The Car That Will Not Start Every Second Morning
A 2017 1.2-litre petrol hatchback with 48,000 km comes in because it will not start every second morning, though it bump-starts or jump-starts every time. The battery is a 12 V, 35 Ah, 300 CCA lead-acid unit, 14 months old, and fails a load test at about 78 % of its rating. Charging checks show 13.9 V at idle and 14.1 V at 2,000 rpm with electrical loads off; alternator ripple is 0.4 V AC against a 0.5 V limit. A low-current clamp meter on the negative cable reads 84 mA after the car has been locked for 45 minutes with the key 5 m away; the manufacturer's limit is 30 mA after 30 minutes. Nothing is plugged into the accessory socket. The owner mentions an aftermarket reversing camera and a USB charger wired into the interior fuse box on a piggyback fuse, fitted about six weeks ago. The battery earth strap measures 0.6 ohm end to end, against a specification below 0.1 ohm. The owner does mostly 8–10 minute trips.
1. How would you measure and locate this parasitic draw properly, and what is wrong with simply pulling fuses one at a time?
Read the worked answer
Set the car up to sleep. Close and latch all doors, bonnet and boot (or tape the bonnet switch closed), leave the key fob at least 5 m away, do not open anything, lock the car, and wait the full sleep time - many modules need 40–60 minutes, so 45 minutes may be too early. Re-measure at 60 minutes before believing 84 mA.
Measure without breaking the circuit. Use a low-current clamp on the negative cable, or a multimeter in series with a bridging switch so you can zero and re-connect without interrupting current. Do not disconnect the battery to "check for a draw" - that wakes every module, resets adaptions and restarts the sleep timer, so you measure the wrong thing.
Locate the circuit by voltage drop across fuses. With the draw flowing and the car asleep, measure millivolts across each fuse and compare against a fuse voltage-drop chart for its rating. No fuse removal, no waking modules.
What is wrong with pulling fuses one by one? Each pull and refit interrupts supply, wakes the network, and restarts the 30–60 minute sleep clock; you also get false results from modules that draw heavily for a few minutes after waking. On some cars, pulling a fuse can also set codes and lose settings. Fuse voltage-drop is faster and non-invasive.
2. Calculate what 84 mA does to a 35 Ah battery over 36 hours, and decide whether the draw alone explains the no-start.
Read the worked answer
0.084 A × 36 h = about 3.0 Ah. On a healthy 35 Ah battery that is roughly 9 % of capacity - annoying, but not enough to stop a healthy battery cranking a 1.2-litre engine.
So the draw does not, on its own, explain the symptom. Three other facts do:
- The battery fails its load test at 78 % of rating, so effective capacity is nearer 27 Ah and cranking ability is already reduced. A 14-month-old battery in this state has probably been deep-discharged more than once.
- The earth strap at 0.6 ohm adds resistance in the charging and cranking circuit. It lowers the voltage the battery actually sees while charging and drops voltage during cranking, making a marginal battery fail.
- The 8–10 minute trip pattern. Each start takes a chunk out of the battery and a ten-minute drive may not put it back, especially with lights, blower and a 0.6 ohm strap in the path. Add the 3 Ah nightly loss and by the second morning the battery is below cranking threshold.
The draw is still a genuine fault - 84 mA against a 30 mA limit - and must be fixed. But fixing only the battery will bring the car back in a few weeks, and fixing only the draw leaves a sulphated battery. Both must be addressed, and the earth strap is the cheapest item with the widest effect.
3. Give the order in which you would repair this vehicle and explain the reasoning behind that order, including what you tell the owner.
Read the worked answer
- Earth strap first. Clean the mounting points and replace the strap, then re-check charging voltage and perform a cranking voltage-drop test. It is cheap, it affects both charging and starting, and a 0.6 ohm resistance can distort every other voltage reading you take afterwards. Do the cheap global fix before chasing local faults.
- The aftermarket wiring. Disconnect the camera and USB charger at the piggyback fuse, let the car sleep, and re-measure. Reversing cameras left in parking mode can draw well over 100 mA; the piggyback may also be tapped into a permanent live. If that circuit is the culprit, move it to an ignition-switched supply and re-test, or remove it.
- If the draw persists, use the fuse voltage-drop method to identify the circuit - a boot lamp, glovebox lamp, stuck switch or a module that will not sleep.
- Battery. It has failed a load test and is only 14 months old - check the warranty position, and be aware a warranty claim is stronger once the parasitic draw is shown to be fixed.
- Verify and hand over. Confirm the sleeping draw is under 30 mA, charging is 13.8–14.4 V, the battery holds a load test, and the car starts after standing two nights if possible.
Tell the owner plainly: the draw is a real fault, the battery has been damaged by repeated deep discharge, the earth strap was adding to the problem, and the aftermarket wiring is a known risk. Also advise longer runs or a maintenance charger given the short-trip pattern.