Overnight Parasitic Drain After Towbar and Dash-Cam Fitment
A 2019 petrol SUV has 62,000 km and a 70 Ah battery that went flat after sitting for two days. The battery is new and passes a conductance test; charging voltage is 14.1 V with loads on. The customer reports the problem began after a dash-cam was hard-wired to the interior fuse box and a towbar was fitted. After locking, the vehicle is left for 65 minutes to allow modules to sleep. A clamp meter then shows 0.46 A key-off draw. The manufacturer’s guideline is less than 0.05 A after sleep. With all doors, bonnet and tailgate latched, a voltmeter is used across fuses; most show 0.0 mV, the 15 A dash-cam fuse shows 2.8 mV and the 20 A towbar fuse shows 0.4 mV. The dash-cam is plugged into its hard-wire lead. The customer needs the vehicle back tomorrow and asks whether the battery is faulty.
1. Calculate the ampere-hour effect of the 0.46 A draw over 48 hours on the 70 Ah battery and explain whether the battery is likely to be the root cause.
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Calculation: 0.46 A × 48 h = 22.08 Ah. That is about 31.5% of a 70 Ah battery’s nominal capacity, and it is on top of normal reserve and any ageing. A healthy battery would be substantially discharged after two days, and repeated deep discharge can damage it.
Conclusion: The battery is probably not the root cause. It is new, passes a conductance test and the charging voltage is normal at 14.1 V. The measured key-off draw of 0.46 A is about nine times the manufacturer’s 0.05 A guideline. The battery is the symptom holder, not the fault. Replacing it again without correcting the draw would only delay the same complaint. The logical next step is to isolate the excessive circuit and confirm the repair by repeating the sleep-current test.
2. Describe how you would use the fuse voltage-drop results to isolate the draw, then state the repair and verification. Include one precaution about pulling fuses.
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Isolation: The fuse voltage-drop test is comparative: a fuse with current flowing through it shows a small voltage drop across its terminals, while a fuse with no current shows 0.0 mV. The 2.8 mV on the 15 A dash-cam fuse is the strongest lead, especially because the dash-cam is hard-wired and the fault began after fitment. The 0.4 mV on the towbar fuse may be a normal keep-alive load, but it should be checked if the draw remains after the dash-cam is isolated.
Repair: Unplug the dash-cam at its hard-wire connector, re-lock the vehicle and wait for sleep. If the draw falls below 0.05 A, rewire the dash-cam to a switched ignition feed or fit a voltage-monitoring shutdown device rather than a constant battery feed. Inspect the towbar wiring for a relay or module staying awake.
Precaution: Do not randomly pull fuses or disconnect modules while the vehicle is awake; that can wake other modules, set codes and give false readings. Verify the final sleep current after 60–65 minutes with everything latched.