A Battery That Dies in Two Days: Drain, Charging or Battery?

Charging voltage: 13.9 V at idle with no load, falling to 13.4 V with headlamps, blower and demister, is within the normal regulated band of roughly 13.5-14.5 V. A small drop at idle with a heavy electrical load is expected, because alternator output depends on engine speed. There is no evidence of a failed regulator or open diode here. Before condemning anything, do a voltage-drop test on the B+ cable and the engine/body earth: more than about 0.3-0.5 V across a cable indicates resistance, which mimics a weak alternator.

Battery: 12.45 V open-circuit after standing is about 60-70 per cent state of charge, not a dead cell and not proof of a failed battery. A 12.45 V reading with a 3-year-old battery is consistent with chronic undercharging on 4-5 km trips with lights on, where the alternator does not replace what the starter and lamps take out.

Verdict: the alternator is not shown to be faulty and should not be replaced on this evidence. The battery should be properly charged and then load-tested or capacity-tested before it is condemned. Testing a partially discharged battery and declaring it bad is a common and expensive error.

Acceptable value: the manufacturer's limit here is 30 mA; many vehicles settle between 20 and 50 mA once all modules sleep. Anything sustained at several times that figure is a real drain.

Locating it:

1. Confirm the meter or clamp is on the correct range and that the vehicle has actually gone to sleep - doors shut, key out, bonnet switch taped closed, 40 minutes elapsed. Opening a door wakes modules and gives a false reading.
2. If using a series multimeter, use a bypass lead so reconnecting does not wake the modules.
3. Pull fuses one at a time, watching the reading; replace each before pulling the next so circuits are not cross-fed.
4. When a fuse produces the drop, identify every circuit on it from the wiring diagram. Here the interior lamp fuse drops the drain to 40 mA, so the boot lamp, glovebox lamp and interior lamps are the targets - a worn boot lid switch holding the boot lamp on is the classic cause, and it is invisible from outside.
5. Fix the switch, then re-measure after a full sleep cycle and confirm the reading falls below 30 mA.

Amp-hour arithmetic: 210 mA for 24 hours is 0.21 x 24 = about 5 Ah per day. Over two days that is roughly 10 Ah, close to a third of a 35 Ah battery, on top of the starting load and the short-trip deficit. That alone explains the no-crank after two days.

Why not a bigger battery: it masks the fault for a few days and then goes flat too. A 45 Ah battery also needs more charge than a 35 Ah one, and this car does 4-5 km trips with the headlamps on - a short trip may put back only a fraction of an amp-hour while the starter and lamps take several. Fitting a larger battery to a charging system that is only just keeping up makes the energy deficit worse, adds weight and cost, and can void the fitment if the tray and clamp do not match.

Recommendations, in order:

1. Repair the boot lamp switch so the quiescent current is below 30 mA. Re-measure after the sleep period.
2. Fully charge the existing battery with a proper charger, then load-test it. If it fails, replace with the specified 35 Ah capacity and correct terminal layout, not a larger one.
3. Confirm the alternator and earth straps with a voltage-drop test, since the alternator is already new and the complaint recurred.
4. Advise the owner that 4-5 km trips with lights on are the hardest duty for a battery: a long run every week or two, or a maintenance charger at home, will prevent a repeat. Also check for any aftermarket accessory such as a dash camera wired to a permanent live.

Document the before-and-after drain readings on the job card, so the repair is evidence-based rather than a parts swap.