Battery Diagnosis and Service
The battery is a chemical device, so its state of charge must be read on its own terms. Open-circuit voltage approximates state of charge on a rested, surface-charge-free battery: roughly 12.6 volts (about 2.1 volts per cell) is fully charged, and each few tenths of a volt of drop represents a substantial loss of charge. Always remove surface charge - a load of a few seconds or a headlight cycle, then a rest - before trusting a voltage reading. Temperature matters too; a cold battery delivers fewer cranking amps, and a battery tested cold may fail a load test it would otherwise pass.
Testing sequence
- Visual inspection: case swelling, terminal corrosion, loose hold-down, electrolyte level on serviceable batteries, and correct group size and CCA rating.
- State of charge; recharge a discharged battery before load testing, because load testing a low battery produces a false failure.
- Capacity test with a conductance tester or a carbon-pile load test to the manufacturer's specification (typically half the CCA rating for 15 seconds while watching for a voltage floor).
Traps and details
A battery with adequate voltage can still have low capacity, and a battery with good CCA can still have low reserve capacity. Hydrometer readings apply only to flooded batteries; never open or probe an AGM or EFB. Hydrogen gas makes sparks and smoking near a charging battery dangerous, and reverse-polarity connections can destroy modules. After replacement, note that many vehicles require relearning idle, window pinch protection, steering angle, and battery registration so the charging system applies the correct profile.
Rule of thumb: diagnose a no-crank with a voltage drop across both battery cables; if the battery passes a capacity test and drops are under 0.5 volt total, the battery is not the problem.