0.09 A Drain on a 35 Ah Battery
A 2020 petrol hatchback with a 35 Ah battery comes in after going flat twice in one week. The battery passes a load test, holds 12.4 V after two hours at rest, and the alternator gives 14.1 V at 2,000 rpm with lights, blower and rear demister on. With a 10 A digital meter in series at the negative terminal, the drain is 0.42 A immediately after locking, falls to 0.09 A at 45 minutes and is still 0.09 A after three hours parked with the key kept 30 m away in the office. The manufacturer's limit is 0.03 A once all modules have gone to sleep. Three months ago the owner had an aftermarket Android head unit and a reverse camera fitted at an accessory shop, tapped into the boot-lamp wiring. The technician's plan is to pull fuses one at a time, with the meter still in series, until the drain disappears. The owner insists the car is under warranty and refuses to pay for diagnosis.
1. Do the arithmetic for 0.09 A on a 35 Ah battery and say whether this drain explains the flat battery; state what else you must check.
Read the worked answer
0.09 A times 24 hours is 2.16 Ah per day, so three days parked is about 6.5 Ah. A healthy 35 Ah battery has roughly 20 to 25 Ah of usable capacity at the 20-hour rate, and a battery already deep-discharged twice may hold much less, but 6.5 Ah alone would not normally flatten it in two nights.
So 0.09 A is about three times the 0.03 A limit and must be fixed, but it is not yet the whole explanation. Check three more things. First, battery state of health and date code: a sulfated battery self-discharges and has little reserve, and two deep discharges will have damaged it. Second, whether the drain is intermittent - log current with a clamp meter over 24 hours rather than trusting one snapshot, because a module that wakes periodically draws far more than the average. Third, the quality of the measurement itself: a meter in series can wake modules and can under-read on some ranges.
Also confirm the boot lamp, glovebox lamp and any USB charger actually go out, and check charging voltage at idle with all loads on, not only at 2,000 rpm.
2. Criticise the fuse-pulling plan and describe a better isolation procedure, including how to keep modules asleep.
Read the worked answer
Pulling fuses one at a time with the meter in series is poor practice here. Every break in the circuit can blow the meter fuse if a module wakes and draws amps, the meter must be re-zeroed each time, and each break resets the sleep timer, so the technician can end up chasing a drain he created himself.
Better: fit a low-current clamp around the negative cable, close all doors, bonnet and boot, lock the car and keep the key well out of range, then wait a full sleep cycle, which can be 20 to 60 minutes and longer on some body computers. Watch the current fall and hold. Once it is stable, read voltage drop in millivolts across each fuse in the engine-bay and cabin boxes, converting using the fuse resistance or a published table; a measurable drop on a circuit that should be dead identifies the branch. Only then unplug that circuit at a connector rather than pulling the fuse.
Start with the non-OEM wiring, since the head unit and camera are the newest variables. Record the current, the conditions and the time at each step.
3. The aftermarket fitment exists and the owner refuses to pay for diagnosis. How do you handle the charge and the customer?
Read the worked answer
Diagnosis is chargeable work, but you say so before starting, not afterwards. Record on the job card the measured drain, the time and conditions of measurement, the battery test printout and the manufacturer's 0.03 A limit, then take signed authorisation for a fixed diagnostic fee, refundable against the repair.
Do not simply blame the accessory shop; prove the circuit. Disconnect the camera and head unit at their protective fuses, repeat the sleep test, and see whether the drain falls to specification. If it does, the aftermarket installation is the cause and the owner has a claim against that shop, which you can support with your written findings. If it does not, the fault is in the vehicle and warranty may apply.
In the meantime charge the battery properly, offer a charged loan battery or jump-start help, and give a written estimate. Keep the tone factual. The owner's objection - refusing to pay for a diagnosis he was never quoted for - is a fair complaint if no authorisation was taken at the counter.