Slow Cranking, Low Charging Voltage and a Parasitic Drain
A 1.6 L petrol sedan with a 12 V, 60 Ah battery (three years old) is in for slow cranking on cool mornings and a belt squeal on start-up. The owner's commute is 6 km each way. Measurements taken by the apprentice: battery open-circuit voltage after standing overnight 12.35 V; cranking voltage at the battery posts 9.4 V; electrolyte specific gravity 1.215–1.225 across the six cells; charging voltage 13.4 V at 2000 rpm with all loads off, and 13.1 V with headlights, blower and rear demister on; voltage drop from battery positive post to the starter terminal during cranking 0.45 V; engine block to battery negative 0.12 V; parasitic drain 0.28 A measured with a clamp meter 60 minutes after lock-up with everything off. The alternator belt is glazed and deflects 15 mm over its longest span against a 10 mm limit.
1. Assess the battery: its state of charge, its condition and whether 9.4 V during cranking is acceptable. Justify each conclusion with the numbers given.
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State of charge. A rested open-circuit voltage of 12.35 V corresponds to roughly 50–60% state of charge (about 12.6–12.7 V is fully charged, 12.2 V about 25%). The specific gravities of 1.215–1.225 against a fully charged figure of about 1.265 confirm the battery is significantly discharged. The spread across cells is only about 0.010, which is small, so there is no evidence of a shorted or dead cell (a spread of about 0.050, or one cell far below the rest, would point that way).
Cranking voltage. 9.4 V at the posts during cranking is below the typical minimum of about 9.6 V. On its own it is marginal rather than conclusive, because a discharged battery will always crank low; the test has to be repeated on a fully charged battery before the battery is condemned.
Condition. Three years old, a 6 km commute, a charging system that is not reaching normal voltage and a 0.28 A drain together mean this battery has lived its whole life part-charged, which sulphates the plates. The correct process is: charge it fully at the recommended rate, let it stand, recheck open-circuit voltage and specific gravity, then load-test at half the cold-cranking rating. Only if it fails that test is it genuinely unserviceable. Replacing it now, without fixing the charging and drain faults, would simply destroy the new battery in the same way.
2. Evaluate the charging circuit and the voltage-drop readings, then calculate what the parasitic drain does to this battery over a week of short trips. State which faults you would repair and in what order.
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Charging voltage. 13.4 V with no load is below the usual 13.8–14.4 V specification, and 13.1 V with accessories on is worse - at that voltage the battery never fully recharges. The glazed belt with 15 mm deflection against a 10 mm limit is slipping, which explains both the squeal and the low output; it must be corrected before the alternator is judged.
Voltage drops. 0.45 V on the positive side is high: the usual working limit is around 0.5 V total for the whole circuit, with each cable and connection expected to contribute only 0.2–0.3 V. 0.12 V on the earth side is acceptable (under about 0.2 V). So even a healthy alternator is losing voltage in the cables and connections before it reaches the battery, and the starter is losing cranking voltage the same way.
Drain calculation. 0.28 A × 24 h = 6.7 Ah per day. A 60 Ah battery at 50% charge has roughly 30 Ah of usable capacity, so about four to five days parked would flatten it to the point of not cranking. A weekend standing takes about 13 Ah, nearly half of that usable capacity, and 6 km trips return very little charge - so the car sits progressively further discharged each week. Acceptable drain is typically below 0.05 A.
Repair order: belt and tensioner first; then clean, tighten or replace the positive cable, terminals and earth straps until the drops are within limits; re-test charging (expect 13.9–14.4 V); fully charge and load-test the battery; then locate the drain. Fixing the drain before the charging circuit leaves the battery still undercharged.
3. Describe the correct method for locating the parasitic drain, and one common mistake that can damage the vehicle or the meter.
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Method:
- Fully charge the battery and confirm it holds charge.
- Switch everything off, remove the key, close all doors, the boot and the bonnet onto their latches (or tape the switches closed) so the vehicle believes it is locked.
- Lock the vehicle and wait 30–60 minutes for all modules to go to sleep; a reading taken too early simply shows normal wake-up current.
- Clamp an inductive low-current clamp around the negative cable and read the drain; compare it with the manufacturer's figure (typically below 0.03–0.05 A).
- If it is high, pull fuses one at a time, or bridge each fuse holder with a fused jumper and watch the meter, until the current collapses. Note which circuit it is and then diagnose that circuit with a wiring diagram.
- Remember that opening a door, pulling a fuse or operating a latch can wake modules; re-baseline and wait again before trusting the next reading.
Common mistake: disconnecting the battery or putting a multimeter in series on a high-current range to "see what is drawing current". Disconnecting the battery on a modern vehicle can lose adaptions and set fault codes, and reconnecting can cause a surge. Putting the meter in series and then operating something (opening a door, cranking, or letting a motor start) sends far more current than the meter's fuse can take, blowing the fuse or the meter, and arcing at the terminal can damage modules. Also never disconnect a battery with the engine running: the load dump can destroy the alternator's rectifier and the engine control unit. Use an inductive clamp, never a series meter, for drain testing.