Battery Flat After Two Nights
A three-year-old 1.2-litre petrol car with a 12 V, 45 Ah battery and a 70 A alternator is brought in with a flat battery after two nights of standing. The owner drives 8 km to work and back. The battery has rested for 12 hours: open-circuit voltage 12.4 V and electrolyte specific gravity 1.235 in all cells (specification 1.265). With all loads off at 2000 rpm the charging voltage is 13.9 V; specification is 14.1-14.8 V. With the key off and the car 'asleep' after 30 minutes, a clamp meter on the negative cable reads 780 mA, against a specification of under 50 mA. The workshop has already sold the customer a new battery, and the car went flat again in three days.
1. Using the battery capacity, estimate how long the car can stand with a 780 mA drain before it will not crank, and judge whether 780 mA explains 'flat after two nights'.
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Calculation. Use only about 50% of a lead-acid battery's rated capacity if it is to survive (cranking also needs a high current at low state of charge). Usable capacity = 45 x 0.5 = 22.5 Ah. Time = 22.5 Ah / 0.78 A = 28.8 hours.
A three-year-old battery may only deliver 60-70% of its rated capacity, so the realistic figure is closer to 17-20 hours - roughly one night plus part of the next day.
Judgement: yes, 780 mA explains a flat battery after two nights, especially with only 8 km trips that never fully recharge what the starter took. It is roughly 15 times the 50 mA limit.
This also explains why the new battery did not fix it: the drain is in the vehicle, not in the battery. Selling a battery without measuring the drain is the error - the fault will flatten any battery fitted. The old battery's 1.235 SG (about 75% charged) and 12.4 V show it was undercharged, not necessarily defective; it still needs a load test to decide whether to replace it as well.
2. Explain the systematic method to find the drain, and name two likely culprits with the current each would draw.
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Method:
- Charge the battery fully, then let the vehicle go to sleep (30-60 minutes; modules keep drawing after key-off).
- Clamp the meter around the negative cable - never put a normal multimeter in series on a high-current circuit; use a fused shunt/jumper if the meter must be in line.
- Pull one fuse at a time, watching the reading; a drop to normal identifies the circuit.
- For circuits without a fuse (alternator B+, starter), disconnect at the component and re-measure.
- Use millivolt drop across fuses to narrow down without pulling them.
Likely culprits and currents:
- Boot or glovebox lamp stuck on: 5 W / 12 V = 0.42 A.
- Aftermarket music system in standby: typically 0.3-0.8 A - this alone can produce the 780 mA.
- Alarm/immobiliser: 20-40 mA (normal, not the cause).
- Stuck relay or a wet connector leaking to earth: tens to hundreds of mA.
A boot lamp (0.42 A) plus a head-unit standby (0.35 A) gives 0.77 A - close to the measured 780 mA, which is why each circuit must be checked separately rather than assuming one fault.
3. Judge the 13.9 V reading: is the alternator good or bad, and what tests separate a weak alternator from a sulphated battery?
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13.9 V is below the 14.1-14.8 V specification at 2000 rpm with loads off, so charging is marginal. At that voltage a 45 Ah battery on 8 km trips may never return to full charge - the alternator must run long enough to replace starting current, and short trips do not allow it.
Tests, in order:
- Belt tension and condition; earth strap and engine-to-body earth (voltage drop under load should be under 0.3 V on both B+ and earth).
- Voltage drop from alternator B+ to battery positive, and check the sense wire.
- AC ripple at the battery: above about 0.5 V AC suggests a failed diode.
- Battery load test: half the CCA for 15 seconds, must stay above 9.6 V at 20 C; charge and check whether specific gravity rises towards 1.265. If it will not rise, the cells are sulphated or aged, so replace the battery.
- Only if the battery passes the load test and the voltage drop checks are clean do you condemn the alternator (regulator or diodes).
Judgement: the immediate causes are the 780 mA drain and the marginal charging voltage; the battery is a possible secondary casualty. Fix the drain, verify charging (aim for 14.2-14.5 V), and then decide on the battery from the load test rather than from age alone.