Batteries
Batteries - what the exam tests
Lead-acid construction, ratings (Ah, CCA, RC), state of charge, load and conductance testing, charging methods, maintenance and safe handling, plus AGM/EFB differences. Expect voltage figures and interpretation questions.
Construction and ratings
- Six cells at about 2.1 V give 12.6 V nominal. Positive plates are lead dioxide, negatives sponge lead, electrolyte sulfuric acid at SG near 1.265 when fully charged.
- CCA: cranking current at −18 °C for 30 s down to 7.2 V. Ah: capacity at the 20-hour rate. RC: minutes at 25 A to 10.5 V.
- Maintenance-free and AGM/EFB batteries take no acid or water. AGM charging voltage is usually lower, around 14.4 V, than flooded at about 14.8 V.
State of charge
- Open-circuit voltage after surface charge is removed: 12.6–12.8 V is 100%; 12.4 V about 75%; 12.2 V about 50%; 12.0 V about 25%; below 11.9 V is deep discharge.
- Load test: half the CCA for 15 s at 20 °C; it must stay above roughly 9.6 V.
Charging
Charge at about 10% of Ah capacity, ventilate the area, never charge a frozen battery, and observe polarity.
Worked example
A 70 Ah, 600 CCA battery rests at 12.4 V. On load test it falls to 8.8 V within 10 s. Interpretation: 12.4 V shows roughly 75% charge, but the collapse under load proves the plates cannot deliver current. The battery has lost capacity and must be replaced, not merely recharged. Reasoning: open-circuit voltage shows charge level; the load test shows capacity.
Traps that cost marks
- Testing straight after charging, when surface charge reads high.
- Calling 12.0 V "good".
- Topping up with acid instead of distilled water.
- Applying a flooded-battery charge profile to an AGM.
- Checking specific gravity immediately after adding water.