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How to test car battery?
To test a car battery, set a digital multimeter to 20V DC, touch the red probe to the positive terminal (+) and the black probe to the negative terminal (-), and verify a resting voltage between 12.6V and 12.8V with the engine off. A resting voltage reading below 12.0V indicates a fully discharged or degraded battery, while a reading under 9.6V during engine cranking indicates internal plate failure requiring immediate battery replacement.
State-of-Charge & Voltage Diagnostic Matrix
Voltage levels directly correlate to internal state-of-charge (SoC) percentages and overall cell health.
| Multimeter Voltage Reading | Battery State-of-Charge (SoC) | System Condition & Action Required |
| 12.6V – 12.8V+ | 100% | Excellent condition; fully charged state. |
| 12.4V – 12.5V | 75% | Good condition; minor discharge from sitting. |
| 12.2V – 12.3V | 50% | Fair condition; recharge battery with dedicated charger. |
| 12.0V – 12.1V | 25% | Discharged; battery needs immediate charging to prevent sulfation. |
| Under 11.9V | 0% | Severely discharged or dead cell; retest after full charge cycle. |
| 13.8V – 14.7V (Engine Running) | Charging Mode | Healthy alternator output voltage range. |