Repeat Battery Flatness and a 0.35 A Parasitic Drain
A 1.0-litre petrol hatchback is brought in after its three-year-old 35 Ah battery went flat for the third time in a month. The battery was replaced six weeks ago. Resting voltage is 11.8 V before charging. With the engine running at 2,000 rpm and headlamps plus blower on, the voltage at the battery terminals is 13.9 V; the specification is 13.8–14.4 V. A clamp meter on the negative battery cable shows 0.35 A after the vehicle has been locked and left for 30 minutes; the specification is less than 0.05 A. Removing the 15 A interior-lamp fuse drops the current to 0.04 A. The customer fitted a dash camera and a phone charger two months ago, and the boot lamp has been glowing faintly when the tailgate is shut. The workshop has a diagnostic scanner but no current probe.
1. Evaluate the battery and charging system results. Is the alternator definitely faulty?
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The charging voltage of 13.9 V is within the stated 13.8–14.4 V range, although it is at the lower end. That alone does not prove the alternator is faulty. The test should be repeated with the engine at 2,000 rpm, all specified loads on, and the voltage measured at the battery posts and at the alternator output. A voltage drop test on the positive and earth cables will show whether resistance is causing a low reading. The alternator should also be checked for rated output and for AC ripple if the equipment is available.
The battery at 11.8 V is deeply discharged, not necessarily defective. A six-week-old battery can be ruined by repeated deep discharge, but it should first be slow-charged and load-tested to the battery's cold-cranking specification. The fact that the original battery was replaced and the problem returned points to a drain or charging fault, not simply a bad battery.
2. Calculate the effect of the 0.35 A drain and use the fuse-pull result to locate the cause.
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A 0.35 A drain for 24 hours is 0.35 × 24 = 8.4 Ah per day. Over three days that is about 25 Ah, which is most of the 35 Ah battery's capacity, so the flat-battery complaint is plausible. The specification is below 0.05 A, so 0.35 A is about seven times the limit.
Removing the 15 A interior-lamp fuse drops the current to 0.04 A, which is within specification. That strongly implicates the interior-lamp circuit. The faintly glowing boot lamp with the tailgate shut is the obvious suspect: the lamp switch or striker may be misadjusted, or there may be a short keeping the circuit live. The dash camera and phone charger are additional suspects if they are connected to an always-live socket, but the fuse-pull test points first to the lamp circuit. The technician should inspect the boot lamp switch, the tailgate alignment, and any interior lamps, then disconnect the accessories one at a time to confirm.
3. What repair and verification steps should be taken before returning the car?
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Repair the boot lamp circuit first: adjust or replace the boot lamp switch or striker so the lamp is off when the tailgate is closed. Check the interior lamp switch and wiring for chafing or a partial short. The dash camera should be moved to an ignition-switched supply, or wired through a relay so it cannot run continuously, and the phone charger should be removed from a permanently live socket. The battery should be slow-charged and load-tested; if it fails, it should be replaced, but the drain must be fixed first or the new battery will fail again.
Verification: lock the car, wait at least 30 minutes for modules to sleep, and measure the drain with the clamp meter. It should be below 0.05 A. Recheck charging voltage under load, and confirm the boot lamp is off with the tailgate closed. Record the before-and-after current readings. If the drain remains high, pull fuses one at a time and note the circuit; the scanner can help identify a module that is not sleeping, but the clamp meter remains the primary test.