60 Milliamps Overnight: Chasing a Parasitic Drain on a 35 Ah Battery
A 2019 petrol hatchback with a 1.2-litre engine and a 35 Ah battery, 18 months old, comes in with hard starting after two days of parking. Battery open-circuit voltage is 12.42 V after a 12-hour rest. Charging voltage is 13.9 V at 2000 rpm with headlamps and blower on. With a clamp meter on the negative cable, key-off current is 0.38 A immediately after locking and falls to 0.06 A after 25 minutes; it does not fall further over the next 20 minutes. A conductance test gives 340 CCA against a 350 CCA rating and 72% state of health. A local accessory shop fitted a dash camera and an amplifier eight months ago, tapping the interior-lamp circuit. The owner does mostly 6 km city trips, and the workshop floor is at 38 °C in May.
1. Is 0.06 A acceptable? Support your answer with a calculation and explain why the customer's starting problem may have more than one cause.
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The calculation. 0.06 A × 24 h = 1.44 Ah per day. Over a three-day park that is about 4.3 Ah. A 35 Ah battery at 72% state of health holds roughly 25 Ah, and useful capacity before cranking suffers is about half of that, around 12 Ah. So the drain alone would not flatten the battery in two days.
But it is still out of spec. Most manufacturers allow roughly 0.02–0.05 A after all modules sleep. 0.06 A is above that band, and the 0.38 A at locking shows a module or accessory that is slow to sleep or never fully sleeps.
Why there is more than one cause. The 6 km city trips are the real problem: each start takes 1–2 Ah out and the alternator may not put it back before the next stop, especially with lights and blower. Add 38 °C under-bonnet heat, which shortens battery life and raises self-discharge, and 72% state of health at 18 months looks like a battery that has been chronically undercharged rather than simply worn out.
Conclusion. Treat the drain as a contributor to fix, and the battery as a separate decision. Replacing the battery without fixing the drain will produce the same complaint in a few months.
2. Describe the diagnostic procedure you would follow to isolate the drain, including the precautions that keep the measurement valid and the car safe.
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Prepare. Charge and test the battery first so the readings are meaningful. Fit a memory saver if you disconnect the battery, and follow the key-off and airbag precautions - never probe airbag circuits.
Measure properly. A clamp meter with 10 mA resolution is fine for 0.06 A but confirm the figure with a digital multimeter in series on the negative cable, on its 10 A range, then switch ranges only after reconnecting. Do not open doors or operate anything during the test: use a screwdriver or latch to keep doors closed, and watch through the glass.
Allow sleep. Lock the car and wait 30–60 minutes. Modules that stay awake at 25 minutes are the usual suspects. Record the stabilised figure.
Isolate. Pull one fuse at a time and refit it before moving on, watching for the drop. Start with the non-original taps - dash camera and amplifier on the interior-lamp circuit - because aftermarket work is the most likely source. Then check the usual residual suspects: boot lamp, glovebox lamp, radio memory, body control module, and alternator leakage measured as AC ripple or a diode leak test.
Confirm. Once the offending circuit is found, measure again after the repair and record the before and after values on the job card. Verify charging again at 2000 rpm with loads on.
3. The drain traces to the accessory wiring. The customer says the accessory shop should fix it free and does not want to pay you. How do you handle this commercially and ethically?
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Separate diagnosis from repair. Your diagnostic time is a real service and is chargeable unless the workshop agreed otherwise; say so at the start, not at the end.
Give evidence, not opinion. Show the customer the measured figures - 0.38 A falling to 0.06 A, the fuse that dropped it, and a photograph of the tap into the interior-lamp circuit. That converts 'your accessory is at fault' into a fact the accessory shop can act on.
Offer options. The customer can take the car back to the accessory shop with your written findings, or authorise you to rewire the accessory to a proper ignition-switched feed with an inline fuse, with a quoted price and a signature before work starts. Do not simply cut the accessory wiring without consent; that leaves the customer with a dead accessory and a dispute.
Advise on the battery. Explain the 72% state of health, the short-trip duty cycle and the heat, and recommend either a battery replacement or a periodic long run with a charger. Record the advice on the job card, including that the customer declined if they do.
No false claims. Do not describe the fault as a vehicle manufacturing defect to push it through warranty; that is a misrepresentation and it will not survive an audit.