Lead-Acid Battery Voltage Chart
Approximate resting voltage values for flooded, AGM, and gel lead-acid batteries across common bank voltages. Use this chart to estimate state of charge from voltage readings.
Approximate relationship for a 12V pack; lead-acid voltage is generally more linear than LiFePO4.
Flooded lead-acid values at 25°C. Actual voltage varies by manufacturer, age, temperature, and measurement conditions.
Flooded Lead-Acid Voltage Chart
Flooded lead-acid batteries have a well-characterized relationship between resting voltage and SOC. Unlike LiFePO4, the voltage drops relatively linearly across the SOC range, making voltage a practical SOC indicator — provided the battery has been resting and temperature is accounted for.
| SOC % | 12V Pack | 24V Pack | 48V Pack | Notes |
|---|---|---|---|---|
| 100% | 12.70V | 25.40V | 50.80V | Fully charged, resting |
| 90% | 12.58V | 25.16V | 50.32V | Slight surface charge possible |
| 80% | 12.46V | 24.92V | 49.84V | Good operating range |
| 70% | 12.34V | 24.68V | 49.36V | Acceptable for daily use |
| 60% | 12.24V | 24.48V | 48.96V | Begin recharge soon |
| 50% | 12.12V | 24.24V | 48.48V | Recharge recommended |
| 40% | 12.00V | 24.00V | 48.00V | Risk of sulfation |
| 30% | 11.88V | 23.76V | 47.52V | Deep discharge zone |
| 20% | 11.76V | 23.52V | 47.04V | Battery at risk |
| 10% | 11.64V | 23.28V | 46.56V | Severe discharge |
| 0% | 11.50V | 23.00V | 46.00V | Fully discharged |
Flooded lead-acid values at 25°C. Actual voltage varies by manufacturer, age, and temperature.
AGM and Gel Voltage Chart
AGM (Absorbent Glass Mat) and gel batteries have slightly different voltage characteristics from flooded cells. AGM batteries typically have lower internal resistance and slightly higher resting voltages at the same SOC. Gel batteries are similar to AGM but with slightly lower charge acceptance.
| SOC % | AGM 12V | Gel 12V | AGM 24V | Notes |
|---|---|---|---|---|
| 100% | 12.90V | 12.85V | 25.80V | Fully charged, resting |
| 90% | 12.75V | 12.70V | 25.50V | Near full |
| 80% | 12.60V | 12.55V | 25.20V | Good operating range |
| 70% | 12.48V | 12.43V | 24.96V | Acceptable |
| 60% | 12.36V | 12.31V | 24.72V | Begin recharge soon |
| 50% | 12.24V | 12.19V | 24.48V | Recharge recommended |
| 40% | 12.12V | 12.07V | 24.24V | Risk of damage |
| 30% | 12.00V | 11.95V | 24.00V | Deep discharge zone |
| 20% | 11.88V | 11.83V | 23.76V | Battery at risk |
| 10% | 11.76V | 11.71V | 23.52V | Severe discharge |
| 0% | 11.60V | 11.55V | 23.20V | Fully discharged |
AGM and gel values at 25°C. AGM rests slightly higher than flooded. Gel rests slightly lower than AGM.
Surface Charge
Surface charge is a phenomenon specific to lead-acid batteries. After charging, the acid concentration near the plate surfaces is temporarily higher than the bulk electrolyte. This elevates the terminal voltage by 0.1–0.3V above the true resting voltage.
Surface charge dissipates within 30–60 minutes as the acid diffuses evenly through the electrolyte. If you measure voltage immediately after charging, you will get a falsely high SOC reading. For accurate voltage-based SOC estimation, always allow the battery to rest for at least 30 minutes after the charger disconnects.
Some chargers include a surface charge removal function — a brief high-current discharge that breaks down the acid concentration gradient. This is common in marine and RV chargers. If your charger has this feature, use it before relying on voltage for SOC estimation.
Temperature Effects
Lead-acid battery voltage is affected by temperature. At cold temperatures, the chemical reaction slows and internal resistance increases, causing lower voltage readings. At high temperatures, the reaction accelerates and voltage readings may be slightly elevated.
| Temperature | Voltage Adjustment | Capacity Effect |
|---|---|---|
| −20°C (−4°F) | −0.4V per 12V pack | ~50% capacity available |
| 0°C (32°F) | −0.2V per 12V pack | ~75% capacity available |
| 10°C (50°F) | −0.1V per 12V pack | ~85% capacity available |
| 25°C (77°F) | No adjustment | 100% capacity |
| 40°C (104°F) | +0.1V per 12V pack | Slightly elevated, faster aging |
Temperature compensation is critical for accurate SOC estimation in extreme conditions.
Common Mistakes
Measuring Immediately After Charge
Surface charge inflates voltage by 0.1–0.3V. A battery that appears at 100% SOC may actually be at 70–80% once surface charge dissipates. Always wait 30+ minutes after charging before measuring.
Using the Wrong Chemistry Curve
Flooded, AGM, and gel batteries have different voltage profiles. Applying flooded voltage values to an AGM battery (or vice versa) leads to incorrect SOC estimates. Identify your battery type first.
Ignoring Temperature
A battery in a cold engine bay or unheated storage unit reads lower than the chart values. A 12V battery at 0°C may read 12.4V at full charge, which appears to be 50% SOC on the chart. Always compensate for temperature.
Discharging Below 50% Regularly
Lead-acid batteries suffer significant capacity loss when regularly discharged below 50% SOC. Flooded batteries may lose 50% of their cycle life at 50% DoD. Use the voltage chart to ensure you recharge before reaching this threshold.
Try It
Use the Battery Sizing Calculator to size a lead-acid battery bank with proper DoD limits and temperature derating.
Open Battery Sizing CalculatorFrequently Asked Questions
What is the resting voltage of a fully charged 12V lead-acid battery?
A fully charged flooded lead-acid battery rests at approximately 12.6–12.8V. AGM and gel batteries typically rest slightly higher at 12.8–13.0V. After charging, surface charge can inflate the reading to 13.0–13.4V, so wait at least 30 minutes before measuring resting voltage.
How does temperature affect lead-acid voltage readings?
Temperature affects the chemical equilibrium of the cells. Cold temperatures increase internal resistance and lower voltage readings. At 0°C, a fully charged battery may read 0.2–0.3V lower than at 25°C. At high temperatures (above 40°C), voltage readings may be slightly elevated. Always compensate for temperature when estimating SOC from voltage.
What is surface charge?
Surface charge is a temporary voltage elevation that occurs after charging. It is caused by uneven acid concentration near the plate surfaces. It dissipates within 30–60 minutes of rest. Measuring voltage immediately after charging gives a falsely high SOC reading. Always let the battery rest before taking a voltage measurement.
Do flooded, AGM, and gel batteries have different voltage curves?
Yes. AGM and gel batteries have slightly higher resting voltages than flooded batteries at the same SOC. AGM typically rests 0.1–0.2V higher than flooded. Gel batteries are similar to AGM. The differences are small but meaningful when estimating SOC from voltage. Use the correct voltage curve for your battery type.