A Four-Year-Old 35 Ah Battery, Low Specific Gravity and an Extra 55 W Lamp
A 12 V, 35 Ah lead-acid battery in a petrol car is four years old. In the morning the engine cranks slowly and the headlights dim during cranking. The battery terminals are corroded, and the electrolyte level in two cells is below the top of the plates. An open-circuit voltage reading after standing overnight is 12.1 V. A hydrometer shows 1.120, 1.125, 1.120, 1.115, 1.130 and 1.125 in the six cells. The alternator belt is tight, and with the engine running at 2,000 rpm the voltage across the battery is 13.9 V. The battery has a 35 Ah capacity and the car's standing load is 20 mA. The owner wants to add a 55 W auxiliary lamp and asks whether the battery can be saved and whether the alternator will cope.
1. Interpret the voltage, hydrometer and electrolyte observations; decide whether the battery should be recharged, load-tested or replaced. Justify.
Read the worked answer
The battery is deeply discharged and probably damaged. An open-circuit voltage of 12.1 V after standing corresponds to a low state of charge, roughly 40–50%, but the hydrometer readings of 1.115–1.130 are far below a fully charged cell, about 1.260–1.280, and are uneven. The exposed plates in two cells mean sulfation and possible plate damage. The first step is not to condemn it immediately: top up the exposed plates with distilled water, then slow-charge at a low rate in a ventilated area and re-check the specific gravity. If the readings remain low and uneven after charging, or if a load test shows a rapid voltage drop, replace the battery. At four years old with electrolyte loss, replacement is the likely outcome, but a load test is the fair decision.
2. Explain why 13.9 V at 2,000 rpm is not proof that the alternator will cope with the new 55 W lamp. Estimate the extra current and state what else must be checked.
Read the worked answer
13.9 V shows the alternator is charging, but it does not show how much current is available at idle or how much reserve remains. A 55 W lamp at about 12 V draws 55/12 ≈ 4.6 A, and at 13.9 V about 4.0 A; with wiring losses, allow about 5 A. The alternator must supply the car's standing load, the lamp, and still charge the battery. At 2,000 rpm the output may be adequate, but at idle it may not. Check the alternator's rated output, measure charging current with a clamp ammeter with all loads on, check belt condition and voltage drop in the positive and earth cables, and confirm the battery is healthy. A 35 Ah battery has little reserve for extra loads when the engine is idling.
3. List the safety precautions for charging and load-testing this battery in the workshop.
Read the worked answer
Lead-acid batteries produce hydrogen, which is explosive, and contain sulfuric acid. Wear safety glasses and acid-resistant gloves; remove rings and watches. Charge in a well-ventilated area away from sparks, flames and switches; connect the charger before switching it on and switch it off before disconnecting. Do not smoke. Keep vent caps in place but ensure vents are clear; do not overfill with acid, use distilled water only. Avoid overcharging, which heats the battery and produces more gas. For load testing, follow the tester's instructions and do not short the terminals. If acid contacts skin or eyes, flush with plenty of water and seek medical help. When jump-starting, connect positive to positive and the final negative to a distant engine earth, not to the battery.