Two Alternators Later, the Battery Is Still Flat
A 2015 1.6 diesel hatchback comes in with the battery flat every second morning and headlamps that dim at idle and brighten when the engine is revved. The battery is a 12 V, 70 Ah, 640 CCA lead-acid unit, 8 months old, and reads 12.1 V after standing overnight. The alternator is rated 90 A and has a remote voltage-sense wire connected at the battery positive post. At 2000 rpm with headlamps and blower on, the following is measured: alternator B+ terminal 15.0 V, battery posts 13.8 V, voltage drop along the B+ cable 1.2 V, charging current 55 A (clamp meter), and AC ripple at the battery 0.12 V AC. The workshop has already fitted two exchange alternators to this vehicle for the same complaint, and the parts supplier will not accept a third warranty claim without test evidence.
1. Is the alternator at fault? Answer with the numbers, including what the 1.2 V drop is doing to the system.
Read the worked answer
No - the alternator is not the faulty part, and the numbers say so.
- 15.0 V at the alternator B+ with the regulator commanded to bring the remote sense point at the battery to about 14.2–14.8 V means the regulator is saturated at its upper limit trying to overcome the cable loss.
- 1.2 V drop at 55 A = 66 W of heat being dissipated in the charging cable and its connections. That is a fire risk (and a classic cause of melted fuse-box terminals) as well as a charging fault.
- 13.8 V at the battery is only just above float; with the lamps and blower on and short-trip use, the battery never recovers. 12.1 V overnight is roughly 50 % state of charge, which matches 'flat every second morning'.
The alternator is doing exactly what a healthy alternator does in a circuit with excessive resistance. Two exchange units behaved identically because the fault was never in the alternator - it is in the B+ cable, its crimps, the fuse-box terminal or the ground path. Any warranty claim should be withdrawn until a voltage-drop test has been done.
2. Describe the voltage-drop test that will localise the bad connection, with pass criteria, and explain why an ohmmeter is useless here.
Read the worked answer
Set-up: engine running at ~2000 rpm, headlamps and blower on to load the circuit to 40–60 A (or use a carbon-pile/electronic load). Keep the meter on the DC volts range and measure across each segment while current flows.
Sequence (positive side):
- Alternator B+ post → battery positive post: total should be ≤0.5 V for a charging circuit (many manufacturers specify ≤0.3 V). You have 1.2 V - fail.
- Alternator B+ post → cable end at the alternator, then cable end → fuse-box/terminal stud, then terminal stud → battery clamp → battery post. Each connection should show ≤0.1–0.2 V. The segment that eats the budget is the culprit - typically a corroded crimp or a loose terminal stud.
Negative side: engine block → battery negative post, expecting ≤0.1–0.2 V. A bad engine-to-body strap causes the same symptoms and must be ruled out.
Why not an ohmmeter: a 25 mm² cable with one strand left still reads ~0 Ω on a meter that pushes a milliamp, so the meter says 'good'. Resistance measurements are meaningless for high-current conductors - the circuit must be loaded and tested under its working current. Repair by replacing the cable or cutting back and re-crimping with a proper crimping tool and heat-shrink, then re-run the drop test and confirm battery voltage at 2000 rpm is within the manufacturer's window (about 13.9–14.8 V) with the load on.
3. What does the 0.12 V AC ripple tell you, and how would you explain the whole case to the customer?
Read the worked answer
Ripple: 0.12 V AC at the battery under load is normal (generally <0.5 V AC is acceptable; a failed rectifier diode typically produces >1 V AC ripple, a rhythmic flicker in the lamps, and often a whining alternator). So the rectifier is sound and the alternator's electrical output is not the problem - it reinforces that replacing the alternator was the wrong move.
Explanation for the customer:
- The alternator is producing full output - it is pushing 15.0 V at its own terminal.
- Only 13.8 V is arriving at the battery because 1.2 V is being lost in a corroded/loose connection in the main charging cable; at 55 A that is 66 W of heat in the loom.
- So the battery never fully charges, which is why it is flat every second morning and why two replacement alternators changed nothing.
- The same bad connection explains the lamps dimming at idle and brightening with revs, because the volt drop changes with current and temperature.
What must be said clearly: this is a fire risk as well as a charging fault, the car should not be driven until the cable/terminal has been replaced, and the repair should be verified with a loaded voltage-drop test plus a battery recharge and a capacity check before the car is released.