Parasitic drain, a degraded AGM battery and added accessories
A 2020 petrol hatchback with stop-start and keyless entry has a 70 Ah AGM battery. After three days parked, the engine cranks slowly and the battery is at 11.8 V. The owner had a dashcam and a third-party telematics unit fitted two months earlier. A clamp meter shows 85 mA after 65 minutes; the manufacturer's limit is 30 mA after 60 minutes. The alternator charges at 14.1 V and passes a ripple test. Battery test: 62% state of health, 680 A CCA rated, measured 410 A. The workshop suspects the dashcam, telematics, or a module not sleeping. With the vehicle locked, the technician measures voltage drop across fuses and finds 0.8 mV at fuse F12 (5 A), 0.2 mV at F27 (15 A) and 0.1 mV at F31 (7.5 A). The dashcam is on F12, telematics on F27. The customer wants the cheapest reliable fix and asks whether the battery must be replaced before testing. The workshop has one bay for two hours and no manufacturer online subscription until tomorrow.
1. Calculate the approximate energy taken by the 85 mA drain over three days and compare it with the 30 mA limit. Decide whether the drain and the battery condition are both faults.
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Over 72 hours, 85 mA × 72 h = 6.12 Ah. Against the 70 Ah nominal capacity that is about 8.7%, although usable capacity is less at low temperature and with a degraded battery. At the 30 mA limit, the same period would take 30 mA × 72 h = 2.16 Ah, so the excess drain is about 3.96 Ah. The drain exceeds the specification and is a fault.
The battery is also a fault: 62% state of health and 410 A measured against 680 A rated CCA is about 60% of rated cranking current. A healthy 70 Ah battery should normally tolerate a 6 Ah parasitic load without slow cranking, so the slow crank is likely the combination of an excessive drain and a degraded battery. The 11.8 V open-circuit reading supports a deeply discharged battery. Both must be addressed, but the drain should be isolated first so a new battery is not cycled and damaged.
2. Interpret the fuse voltage-drop readings and describe a reliable isolation procedure. Recommend the cheapest reliable fix for the accessories.
Read the worked answer
The fuse readings cannot be ranked directly by millivolts because different fuse ratings have different resistance and the voltage drop depends on current. F12 (5 A) showing 0.8 mV is suspicious and is consistent with the dashcam drawing current, but it is not proof. F27 (15 A) at 0.2 mV and F31 at 0.1 mV are lower, but the 15 A fuse may pass more current for the same millivolt reading. Use a current clamp or remove one fuse at a time and measure total drain.
Procedure: close and latch all doors, boot and bonnet, keep the key fob away, disconnect any charger or scan tool, lock the car and wait at least 60 minutes. Measure the baseline 85 mA. Unplug the dashcam and recheck after sleep; if the drain falls below 30 mA, the dashcam is the culprit. Reconnect it and unplug the telematics to test that circuit. If neither changes the drain, pull fuses one at a time and watch for the drop, then trace the relevant module.
Cheapest reliable fix: supply the dashcam and telematics from an ignition-switched circuit, or fit a voltage-sensing battery guard that cuts them off before cranking reserve is lost. Do not simply fit a larger battery. After the drain is repaired, recharge and retest the AGM battery; if it remains at 62% SOH, replace and register it. The two-hour bay is enough for the clamp and unplug tests; the online subscription is not needed for this isolation.