0.38 A Drain That Flattens an AGM Battery at the Airport
A 2019 SUV with smart charging and an 80 Ah AGM battery (680 CCA) comes in for the third time: it will not start after standing for two to three days. The battery is 4 months old and passed a capacity test; charging voltage is a steady 14.3 V and the charging circuit has been checked. With a low-current clamp on the battery negative and everything switched off (doors closed and latched, key kept away from the car, bonnet switch latched with a screwdriver), the reading is 0.62 A immediately after locking, falling to 0.38 A after 45 minutes, where it stays. The manufacturer's specification is less than 40 mA after 60 minutes. Fitted extras: a hard-wired dash camera, an aftermarket radio and a towbar module. Pulling the 15 A fuse for the wiper motor drops the drain to 0.04 A. The workshop has a low-current clamp, a fuse voltage-drop chart, a DMM and a power supply.
1. Work out what 0.38 A does to this battery and decide whether it explains the complaint.
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Arithmetic:
- 0.38 A × 72 h = 27.4 Ah consumed over a weekend.
- 0.38 A × 168 h (one week at the airport) = 63.8 Ah.
Against the battery: nominal capacity is 80 Ah, but a lead-acid/AGM battery should not be taken below about 50 % state of charge if it is to keep cranking and survive. That leaves roughly 40 Ah of usable capacity. A three-day stand therefore uses about two-thirds of the usable capacity, leaving the battery around 12.0–12.1 V - marginal for cranking, and far worse in cold weather when capacity falls and cranking current rises. A week away takes it essentially flat and contributes to sulphation, which is why the previous batteries died young.
Verdict: yes, 0.38 A (nearly ten times the 40 mA limit) fully explains the complaint. It is a genuine parasitic drain, not a battery or charging fault, and the 'third visit' pattern is typical of a drain that is only measured too early (0.62 A is the modules still awake) or measured with the wrong technique. The 0.04 A measured after removing the wiper fuse is in spec, which shows the harness and the rest of the vehicle are capable of sleeping correctly.
2. List the measurement mistakes that commonly produce a wrong parasitic-drain result, and how you avoid each one here.
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Common mistakes and the fix:
- Reading too early. Modules stay awake for 20–60 minutes (some up to 90). This car only settles at 45 min. Fix: wait the full sleep time, then record min/max over at least 30 minutes with the meter's min/max function to catch intermittent wake-ups (a module that wakes every few minutes averages out to a low reading).
- Breaking the circuit with an in-line ammeter. Unplugging the battery wakes every module, and if they all wake at once the inrush can blow the meter's fuse - after which you are reading zero and believing it. Fix: use a low-current clamp around the negative cable, or an in-line meter with a bridging switch.
- Opening doors, touching handles, or leaving the key nearby. Any of these wakes the network. Fix: doors closed and latched, windows down for access, key several metres away in a screened tin, and no central-locking activity during the test.
- Bonnet switch open. The car thinks the bonnet is up and holds modules awake. Fix: latch the switch (as done here with a screwdriver).
- Not zeroing the clamp / wrong range. A low-current clamp drifts; zero it with no conductor inside, and keep it away from other cables.
- Ignoring the extras. Hard-wired dash cameras, aftermarket radios and towbar modules are prime suspects and are often on permanent live. Measure them individually rather than assuming the vehicle's own circuits are at fault.
A drain is only confirmed when the reading is stable, below spec, and reproducible after a full sleep cycle.
3. The wiper fuse test dropped the drain to 0.04 A. What does that prove, what are the possible faults, and how do you distinguish them before replacing anything?
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What it proves: the 0.38 A is flowing through the wiper-motor fuse circuit, i.e. that branch is the drain. Note the fuse also feeds the wiper relay coil on many vehicles, so the fault is either the load or the control of the load.
Possible faults:
- Wiper motor not parking / stuck park switch or internal short - the motor keeps drawing after the switch is released.
- Wiper relay stuck closed, or its coil held energised by the BCM - a BCM output held on, or a short to voltage on the control wire.
- Chafed wiring to the motor or through the tailgate/bulkhead grommet leaking to ground.
How to distinguish:
- Check whether the wiper relay is warm after the car has been standing - a stuck-closed relay is a giveaway.
- Unplug the wiper motor connector and re-measure the drain after a full sleep cycle. Drain gone → motor/park switch. Drain still present → relay, control wire or BCM output.
- If the relay is the suspect, pull the relay and repeat; then voltage-check the control wire for a short to battery voltage with the ignition off.
- Inspect the loom where it flexes.
Then verify the whole job: refit the fuse, re-measure quiescent current after 60 minutes to confirm it is under 40 mA, confirm the dash camera's parking-mode draw (a few tens of milliamps is normal; a few hundred is not), charge and test the battery, and only then release the car. If the total settles at, say, 0.08 A, there is a second smaller drain and the job is not finished.