Three-Day No-Start: Is 120 mA the Fault or the Battery?
2018 hatchback, 60 Ah battery, four years old. Complaint: the engine will not start after the car sits for three days in the customer's driveway; a jump-start always works. Your technician measured parasitic draw with a clamp meter: 480 mA ten minutes after locking, 60 mA at forty minutes, 120 mA at ninety minutes. He also left the driver's door latch taped closed, kept the key fob in his pocket inside the car, and left a phone charger plugged into the accessory socket. A battery tester set to 600 CCA printed "REPLACE - 62% state of health"; the battery label says 480 CCA. Charging voltage at idle with loads on is 13.9 V. The customer wants to know whether to pay for a battery or for a "wiring fault".
1. Can the 120 mA figure be relied on? List what could distort it and describe the correct test method.
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Not yet - the number is unverified.
Artefacts inflating it:
- Key fob inside the car: keyless-entry vehicles keep polling and may never fully sleep.
- Charger left in an accessory socket that stays live.
- A taped latch is not the same as "closed and locked" to the body control module; many vehicles need the lock signal, bonnet switch and tailgate state correct before sleep.
- An aftermarket dashcam in parking mode draws tens to hundreds of mA by design and looks exactly like a parasitic drain.
- Clamp meters are poor below ~200 mA; on a ±50 mA clamp, a 120 mA reading carries over 40% uncertainty.
Artefacts deflating it: modules waking when the meter is connected, or reading before the full sleep delay, which can be 40-60 minutes.
Correct method: fob at least 5 m away, all doors/bonnet/tailgate closed and the car locked with the key, wait a full 60 minutes, then measure with a series ammeter or a low-range clamp, and repeat to confirm the trend flattens.
2. Do the arithmetic on whether 120 mA can flatten this battery in three days, and say what that implies about the diagnosis.
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120 mA × 72 h = 8.64 Ah, which is about 14% of a 60 Ah battery.
A healthy battery at 25 °C should start an engine from around 60% state of charge without difficulty, so a 120 mA drain does not, by itself, explain a three-day no-start. Rule of thumb: for a vehicle that must sit for weeks, keep the drain under about 1% of capacity per day, roughly 25 mA; for a three-day park, ~50 mA is comfortable and 120 mA is high but survivable.
So the arithmetic turns the diagnosis around: the draw is borderline, while the battery's ability to hold and deliver charge is unproven. Four years old, reported at 62% - but that test was run against 600 CCA when the label says 480 CCA. Testing a 480 CCA battery to a 600 CCA standard makes it look worse than it is, so the true state is unknown, not proven good.
Both variables need verifying before anything is sold: a corrected drain test and a corrected battery test.
3. What do you tell the customer, and what must be done before a battery or any wiring repair is sold?
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Before selling either, do three things.
- Re-run the drain test properly. If it returns 20-40 mA, the "wiring fault" story disappears and you have saved the customer a diagnostic bill.
- Re-test the battery properly. Recharge, remove surface charge, test at 480 CCA, and measure charge acceptance. Check charging at 2,000 rpm with lights, blower and rear screen on - 13.5-14.5 V is the window; 13.9 V at idle is encouraging but not conclusive.
- Interrogate the dashcam. Ask how it is wired and how parking mode is configured; a hardwired camera can be the entire 120 mA, and that is a customer choice rather than a fault.
Then say it plainly: the battery is four years old and near end of life; the drain is borderline but not fatal for a three-day park; the camera may be adding to it. Fit the battery if the retest supports it, note the camera wiring on the invoice, and re-check the drain afterwards so the new battery is not flat again in a fortnight. Never fit a battery without verifying the charging system - a new battery on a 13.2 V system comes straight back.