Battery Drain on a Vehicle With a New Alternator
A 2019 diesel pickup will not start after standing for two days. The battery is 18 months old and passes a conductance test at 78% state of health. The alternator was replaced three months ago and charges at 14.2 V at idle with loads off, dropping to 13.6 V with headlights and blower on. The customer has fitted an aftermarket reversing camera and a dash camera. A parasitic drain test with a clamp meter shows 480 mA after the vehicle has been locked for 40 minutes; the manufacturer's limit is 50 mA after 60 minutes. Removing the dash camera fuse drops the reading to 60 mA. The reversing camera is wired to a permanent live.
1. Evaluate the charging system and battery evidence, and decide whether the no-start is a charging fault or a drain fault.
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The evidence points to a parasitic drain fault, not a charging fault.
- Charging voltages are within normal range: 14.2 V at idle is typical for a smart-charging diesel, and 13.6 V under load is acceptable. A failed alternator would typically show below 13 V or an unstable reading.
- Battery state of health 78% means the battery retains useful capacity; it is not the primary cause, though it is ageing.
- Drain of 480 mA is roughly ten times the 50 mA limit. Over 48 hours that is about 23 Ah removed, enough to prevent cranking on a typical 70-90 Ah battery.
- Removing the dash camera fuse drops the drain to 60 mA, close to the limit and consistent with the camera being the dominant load. The remaining 60 mA is slightly above spec and should be investigated after the camera is corrected.
Conclusion: correct the aftermarket wiring first, then re-test the drain and battery.
2. Describe how you would perform a systematic parasitic drain test, including the precautions needed to avoid disturbing modules.
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Systematic procedure:
- Prepare the vehicle: fully charge the battery, then run the engine briefly and switch off. Close all doors and lock the vehicle with the remote so modules enter sleep mode.
- Allow the sleep delay: typically 40-60 minutes; do not open doors or operate anything during this time.
- Measure with a clamp meter around the negative cable rather than breaking the circuit, to avoid waking modules. If a series ammeter is used, connect a memory-saver device first to preserve radio and module settings.
- Record the baseline and compare with the manufacturer's limit (here 50 mA).
- Isolate circuits one at a time by removing fuses, watching for the reading to fall. Note that pulling a fuse can wake other modules, so allow the sleep delay again after each change.
- Investigate the identified circuit with a wiring diagram; check for relays stuck closed, aftermarket accessories on permanent live, and damaged looms.
- Re-test after repair to confirm the drain is below the limit.
Precautions: never disconnect the battery with the ignition on; use a memory saver; avoid probing airbag circuits.
3. Recommend how the aftermarket accessories should be rewired, and state what you would tell the customer.
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Recommended rewiring:
- Move both cameras to an ignition-switched or accessory live so they are off when the vehicle is parked. Use a fused supply taken from an appropriate circuit, not a direct battery feed.
- Use an ignition-sensed relay if the camera requires a permanent memory feed; the main load should be switched.
- Fit an inline fuse correctly rated for the camera circuit and route the wiring through grommets, away from sharp edges and moving parts.
- Secure and label the installation, and record it on the job card.
What to tell the customer:
- The no-start was caused by the dash camera drawing about 480 mA continuously, roughly ten times the vehicle limit.
- The battery is serviceable at 78% health but should be re-tested after the repair.
- The rewiring will stop the drain; confirm with a repeat drain test.
- Advise that any future accessories be fitted with a switched supply and that the work be documented, since aftermarket wiring can affect warranty and insurance.