging) for at least 6 hrs, preferably 24 hrs." U.S. Battery asks for at least 4 to 6 hours on flooded batteries and 24 hours on AGM. Lifeline asks for a minimum of four hours. This page uses six hours as its rule throughout, and a reading taken sooner is a guess wearing a number.

If waiting six hours is not practical, the answer is not a faster voltage reading. It is a battery monitor with a shunt, which counts the amp-hours going in and out and does not care whether the battery has settled.