Batteries
What the exam tests
Lead-acid construction, ratings, state of charge, testing, charging and safety.
Core
- A 12 V battery has six cells at about 2.1 V each. Positive plates are lead dioxide, negative plates are sponge lead, electrolyte is sulfuric acid at about 1.265 SG when fully charged.
- Ratings: Ah (capacity), CCA (cold cranking at −18 °C), RC (reserve capacity in minutes). A 60 Ah battery delivers 3 A for 20 h, not 60 A for 1 h.
- Open-circuit voltage after surface charge removal: about 12.6 V = 100%, 12.4 V = 75%, 12.2 V = 50%, 12.0 V = 25%. Below 11.9 V is deeply discharged.
- Test with a conductance tester or a load test at half CCA; a healthy battery should stay above about 9.6 V at −18 °C during the specified load.
- AGM, EFB and gel batteries need the correct charge profile; do not use a hydrometer on sealed types.
- Hydrogen is produced during charging; vent the area and avoid sparks. Never charge a frozen battery.
Worked example
A battery reads 12.5 V immediately after a charger is removed. That is surface charge. Switch on headlights for two minutes, wait, then read again: 12.2 V indicates roughly 50% state of charge. A load test then falls to 9.0 V, below specification, so the battery has high internal resistance and must be replaced. Recharging first would not fix a failed load test.
Traps
Judging a battery by voltage alone; testing straight after charging; fitting a larger Ah battery to fix a charging fault; using the wrong charger on AGM; assuming a 12 V reading means 12 V available under cranking.