Battery Voltage → Charge Calculator

12 V battery voltage chart: what voltage is 50% charged?

By the makers' own tables, 12.10 V for a flooded battery and 12.18 V for AGM, rested, at room temperature. Trojan publishes its flooded table in the Trojan Battery User's Guide (Table 7, with specific gravity beside each voltage), and Lifeline publishes its AGM table in its technical manual; those are the two printed above. A rested 12.4 V, often given as the halfway mark, is about 72% on Trojan's table and 68% on Lifeline's. What common readings mean:

Straight-line interpolation between the makers' rows: Trojan's 10% steps (Table 7, 12 V column) and Lifeline's 10% steps with 25% and 75% added (Rev. F, p. 32).

Fifty percent matters because it's where lead-acid stops being a good trade: the planning floor on every calculator here is 50% of the bank, which is 12.10 V on a flooded battery and 12.18 V on AGM. Read it in the morning, before the solar wakes up; if it's at or under that line, recharge first and run the runtime numbers from what's left.

What voltage is a dead 12 V battery?

At rest, below about 11.5 V flooded and 11.6 V AGM. Trojan's table ends at its 10% row, 11.51 V, and prints nothing lower, so a flooded battery reading under that is past the bottom of its maker's chart; Lifeline calls anything under 11.58 V empty. The "10.5 V is dead" figure is a different thing. It's the voltage under load where an inverter shuts off (Battle Born's data sheet gives 10.5 V as its recommended low-voltage disconnect, too), and a battery that sags to 10.5 V under a microwave can bounce back to 12 V when it's switched off, which tells you the battery is nearly empty and the load has stopped pulling it down. A resting lead-acid battery below 11.5 V has been deeply discharged; charge it now, because it sulfates while it sits. A lithium battery is effectively empty below about 12.0 V at rest, and its BMS disconnects around 10 V to protect the cells.

LiFePO4 voltage chart: why lithium's voltage tells you so little

Because the curve is nearly flat, and the published charts disagree about where on it you are. A rested LiFePO4 battery spends most of its capacity between about 13.0 and 13.3 V. Battle Born, a brand many RVers look to for a lithium chart, prints no resting state-of-charge table in its BB10012 data sheet; the sheet's only graph of voltage against charge is a charging curve. The charts that circulate come from Renogy, which publishes two that disagree with each other, and LiTime, and they put 13.2 V anywhere from 54–70% and 13.1 V anywhere from 31–60%:

Each chart interpolated between its printed points; LiTime's ranges taken at their middles. Resting, 12.8 V nominal battery.

How long should a battery rest before you check the voltage?

6 hours for flooded and 4 for AGM, by their makers. In an RV the practical version is "first thing in the morning, before the sun hits the panels and before the furnace runs." Lithium settles much faster; the charts that state a figure say 15–30 minutes. Anything sooner and the tool gives you a ceiling instead of a number: a lead-acid battery that just came off the converter reading 12.8 V may be well short of full, and there's no way to know until the surface charge is gone.

Why does the RV's battery monitor say full when the battery isn't?

Because it's reading the charger, not the battery. The four-light panel by the door (and many "battery %" displays) measures voltage at the panel, and on shore power or in the sun the converter or controller is holding the system at 13.3–14.4 V. Every resting table calls that full, so the tool treats it only as a ceiling: an AGM reading 13.4 V while charging is "at most 100%", which tells you nothing. The same panel then reads "low" the moment the furnace blower starts, because now it's reading voltage under load through thin wire. For a reading you can use, switch off the converter and the solar, turn off the loads, and read at the battery posts in the morning. To stop doing that, fit a shunt monitor: it counts amps in and out at the negative terminal, and it reads the same whether anything is running or not.

How do you check a 24 V bank or a pair of 6 V batteries?

Divide to 12 V and read the table; the tool does it when you say what the meter is across. A 24 V bank at 25.0 V is 12.5 V per 12; a 48 V bank at 51.2 V is 12.8. A bank reading hides a weak battery, though, and the classic RV case is two 6 V golf-cart batteries in series: 12.4 V across the pair looks like 72%, but if one battery reads 6.3 V and the other 6.1 V, one is at about 88% and the other about 57%. The weaker battery sets the pair's floor, and a gap that size that persists after a full charge is a battery on its way out. Measure each one, rested, a couple of times a season.

Hydrometer or voltmeter?

On a flooded battery, the hydrometer. It reads each cell's acid directly, so it finds the one weak cell a voltmeter averages away, and Trojan publishes a specific-gravity column beside its voltages: 1.277 or more is full at 80 °F. The catch is temperature. The electrolyte's density changes with heat, so Trojan corrects by 0.004 for every 10 °F away from 80 °F. A reading of 1.200 in a 100 °F battery box is 1.208 corrected, about 66%, and the same 1.200 at 50 °F is 1.188, about 57%. The hydrometer mode above applies it. Sealed AGM and lithium batteries have nothing to draw from, so the voltmeter (or a monitor) is all there is.

What voltage should a 12 V battery charge at?

Higher than any resting figure above, which is why a reading taken while the converter or solar is running says nothing about the charge. The makers' charging voltages for a 12 V battery, from Trojan's T-105 data sheet, Lifeline's manual and Battle Born's data sheet:

Per 12 V battery; double for 24 V. Lead-acid charging voltages are temperature-compensated, lower when hot and higher when cold: Trojan's figure is 0.005 V per cell per °C from 25 °C, 0.03 V per °C on a six-cell 12 V battery. Trojan's older (2014) T-105 sheet gave a 13.2 V float. Battle Born calls a lithium float unnecessary and lists 13.4–13.8 V where a charger has one.

How long the charge takes, and why a lithium bank on a converter that sits at 13.6 V never reaches the 14.2–14.6 V its maker specifies, is the charge time calculator's job.