Flat Battery After Two Days Parked
A 2017 pickup with a 2.4 L diesel engine comes back for the second time in a month with a flat battery. The battery is a 70 Ah, 650 CCA unit fitted three weeks ago. The customer reports that the truck starts fine, but after two days parked it will not crank. The alternator was replaced with a surplus unit before the battery. Your measurements: charging voltage at 2,000 rpm with headlights and blower on is 13.2 V at the battery posts and 13.6 V at the alternator B+ stud; a voltage-drop test on the positive cable between the alternator and the battery shows 0.4 V. A clamp meter on the negative cable shows 0.35 A ten minutes after the doors were closed and the key removed; the manufacturer's specification for sleep current is under 0.05 A. The truck has an aftermarket alarm and a dash camera spliced into the interior lamp circuit.
1. Evaluate the 0.35 A reading. What would it do to a healthy 70 Ah battery over 48 hours, and what must you check about the way the reading was taken?
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The arithmetic: 0.35 A × 48 h = 16.8 Ah removed. That is about a quarter of the battery's 70 Ah rating, and a healthy battery would normally still crank after losing that much. So the draw alone does not fully explain a no-crank in two days - it is roughly seven times the 0.05 A limit, which is definitely excessive and must be fixed, but something else is also wrong: either the battery is already degraded from repeated deep discharge, or it is not being fully charged, or the true draw is higher than measured.
The measurement is suspect. Ten minutes after closing the doors is usually too early; body control modules, the alarm and the audio unit can take 20 to 60 minutes to go to sleep, so 0.35 A may be a false high - or, if the alarm woke the network when the meter was connected, it may be a false low. Procedure: charge the battery fully, connect the meter in series on the 10 A range with all doors latched (trip the latch with a screwdriver or tape the switch) and all loads off, then wait until the current drops to its sleep level and stabilises. Never crank with the meter in line. Pull one fuse at a time and note the drop; re-baseline after each reconnection because reconnecting a module wakes the network again. The spliced dash camera and alarm on the interior lamp circuit are prime suspects - disconnect them and repeat.
2. Is the alternator good? Interpret 13.6 V at the alternator and 13.2 V at the battery, and state what you do about the 0.4 V drop.
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13.2 V at the battery with lights and blower on at 2,000 rpm is low. A healthy charging system should hold roughly 13.8–14.4 V at the battery, and certainly above about 13.5 V with loads at that engine speed. The surplus alternator is not proven good yet, but you cannot condemn it either, because there is a measurable loss in the circuit.
The 0.4 V drop is the first thing to fix. A 0.4 V difference between the alternator output stud and the battery post means resistance in the B+ cable, the fusible link, a terminal or a ground. At a realistic 60 A charging current, 0.4 V is about 24 W of heat at a bad joint - that is a fire risk as well as an undercharge. Typical acceptance for a charging circuit is around 0.2 V or less under load. Inspect and clean the battery terminals, the alternator B+ nut, the fusible link and the engine-to-chassis ground strap; measure again after repair.
Only after the drop is fixed do you judge the alternator: if it still cannot reach specification at the battery with the engine at 2,000 rpm and loads on, the surplus unit's regulator or rectifier is faulty and it is replaced. Order of work matters: repairing the cable is cheap and fast, and it may make the difference between a battery that lasts two years and one that dies in six months.
3. Set out the diagnostic order for this vehicle and say which single repair you would do first if the customer could only approve one item today.
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Order:
- Repair the charging circuit voltage drop - cables, terminals, grounds, fusible link. It is cheap, it can be a fire hazard, and undercharging both masks and mimics a parasitic draw.
- Test the battery properly - full charge, then a conductance or load test to roughly half its CCA rating for the specified time. Three weeks old does not mean healthy; a battery flattened twice has been sulfated. Check the date code and whether it was ever charged before fitting.
- Re-measure sleep current using the corrected procedure, then isolate by fuse-pulling, starting with the alarm and dash camera.
- Road test and recheck charging voltage and restart after a 48-hour soak if the customer can leave the truck.
If only one item can be approved today: the charging circuit repair. It is the cheapest intervention, it removes a heat/fire risk at a resistive joint, and it addresses the root cause of chronic undercharging. The parasitic draw will still flatten the truck if it is left for days, so tell the customer plainly: the truck will be reliable for daily use but may still go flat after a long park until the draw is found - and advise disconnecting the aftermarket accessories as an interim measure. Document both findings so the second visit starts from evidence, not from scratch.