Battery Drain After Two Days Parked
A 2018 sedan with 62,000 miles and a 70 Ah AGM battery dies after sitting 48 hours. Battery rest voltage is 12.6V and it load-tests at 610 CCA against a 640 CCA rating. The customer hardwired a dash cam four months ago. After 40 minutes, a clamp meter shows 220 mA. With the dash cam connected, the draw periodically rises to 1.4 A for 5 minutes every 15 minutes. Unplugging the dash cam after 70 minutes leaves 35 mA. Removing fuse F13, which feeds the dash cam, stops the cycling. Removing the BCM fuse drops the draw to 0 mA, but reconnecting it wakes several modules. Alternator ripple is 0.4V AC at 2,000 rpm. No DTCs are stored.
1. Calculate the average parasitic draw with the dash cam connected and decide whether it explains a no-start after 48 hours. Show your reasoning.
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With the dash cam connected, the average draw is the baseline plus the duty-cycled wake load: 220 mA + 1.4 A × (5 min / 15 min) = 0.220 A + 0.467 A = 0.687 A. Over 48 hours that is 0.687 A × 48 h = 32.98 Ah, or about 47% of a 70 Ah battery. A healthy battery may still start, but a battery at 610/640 CCA with unknown age has little margin, especially if the 48-hour park ends in cool weather. So the dash cam circuit can plausibly cause the no-start, but the calculation does not prove the battery is good. I would recharge and load-test the battery after repairing the draw; if it still fails, replace it.
2. Explain how you interpret the BCM fuse result and how you avoid a false diagnosis from fuse-pull testing.
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Removing the BCM fuse and seeing 0 mA is not proof that the BCM is the culprit. Reconnecting the fuse wakes the network, and several modules may draw high current for minutes before sleeping. Fuse-pull testing can also reset timers and make a normal module look guilty. The valid comparison is the amp clamp over time: with F13 removed, the cycling stops, and with the dash cam unplugged the draw falls to 35 mA. That correlates the cycling with the dash cam circuit, not the BCM. To avoid a false diagnosis, reconnect everything, wait for the specified sleep time, and monitor current for at least 30–60 minutes; use a low-current clamp or a fused jumper, not a test light.
3. What is the next step: replace the battery, repair the dash cam circuit, or both? Justify using the ripple and battery data.
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Repair the dash cam circuit first. The 0.4V AC ripple at 2,000 rpm is within a normal range and does not indicate a failed alternator diode. The battery’s 12.6V rest voltage and 610 CCA are serviceable but marginal enough that repeated deep discharges may have damaged it. I would correct the dash cam so it is ignition-switched or shuts down fully, confirm sleep draw below about 50 mA, recharge the battery, and conduct a load test. Replace the battery only if it fails after recharge or if its age and test data show it cannot hold a charge. Replacing the battery alone would leave the 0.687 A average draw in place and the problem would return.