Battery Flat After Two Nights on a Petrol MPV
A 2020 petrol MPV has covered 45,000 km. Its battery goes flat after standing for two nights, about twice a week. The alternator was replaced two weeks ago after a no-charge complaint. Measurements: battery rest voltage 12.5 V; cranking voltage 10.8 V; idle charging voltage 13.0 V; at 2,000 rpm 13.2 V; AC ripple at the battery 0.6 V. A clamp meter shows 420 mA drain after the vehicle has been locked for 40 minutes; the manufacturer's limit is 50 mA. Removing fuse F10, which feeds interior lamps, audio, and the body-control-module keep-alive circuit, drops the drain to 35 mA. The battery is a 12 V, 65 Ah, three-year-old unit. The owner has fitted a dash camera and a phone charger plugged into the accessory socket.
1. Is the replacement alternator charging correctly? Justify your answer with the measured values and list the checks you would make.
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No. At 2,000 rpm the charging voltage should normally be about 13.8–14.4 V, and 13.2 V is low. The 0.6 V AC ripple is also excessive; a good rectifier normally gives a much smaller ripple, often below about 0.1–0.2 V. Together these suggest a faulty rectifier or regulator, a poor connection, or an incorrect replacement unit.
Check the voltage drop on the positive cable from alternator B+ to battery positive and on the ground side; each should be low, generally under about 0.2 V under load. Confirm belt condition and tension, pulley ratio, and that the alternator rating matches the vehicle. Check the sense wire and charging-lamp circuit, because a bad sense connection makes the regulator undercharge. Load-test the alternator with headlamps and blower on.
A new alternator is not proof of a good one. If the voltage drop and wiring are good, remove and bench-test the alternator or fit a known-good unit before blaming the battery.
2. How do you isolate the 420 mA drain, and what does the fuse F10 result prove?
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The 420 mA drain is far above the 50 mA limit. A 65 Ah battery losing 0.42 A continuously loses about 10 Ah a day, so two nights of standing can indeed prevent starting. First disconnect owner-fitted accessories, because a dash camera or charger left live can account for the drain. Then, with the vehicle locked and modules allowed to sleep for 40 minutes, use a clamp meter or a series ammeter and pull fuses one at a time, watching for the drain to fall.
The F10 result proves that about 385 mA is flowing through the F10 circuit. It does not prove which component on that circuit is at fault. Unplug the audio unit, interior lamps, and any body-control-module accessory on that feed one by one. If the drain remains with all consumers disconnected, check for a module that is not sleeping, a stuck relay, or a chafed wire. The residual 35 mA is within the limit and is normal keep-alive current.
Repair the parasitic drain and the charging fault, then recharge and load-test the battery. A three-year-old battery that has been deep-discharged repeatedly may also need replacement, but only after the electrical faults are fixed.