Repeat Flat Battery on a Smart-Charging SUV
A 2019 compact SUV with a 70 Ah AGM battery, smart charging and start-stop comes back for the third time in five weeks with a flat battery after standing for two days. The battery is three years old, has been charged twice, and the alternator was replaced last month with a genuine unit. The customer's only accessory is a dashcam plugged into the OBD socket. A clamp meter on the negative cable shows 1.9 A immediately after locking, 0.15 A after 10 minutes, and 48 mA after 45 minutes; the manufacturer's sleep-current limit is 30 mA after 60 minutes. Charging voltage at idle is 13.9 V with electrical loads on, and 12.4 V with loads off and the battery at 80% state of charge. Battery registration was performed after the alternator replacement. The workshop has a 60-minute slot and the customer wants the battery replaced under warranty.
1. Calculate what the 48 mA reading means for this battery and decide whether that drain alone explains a flat battery after two days.
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At 48 mA, the daily consumption is 0.048 A × 24 h = 1.15 Ah. Over two days that is only about 2.3 Ah, roughly 3% of a 70 Ah battery. Even allowing for a battery that is not at full charge, that cannot explain a no-crank after 48 hours. At 48 mA, a healthy 70 Ah battery would take about 1,450 hours, or 60 days, to reach zero in theory, and around 30 days to reach 50% depth of discharge. The 30 mA limit means the vehicle is out of specification, but the excess is 18 mA, or about 0.43 Ah per day. That is a long-term drain issue, not the immediate cause. The likely explanations are a periodic wake-up drawing a much larger current, a battery that has lost capacity, or charging that does not replace what start-stop and short trips consume. The 12.4 V reading with loads off at 80% state of charge is plausible for smart charging and is not proof of alternator failure; 13.9 V under load shows the system can charge, but current output still needs checking.
2. What diagnostic plan would you run in the available time, and what would you tell the customer about warranty replacement?
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Fully charge the AGM battery with a suitable charger, then conductance-test it on the AGM setting and record state of health, state of charge and date code. Check battery registration and the intelligent battery sensor readings. Remove the dashcam from the OBD socket, because that socket may remain live, and check for other aftermarket wiring. Confirm all modules go to sleep using the scan tool, and inspect boot, glovebox and door switches. Repeat the current measurement after a full 60-minute sleep, then use fuse voltage-drop measurement or careful fuse removal to locate any circuit drawing current. Because 60 minutes cannot catch an intermittent wake-up, connect a current/voltage data logger for 24–48 hours and ask the customer to leave the vehicle undisturbed. Tell the customer that 48 mA is above the 30 mA limit and will be investigated, but the arithmetic shows it cannot flatten a healthy 70 Ah battery in two days. A warranty claim depends on the battery test result and age, not on the complaint alone. Do not replace the battery without evidence, and explain that the logger may be needed to find the real cause.