A 12-volt car battery resting voltage is the quickest read on its state of charge. Measure with the engine off and the battery undisturbed for a few hours, then compare against this chart.
| Resting voltage | State of charge |
|---|---|
| 12.6 – 12.8 V | 100% (fully charged) |
| 12.4 V | 75% |
| 12.2 V | 50% |
| 12.0 V | 25% |
| 11.9 V or below | Discharged |
AGM batteries read slightly higher at rest (around 12.8 – 13.0 V fully charged). Surface charge right after driving can inflate the reading, so let it settle first.
FAQ
Why does battery voltage drop under load compared to its rated voltage? Internal resistance causes a voltage drop proportional to current draw, so a battery reads closer to its rated voltage at rest than while powering a device. This is normal and part of why load testing gives a more accurate picture of battery health than a resting voltage reading alone.
Match a replacement with the Battery Group Size Chart.