Overnight Battery Drain After an Aftermarket Central-Locking Kit
A 2017 Maruti Swift petrol, 62,000 km, has a battery that goes flat overnight twice in one week. The battery is 18 months old, 35 Ah, and its resting voltage after 2 hours is 12.4 V. With the engine idling and all loads off, the alternator output at the battery terminals is 13.2 V; at 2,000 rpm it rises to 13.5 V. A clamp meter on the negative battery cable shows 0.38 A after all doors are closed and the key is removed, waiting 30 minutes. An aftermarket central-locking kit and a horn relay were fitted two weeks ago. Fuse voltage-drop testing shows 0.02 V across the alternator fuse and 0.31 V across the central-locking fuse. The manufacturer's acceptable parasitic draw is below 0.05 A.
1. Calculate whether the 0.38 A draw is acceptable and estimate its effect on the 35 Ah battery.
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The limit is 0.05 A, so 0.38 A is about 7.6 times too high. Over 24 hours the draw removes 0.38 × 24 = 9.1 Ah from a 35 Ah battery, about 26% of capacity. Over a weekend it removes about 27 Ah, leaving too little reserve for cranking, especially in cool weather. The theoretical time to zero is about 92 hours, but a battery cannot crank an engine at low charge, so the practical effect is a flat battery much sooner.
The 30-minute wait is correct because modules need time to sleep. The test should be repeated with a current clamp that can resolve low currents, and all doors, boot and bonnet switches must be closed. A 12.4 V resting voltage is also only partly charged, so the battery should be charged and retested before condemning it.
2. Interpret the charging voltages and decide whether the alternator alone explains the problem.
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13.2 V at idle and 13.5 V at 2,000 rpm are low for a 12 V system; normal charging is roughly 13.8–14.4 V, with some systems slightly higher. Low charging can leave the battery partly discharged, but it does not explain a 0.38 A draw when everything is off. There are two faults to investigate.
Check the alternator belt, the sense wire, the battery terminals and engine/chassis earths, and load-test the alternator. A faulty regulator or a high-resistance sense circuit can cause low output. Do not replace the alternator on voltage alone. Fix the parasitic drain and the charging system, then recharge and retest the battery; an 18-month-old battery may have been damaged by repeated deep discharge.
3. Use the fuse voltage drops to identify the likely circuit and outline the repair and retest.
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The 0.02 V drop across the alternator fuse is negligible. The 0.31 V drop across the central-locking fuse shows current is flowing through that circuit and that the fuse has resistance. This points to the aftermarket central-locking kit or its relay as the likely drain, especially because it was fitted two weeks ago.
Disconnect the kit and re-measure the current; if the draw falls below 0.05 A, repair or replace the kit's relay, splices and chafed wiring. Inspect the horn relay too, since it was fitted at the same time. Retest after the modules have gone to sleep, with the doors closed. Also recharge the battery and check charging output. Record the before-and-after current so the customer can see that the repair was verified, not just assumed.