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Battery voltage chart and state-of-charge calculator
Convert a resting battery voltage into state of charge for LiFePO4, AGM, gel and flooded lead-acid at 12, 24 or 48 V, with sourced voltage charts for each.
Results
| Estimated state of charge | 70 % |
|---|---|
| Voltage per cell | 3.3 V |
| Position in the chart | Between 70% (13.20 V) and 80% (13.28 V) |
| Rested voltage at 100% | 13.6 V |
| Rested voltage at 50% | 13.04 V |
- LiFePO4's voltage curve is very flat: between about 20% and 90% the whole range spans just 0.15 V per cell (0.6 V on a 12 V battery). A small meter error, temperature or recent charging can shift this estimate by 10-20 points. A shunt battery monitor or the battery's BMS app is far more reliable.
About this calculator
A voltmeter is the cheapest battery gauge there is, but only if you know what the numbers mean for your chemistry. A 12 V lead-acid battery reading 12.2 V is about half empty. A 12 V LiFePO4 battery reading 12.2 V is nearly flat, and one reading 13.2 V could be anywhere from about 60% to 75% full. This page converts a resting voltage into an estimated state of charge, and below the calculator you'll find the full voltage charts for LiFePO4, AGM, gel and flooded batteries at 12, 24 and 48 V.
Two cautions apply to every chart like this. First, the voltage must be measured at rest: with chargers and loads disconnected long enough for the surface charge or sag to settle. Second, resting voltage varies with chemistry, cell brand, temperature, age and recent history (LiFePO4 in particular reads differently after charging than after discharging). Treat the result as an estimate. A coulomb-counting battery monitor is the accurate way to track state of charge.
How to use it
- Pick your chemistry. Most lithium leisure batteries and power stations sold today are LiFePO4. If unsure, check the label or datasheet.
- Select the battery or bank voltage. A '12 V' LiFePO4 battery has 4 cells in series and a 12 V lead-acid battery has 6.
- Turn off chargers, solar and loads. Wait at least 4 hours for LiFePO4 (longer is better) and 6-24 hours for lead-acid, then measure across the battery terminals with a decent multimeter.
- Enter the reading. The calculator interpolates between the chart points and shows where your reading falls.
The formula
Voltage per cell = measured voltage ÷ cells in series (12 V = 4 LiFePO4 cells or 6 lead-acid cells; 24 V and 48 V are 2× and 4×).
State of charge is found by straight-line interpolation between the two nearest points in the chart for your chemistry.
LiFePO4 resting voltage chart
| State of charge | Per cell (V) | 12 V (V) | 24 V (V) | 48 V (V) |
|---|---|---|---|---|
| 100% | 3.40 | 13.60 | 27.20 | 54.40 |
| 90% | 3.35 | 13.40 | 26.80 | 53.60 |
| 80% | 3.32 | 13.28 | 26.56 | 53.12 |
| 70% | 3.30 | 13.20 | 26.40 | 52.80 |
| 60% | 3.27 | 13.08 | 26.16 | 52.32 |
| 50% | 3.26 | 13.04 | 26.08 | 52.16 |
| 40% | 3.25 | 13.00 | 26.00 | 52.00 |
| 30% | 3.22 | 12.88 | 25.76 | 51.52 |
| 20% | 3.20 | 12.80 | 25.60 | 51.20 |
| 10% | 3.00 | 12.00 | 24.00 | 48.00 |
| 0% | 2.50 | 10.00 | 20.00 | 40.00 |
When charging, LiFePO4 cells reach 3.60-3.65 V (14.4-14.6 V on a 12 V battery), but they settle back to about 3.40 V per cell (13.6 V) within hours of the charger stopping. The 2.50 V per cell (10.0 V) point is the cell maker's discharge cut-off, not a level to run down to routinely.
