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 |
Engine running: the charging system should read 13.7 – 14.7 V. During cranking: voltage should stay above about 9.6 V — lower means a weak battery.
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.
Match a replacement with the Battery Group Size Chart.