Overnight Battery Drain on a Petrol Saloon
A 2015 petrol saloon with a 60 Ah, three-year-old lead-acid battery will not start after standing overnight. The battery was recharged and reads 12.4 V at rest. The charging system produces 14.3 V at the battery with the engine running and the battery warning lamp is off. After the car sits for 10 hours, the battery voltage falls to 11.8 V and the engine cranks slowly. A clamp meter on the negative battery cable shows 420 mA after the modules have gone to sleep; the manufacturer's limit is 50 mA. Pulling the 7.5 A radio fuse drops the draw to 190 mA. Pulling the 10 A interior-lamp fuse as well drops it to 60 mA. Pulling both fuses plus the 15 A door-lock fuse gives 45 mA. The battery conductance tester reports 65% of rated capacity.
1. Calculate the ampere-hour consumed overnight at 420 mA and compare it with the battery's remaining capacity. Decide whether the drain alone explains the no-start.
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Calculation
0.420 A × 10 h = 4.2 Ah consumed overnight.
The battery is rated 60 Ah, but the tester reports 65% of original capacity: 0.65 × 60 = 39 Ah remaining capacity.
4.2 Ah is about 11% of that remaining capacity. On a fully charged healthy battery, that would not normally prevent cranking.
Judgement
12.4 V at rest is only about 50–60% state of charge, and 11.8 V after standing is very low. The excessive drain must be repaired, but it is not the whole story. The battery is also undercharged or degraded. Both faults need correction: fix the drain and recharge or replace the battery after testing.
2. Interpret the fuse-pull results and identify the circuits that need repair, including why pulling the door-lock fuse changes the reading.
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Interpretation
420 mA is 8.4 times the 50 mA limit.
- Radio fuse: 420 − 190 = 230 mA. This is a major drain, likely a radio/amplifier memory feed, a boot lamp on that circuit or a stuck accessory.
- Interior-lamp fuse: 190 − 60 = 130 mA. A lamp - probably a boot or glovebox lamp - is staying on.
- Door-lock fuse: 60 − 45 = 15 mA. This is normal module standby and is not the fault.
Conclusion
The two real faults are the radio circuit and the interior-lamp circuit. The door-lock circuit should be left alone; 15 mA is not a reason to condemn it. After repair, the total draw should fall below 50 mA after the modules sleep.
3. State the repair and retest plan, including how you would protect the vehicle's electronics during testing.
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Repair
Check the radio/amplifier remote feed, memory wiring and any boot lamp sharing that fuse. Check the boot-lamp switch, glovebox lamp and interior-lamp timer. Repair or replace the faulty component.
Retest
Reconnect everything, close all doors and wait for the modules to sleep - often 20–40 minutes. Confirm the clamp-meter reading is below 50 mA. Recharge the battery fully at a controlled rate, then load-test or conductance-test it. Replace it if capacity remains below about 70% or if it will not hold charge.
Electronics protection
Use a memory saver, keep the ignition off when pulling fuses, avoid sparks near the battery, and use a clamp meter rather than repeatedly breaking circuits. Recheck charging voltage and clean the battery terminals before release.