A Battery That Goes Flat Overnight: Reading a Parasitic Drain Test
A 2016 1.5 L hatchback arrives with a battery that went flat twice in one week. A 60 Ah battery was fitted three weeks ago by another workshop, and the alternator was replaced at the same time. Measured at 20 °C: battery open-circuit voltage 12.35 V after standing overnight; 9.4 V while cranking; charging voltage 13.6 V at 2000 rpm with all loads off and 13.2 V with headlamps, blower and rear demister on. The manual specifies 13.8–14.4 V at 2000 rpm with loads off, minimum 9.6 V during cranking, and a maximum key-off drain of 50 mA after the modules go to sleep. Using an inductive clamp meter the technician records 2.1 A just after locking the car, 0.42 A after 10 minutes and 0.38 A at 45 minutes. During that test the driver's door was open, the key fob was in the technician's pocket on the seat, and the hood lamp was disconnected. Pulling the 15 A fuse for the body control module drops the reading to 0.04 A.
1. Critique the parasitic drain test as performed. What conditions must be met before any current reading is valid?
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Faults in the test as performed
- Driver's door open. The body control module stays awake, interior lamps are live and the module may never enter sleep. The reading measures the workshop, not the fault.
- Key fob on the seat. Keyless-entry systems wake on fob presence. The fob should be several metres away or in a shielded pouch.
- Magazine clamp meter. Many clamps are specified ±(2 % + 10 mA); at 40–50 mA that is meaningless. The 0.04 A result after the fuse pull cannot be trusted without checking the meter's low-range accuracy and zeroing it.
- 45 minutes may be too short. Some modules take longer than an hour to sleep, and the 2.1 A → 0.42 A → 0.38 A trend is a normal sleep-down curve that was still falling.
- Pulling fuses can wake modules, so a drop after pulling a fuse does not by itself prove that circuit is the drain.
Validity conditions Doors, boot and bonnet closed and latched (tape the latch or disconnect the lamp), all accessories off, key out and away, battery fully charged and supported by a maintainer, meter in series or a low-current clamp rated for mA, and readings taken after the manufacturer's stated sleep time until the current is stable. Measure twice, and confirm the drop by a fused jumper across the fuse rather than by removal alone.
2. Using the numbers given, set out your diagnostic path and state where you think the root cause lies.
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Charging side first 13.6 V at 2000 rpm with no load is below the 13.8–14.4 V specification and falls to 13.2 V under load, so the battery is never fully recharged. Check the voltage drop on the alternator B+ cable and the engine earth strap under load (typically under 0.3 V and 0.1 V), the alternator sense wire, and the battery current sensor on the negative terminal if the car has smart charging - a faulty sensor or a corroded sense connection commands low output. Confirm the replacement alternator's rating and pulley ratio match the original.
Battery condition 12.35 V open-circuit after an overnight stand is roughly 60 % state of charge, and 9.4 V cranking is under the 9.6 V minimum. Three weeks old and already deep-discharged twice means sulphation is likely, so conductance-test it and recharge before judging it.
Drain The 0.38 A figure is not admissible as measured; repeat it properly. Only then chase a module. Meanwhile ask about a hard-wired dashcam or tracker, a very common cause on this type of car.
Root cause Most probable is undercharging from the replacement alternator or its sensing circuit, possibly with a genuine drain on top. Diagnose before fitting a third battery.
3. What do you tell the customer about the three-week-old battery and the other workshop's work?
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Explain, do not accuse State the measurements plainly: the charging voltage is below the manufacturer's specification, the battery is only partly charged and its cranking voltage is under the minimum, so the flat battery is a symptom, not necessarily the faulty part. Avoid blaming the other workshop by name; describe what the readings show and let them draw the conclusion.
Set expectations
- The charging fault must be found first, or any new battery will fail the same way.
- The battery may not recover from two deep discharges; it will be conductance-tested and recharged, and only replaced if it fails. Warranty on that battery belongs to the supplier who fitted it, so give the customer the test printout to support a claim.
- The drain test needs to be redone properly, and that takes time; give a written estimate for diagnosis and get authorisation before replacing parts.
- Ask about accessories, parking habits and short-trip use, since a hard-wired dashcam or a week of 3 km journeys changes the advice.
Record it Put all readings, the customer's description, the advice given and the authorisation on the job card, and give a copy on the invoice. If the customer declines the charging repair, note that too.