Batteries
Batteries covers lead-acid construction, capacity and cold-cranking ratings, state of charge, charging methods, failure modes and safety. The test expects you to choose the right test and interpret results, rather than simply declaring a battery flat.
Core rules
- A 12 V lead-acid battery has six cells at about 2.1 V each; open-circuit fully charged is about 12.6–12.8 V, and about 12.2 V at 50% state of charge.
- Capacity is Ah at a stated rate; CCA is cranking current at low temperature. They are different ratings.
- Maintenance-free, EFB and AGM batteries suit stop-start and high cycling; AGM must not be charged above the manufacturer's voltage limit.
- Charging current should be limited, typically about 10% of Ah for conventional lead-acid unless a smart charger controls the profile.
- Sulphation, stratification, grid corrosion and dry-out are common failures. Specific gravity tracks state of charge on serviceable batteries.
Worked example
A 60 Ah battery is at 50% state of charge, so about 30 Ah must be replaced. Allow 20% charging inefficiency: 30 × 1.2 = 36 Ah. At a 6 A charger, time ≈ 36/6 = 6 hours, plus absorption time. This is an estimate; voltage and temperature affect acceptance. It is not a fast-charge instruction.
Traps
Checking specific gravity immediately after topping up with water. Adding acid instead of distilled water. Using CCA as a capacity figure. Charging a frozen battery. Assuming a battery is good because it shows 12 V with no load; a surface charge can mislead.