A multimeter reading tells you the voltage, but what you actually want to know is how much charge is left. This calculator converts that voltage reading into a percentage based on your battery chemistry and system voltage. The catch: you must measure resting voltage — a reading taken during charging or discharging gives a misleading number.
12V Battery State of Charge by Chemistry
| SOC % | Flooded Lead-Acid | AGM / Gel | LiFePO4 |
|---|---|---|---|
| 100% | 12.70V | 12.80V | 13.60V |
| 90% | 12.50V | 12.60V | 13.40V |
| 80% | 12.42V | 12.50V | 13.30V |
| 70% | 12.32V | 12.40V | 13.28V |
| 60% | 12.20V | 12.25V | 13.26V |
| 50% | 12.06V | 12.10V | 13.20V |
| 40% | 11.90V | 11.95V | 13.16V |
| 30% | 11.75V | 11.81V | 13.10V |
| 20% | 11.58V | 11.66V | 12.80V |
| 10% | 11.31V | 11.51V | 12.00V |
| 0% | 10.50V | 10.50V | 10.00V |
Notice how LiFePO4 stays between 13.1V and 13.4V from 30% to 90% SOC. That flat curve makes voltage a poor SOC indicator for LiFePO4 — a battery monitor or BMS with coulomb counting is far more accurate.
Resting Voltage vs Loaded Voltage
Voltage measured under load is always lower than resting voltage due to internal resistance. A 12V lead-acid battery at 50% SOC reads 12.06V at rest, but might show 11.5V while powering a 10A load. If you check voltage while the battery is working, you will underestimate the state of charge.
Resting voltage requires the battery to sit disconnected (or with very low parasitic loads) for 1-4 hours after the last charge or discharge event. Lead-acid needs the longest settling time — up to 4 hours for an accurate reading. LiFePO4 settles within 30-60 minutes.
Charging also skews readings upward. Immediately after disconnecting a charger, a 12V lead-acid battery may show 13.0-13.2V even though true SOC is only 80-90%. This "surface charge" dissipates after 1-2 hours, settling to the real resting voltage.
For the most accurate SOC reading: disconnect all loads, wait 2 hours, then measure. Or use a battery monitor (like a Victron SmartShunt) that tracks energy in and out — no voltage guessing needed.
Frequently Asked Questions
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Voltage gives you a rough SOC estimate, but resting conditions and battery chemistry significantly affect accuracy. Measure after a 2-hour rest, use the correct chart for your battery type, and accept that voltage-based SOC is an approximation — not a precise reading.
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Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.