Flat Battery After Two Days: Charging System or Something Else?
A 1.2-litre petrol hatchback with a 12 V, 35 Ah battery and an integral alternator comes in with hard starting in the morning and a flat battery after two days of parking. A replacement alternator was fitted at another garage three weeks ago. After 12 hours at rest the battery open-circuit voltage is 12.35 V, and a hydrometer reads 1.200 in two cells against 1.255 in the other four. With the engine at 2,000 rpm and the headlamps on, the battery terminal voltage is 13.80 V while the alternator B+ terminal itself reads 14.25 V. The voltage drop along the B+ cable from alternator to battery is 0.45 V at 40 A, and from engine block to battery negative 0.05 V. AC ripple at the battery is 0.15 V. After 40 minutes of sleep the parasitic drain is 0.38 A, and a clamp meter shows 0.35 A flowing in the boot-lamp circuit. A load test pulls the battery down to 9.2 V at half its rated cold-cranking current.
1. Is the alternator defective? Justify your answer with the measured figures and state what you would test or repair next.
Read the worked answer
No - the alternator is doing its job; the cable is not.
- The alternator's own B+ terminal shows 14.25 V at 2,000 rpm with the headlamps on, which is inside a normal regulated range (roughly 13.9–14.5 V).
- At the battery the same circuit shows only 13.80 V, so 0.45 V is being lost between the two points. A healthy B+ cable and terminals should drop well under 0.2 V at 40 A. 0.45 V × 40 A ≈ 18 W of heat in a joint or cable, which is a corroded terminal, a damaged cable or a bad crimp.
- The ground side is fine (0.05 V from block to battery negative), so the fault is on the positive side.
- 0.15 V AC ripple is within acceptable limits for a diode-rectified alternator, so there is no diode failure to explain the low reading.
- The 0.45 V drop means the battery sees about 13.8 V, marginal for a battery that is already weak, and at night with lights, blower and wipers the net charge can fall close to zero.
Next: clean and re-crimp or replace the B+ cable and terminals, then re-measure - target above 13.9 V at the battery at 2,000 rpm with loads on, and a drop below 0.2 V at 40 A. Only if the B+ figure itself is low do you condemn the regulator. Record before-and-after figures so the customer can see the alternator was never the problem.
2. Calculate whether the parasitic drain explains the two-day flat battery, and explain how you would isolate it.
Read the worked answer
Yes, it explains it. 0.38 A for 48 hours = 18.2 Ah. A healthy 35 Ah battery should not be discharged below about 50% state of charge, i.e. only about 17.5 Ah is usable before cranking becomes unreliable - and any real battery loses capacity with age. So a two-day park is exactly the failure window this drain would produce. The 0.35 A measured in the boot-lamp circuit accounts for almost the whole drain and points to a lamp that stays on because of a misadjusted or sticking switch, a chafed wire or a boot lid that does not press the switch.
Isolation method:
- Charge and test the battery first, so the drain measurement is not distorted.
- Close all doors, bonnet and boot, lock the vehicle and wait 30–40 minutes for all modules to sleep - reading immediately gives a false 0.5–2 A figure.
- Connect a clamp meter around the negative cable, or use a voltage-drop reading across the fuses in the main and interior fuse boxes; the highest drop identifies the circuit.
- Confirm by disconnecting the boot-lamp branch: the drain should fall to a residual (target below 0.05 A, typically 0.02–0.03 A).
- Recheck after the repair and after a second sleep cycle, then record the figure.
Do not overlook a second small drain hidden behind the lamp current; re-measure the residual once the boot circuit is isolated.
3. The customer wants to know why the earlier alternator replacement did not fix the car. Set out the repair order and what you would tell him about the battery.
Read the worked answer
Repair order (cheapest root cause first, but everything verified):
- Repair or replace the B+ cable and terminals; re-measure charging at the battery with loads on.
- Repair the boot-lamp switch or wiring; re-measure the sleep drain to below 0.05 A.
- Replace the battery, then repeat the charging and drain tests with the new battery.
- Road test with lights and accessories on, and give the customer a written record of the voltages and drain currents before and after.
The battery must go. A hydrometer spread of 1.200 against 1.255 across the cells (0.055, well beyond the usual 0.030 tolerance) means two cells are weak, and a load test falling to 9.2 V at half the cold-cranking rating is a fail. Such a battery cannot be recovered by charging - the weak cells limit the whole pack, and they will keep it from accepting a full charge even after the cable repair.
What to tell the customer: the alternator was replaced but the actual faults were a high-resistance charge cable, a boot lamp that never switched off and an aged battery. That is why the same symptom returned. Be factual rather than critical of the other garage: quote the B+ voltage against the battery voltage and the drain current, and explain that the alternator itself is producing correct voltage. A 35 Ah battery on a small hatchback is sensitive to a 0.38 A drain; even a smaller residual drain should be rechecked at the next service.