The Battery That Keeps Getting Replaced: Chasing a 340 mA Key-Off Draw
A taxi operator brings in a 2019 1.5 L gasoline sedan. The complaint is that the battery goes flat overnight roughly every three weeks, and this is the third battery in two months - the previous shop simply replaced the battery each time under warranty. The car starts fine after a jump-start, and the charging warning lamp is off. Your first measurements: battery open-circuit voltage 12.42 V after sitting 12 hours; a conductance test returns 'replace'; alternator output at idle 13.9 V, rising to 14.2 V at 2,000 rpm with headlights and A/C on; ripple measured at the battery terminals is 0.35 V AC. You connect an ammeter in series with the negative cable after the car has been locked for 45 minutes and read 340 mA. The service information gives a maximum key-off draw of 50 mA after 30 minutes. Pulling the 10 A interior lamp fuse drops the reading to 25 mA. The owner mentions an aftermarket dash camera and alarm fitted six months ago.
1. Explain why replacing the battery kept failing, and describe how to measure key-off draw correctly and safely.
Read the worked answer
Why battery replacement never worked: the battery was the victim, not the cause. A 340 mA draw that never sleeps is about 0.34 A × 24 h ≈ 8 Ah per day. On a taxi doing short trips, the alternator never gets enough running time to put that back, so the battery is repeatedly taken to a low state of charge and sulphates. The conductance test then says 'replace' - correctly, for the condition - but the new battery is subjected to the same abuse. You cannot test a battery fairly after a deep discharge; charge it fully first, then re-test.
Measuring draw correctly:
- Charge the battery and confirm it is good before drawing conclusions.
- Use a clamp meter around the negative cable where possible - it avoids breaking the circuit.
- If using a series ammeter, bridge the connection with a jumper or battery-saver tool while disconnecting, so modules are not woken or timers reset by the break. Breaking and reconnecting the battery can restart the 30–60 minute sleep countdown and make a faulty circuit look normal.
- Close and latch all doors and the boot/hatch, tape or latch the bonnet switch, lock the car with the remote, and keep the key fob away from the vehicle.
- Wait the full sleep period (30–60 minutes, sometimes longer) and watch the current fall and stabilise. Note both the final value and the time it took - a module that never sleeps is as much a fault as one drawing too much.
- Never crank or switch on loads with the meter in series; use the correct range and fuse.
2. Interpret the readings: is the charging system at fault, and what does the fuse-pull result tell you?
Read the worked answer
Charging system: not the primary fault. 13.9 V at idle and 14.2 V at 2,000 rpm with lights and A/C on is within normal regulated range; ripple of 0.35 V AC is below the commonly cited 0.5 V AC threshold, so a failed diode is unlikely. Neither figure explains a battery going flat overnight. Do not condemn the alternator on this evidence.
Battery: 12.42 V open-circuit after 12 hours is roughly 60–70% state of charge and, combined with the 'replace' result, confirms the battery has been damaged by repeated deep discharge - but the test was done on a discharged battery, so charge and retest before writing it off. If it fails after a full charge, it is condemned on its own merits, not as the cause of the complaint.
The real finding - 340 mA against a 50 mA limit: that is nearly seven times the specification, a genuine parasitic draw.
The fuse-pull result: dropping the interior lamp fuse takes the draw from 340 mA to 25 mA, so about 315 mA is flowing on that circuit. But the interior lamp circuit is a constant battery feed that aftermarket installers love, and the owner has a dash camera and alarm fitted six months ago - the same age as the start of the problem. Before condemning the circuit, disconnect the dash camera and the alarm one at a time and re-measure. A hardwired dash camera on a permanent feed drawing around 300 mA is the classic fit for this pattern.
Note the residual 25 mA is normal; that is module memory, not a fault.
3. What repair and verification steps do you complete, and what do you tell the customer to prevent a comeback?
Read the worked answer
Repair:
- Re-route the dash camera (and check the alarm) to an ignition-switched circuit using a proper fuse tap on an appropriate circuit - not a wire wrapped around a fuse blade, and not the interior lamp feed. Confirm the camera's own parking-mode setting is either off or set to a low-draw timeout the customer actually wants.
- Remove any Scotchlok or twisted-and-taped joints left by the installer; solder or crimp with proper terminals and protect with loom.
- Charge the battery fully and re-run the conductance test. Replace only if it fails when fully charged.
Verification:
- Repeat the key-off test with the same method: lock the car, wait the full sleep time, and record a stable reading of 50 mA or less. Print or write the before/after figures on the job order.
- Re-check charging output under load and confirm no warning lamp.
- Confirm the camera still works and that the alarm arms and disarms correctly - a 'fix' that disables a customer accessory creates a new complaint.
Tell the customer: the battery was being drained by an accessory, not by a faulty battery, so the previous replacements could never have fixed it. Explain what was re-wired and why, advise that short trips plus a heavy electrical load are hard on any battery, and ask them to return after a week so you can re-check the key-off draw and the battery state of charge. That follow-up is what separates a diagnosis from a guess.