Flooded (wet) lead-acid resting voltage chart
| State of charge | Per cell (V) | 12 V (V) | 24 V (V) | 48 V (V) |
|---|---|---|---|---|
| 100% | 2.122 | 12.73 | 25.46 | 50.93 |
| 90% | 2.103 | 12.62 | 25.24 | 50.47 |
| 80% | 2.083 | 12.50 | 25.00 | 49.99 |
| 70% | 2.062 | 12.37 | 24.74 | 49.49 |
| 60% | 2.040 | 12.24 | 24.48 | 48.96 |
| 50% | 2.017 | 12.10 | 24.20 | 48.41 |
| 40% | 1.993 | 11.96 | 23.92 | 47.83 |
| 30% | 1.969 | 11.81 | 23.63 | 47.26 |
| 20% | 1.943 | 11.66 | 23.32 | 46.63 |
| 10% | 1.918 | 11.51 | 23.02 | 46.03 |
AGM lead-acid resting voltage chart
| State of charge | Per cell (V) | 12 V (V) | 24 V (V) | 48 V (V) |
|---|---|---|---|---|
| 100% | 2.140 | 12.84 | 25.68 | 51.36 |
| 75% | 2.090 | 12.54 | 25.08 | 50.16 |
| 50% | 2.040 | 12.24 | 24.48 | 48.96 |
| 25% | 1.990 | 11.94 | 23.88 | 47.76 |
| 0% | 1.940 | 11.64 | 23.28 | 46.56 |
Gel lead-acid resting voltage chart
| State of charge | Per cell (V) | 12 V (V) | 24 V (V) | 48 V (V) |
|---|---|---|---|---|
| 100% | 2.140 | 12.84 | 25.68 | 51.36 |
| 75% | 2.110 | 12.66 | 25.32 | 50.64 |
| 50% | 2.060 | 12.36 | 24.72 | 49.44 |
| 25% | 2.000 | 12.00 | 24.00 | 48.00 |
| 0% | 1.970 | 11.82 | 23.64 | 47.28 |
Sources
- LiFePO4 values: EVE Energy LiFePO4 state-of-charge chart (rested cell voltages), and the EVE LF80 cell specification for the 3.65 V charge and 2.5 V discharge limits.
- Lead-acid values: Trojan Battery User's Guide (flooded, AES/AGM and gel), Table 7, open-circuit voltage after 6-24 hours of rest at 25 °C / 77 °F.
- Battery monitoring: Victron BMV / SmartShunt manual, which treats the shunt's state-of-charge reading as the best way to monitor a battery.
Worked example (default values)
Inputs
| Battery chemistry | LiFePO4 (lithium iron phosphate) |
|---|---|
| Battery or bank voltage | 12 V |
| Measured resting voltage | 13.2 V |
Results
| Estimated state of charge | 70 % |
|---|---|
| Voltage per cell | 3.3 V |
| Position in the chart | Between 70% (13.20 V) and 80% (13.28 V) |
| Rested voltage at 100% | 13.6 V |
| Rested voltage at 50% | 13.04 V |
Tips and common mistakes
Resting means resting. Right after charging, a battery reads high because of surface charge. Under load it reads low because of voltage sag. A LiFePO4 battery fresh off the charger can show 13.8 V and settle to 13.4 V; one powering a 1,000 W inverter can sag below 12.8 V while still half full. Neither reading belongs in this chart.
Temperature shifts the numbers. These charts are referenced to about 25 °C (77 °F). Cold batteries read slightly differently and deliver less capacity, and flooded battery hydrometer readings need temperature correction. LiFePO4 must not be charged below 0 °C (32 °F) unless the battery has built-in heating.
Brands differ. LiFePO4 cells from different makers, and batteries with different BMS settings, sit a few hundredths of a volt apart on the flat part of the curve. That is enough to move an estimate 10-20 points. If you need accurate numbers every day, fit a shunt-based battery monitor and use voltage only as a sanity check.
Use voltage to spot problems. In a series bank, measure each battery at rest. Lead-acid batteries that differ by more than about 0.3 V from the rest of the bank point to an imbalance or a failing battery (Trojan's guidance), and LiFePO4 batteries that disagree point to a balancing issue.
Frequently asked questions
What voltage is a fully charged 12V LiFePO4 battery?
About 13.6 V at rest (3.40 V per cell) once the surface charge settles, and 14.4-14.6 V while it's on the charger. A reading around 13.3-13.4 V after a day of rest is still close to full.
What voltage is 50% for a 12V lead-acid battery?
About 12.10 V for flooded batteries and 12.24 V for AGM, measured after at least 6 hours of rest (Trojan's charts). Lead-acid batteries last longest if you recharge around this point rather than going deeper.
At what voltage is a 12V LiFePO4 battery dead?
About 12.0 V at rest is roughly 10% left, and the cells' discharge cut-off is 2.5 V each, or 10.0 V. Most batteries' BMS disconnects at about 2.5-2.8 V per cell (10-11.2 V) to protect the cells.
Why is my LiFePO4 voltage chart reading inaccurate?
Because the curve is so flat. From about 20% to 90% charge, a 12 V LiFePO4 battery only moves about 0.6 V, so tiny differences in temperature, rest time, meter accuracy and brand matter. Use a shunt battery monitor or the BMS app for state of charge.
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Estimates for planning only, based on the assumptions shown. Real-world performance varies with equipment, temperature and installation. Updated October 19, 2026. See our methodology.