Battery flat after two days on a car with keyless entry
A 2022 1.2-litre petrol sedan with 28,000 km, keyless entry, factory telematics and a dashcam spliced into the interior lamp circuit goes flat twice in three weeks - both times after two days of not being used. The battery was replaced under warranty after the first event, and the car was flat again within 48 hours. Resting voltage 12.6 V, capacity test 640 CCA against a 600 CCA rating, charging 13.9–14.1 V, no warning lamps. With a clamp meter around the negative cable you measure 2.4 A just after locking, 380 mA at 10 minutes, 210 mA at 30 minutes and 205 mA at 60 minutes. The manufacturer's specification is 30 mA or less after 40 minutes. The battery is 45 Ah. The owner is convinced the replacement battery is faulty.
1. Use the numbers to explain what is happening and why the new battery also went flat.
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A drain of 205 mA is roughly seven times the 30 mA limit. Left for 48 hours it consumes about 0.205 A × 48 h = 9.8 Ah, which is over a fifth of a 45 Ah battery, and on repeated cycles the battery sits at a low state of charge long enough for sulfation to start; capacity and cranking performance then fall even though the battery passed its first test. Resting voltage of 12.6 V only shows it is charged now, not that it holds charge. The current trace matters as much as the end value: 2.4 A just after locking is normal module activity, the fall to 380 mA shows some modules going to sleep, and the plateau at 205–210 mA shows one circuit never sleeping. That plateau is the fault. Replacing the battery was reasonable as a safety measure but could never fix a parasitic drain, which is why the second battery behaved exactly like the first.
2. Describe your isolation procedure, including why simply pulling fuses is a poor first move.
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Test only with the car properly asleep: doors closed and latched, hood switch closed, key fob at least 10 m away, no scan tool connected, and 40–60 minutes of no activity, because opening a door or pulling a fuse wakes the body control module and resets the timer. Pulling fuses therefore both disturbs the test and may restart the sleep sequence. Start with the most likely known intrusion: the dashcam splice on the interior lamp circuit, which is often permanently live; disconnect it and re-measure. Then check for a technical bulletin on the telematics unit, a known drain source on many connected cars. If the drain persists, use the millivolt-drop method across fuses with a high-resolution meter and a fuse resistance chart - a fuse reading near zero is not carrying the current, whereas a few millivolts on a 10 A mini fuse is consistent with a couple of hundred milliamps. Also check alternator AC ripple and diode leakage by disconnecting the B+ cable.
3. What evidence confirms the repair, and what will you tell the owner about the dashcam?
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After the repair, repeat the full sleep test and show 30 mA or less at 60 minutes, then leave the car parked for 24–48 hours and confirm resting voltage stays at 12.4 V or above and the engine cranks normally. Record the before-and-after current traces, the circuit identified and the repair, so a repeat visit starts from data instead of guesswork. Explain that 205 mA was an abnormal drain, not a bad battery, and that the warranty replacement was a consequence rather than the cure. On the dashcam, state that splicing into the interior lamp circuit bypasses the vehicle's power management and can both flatten the battery and affect the body control module; offer a proper ignition-switched supply or a voltage-sensing hardwire kit, and note the modification on the job card so the drain history is documented.