The alternator or the battery: telling them apart
The car would not start this morning. Three voltage readings say which one to replace — and they cost nothing.
One symptom, two possible culprits, and they cost very different money. Ten minutes with the app — or a multimeter, if you have one — settles it.
Three readings
First: ignition on, engine off. A healthy battery shows 12.4–12.7 V. Below 12.2 it is discharged. Below 12.0 it is deeply discharged and may not survive.
Second: while cranking. Watch how far the voltage sags. Below 9–10 V the battery is not coping, even if the resting figure looked decent. This is the most honest of the three readings.
Third: engine running. Usually 13.8–14.6 V. Lower means the alternator is not charging. Above 15 V is the voltage regulator, and that is dangerous: overcharging boils the battery and kills it in weeks.
Reading them together
| What you see | Who is to blame |
|---|---|
| Low at rest, normal running | The battery |
| Normal at rest, low running | The alternator or the wiring |
| Low at rest and low running | The alternator has not charged for a while, and the battery went with it |
| Above 15 V running | The voltage regulator |
The load test
With the engine running, switch on the headlights, the heated screen and the blower at full. The voltage will sag — that is normal — but it should stay above 13 V. If it drops below, the alternator is short of capacity, and in winter that becomes a morning no-start.
What else happens
The belt. Stretched or oily, it slips under load: the voltage jumps and there may be a squeal.
Terminals and earth. Corroded contacts give exactly the picture of a dead alternator and are fixed with a wire brush in five minutes.
A drain while parked. Readings normal, yet the battery goes flat in two days — something is eating it. The first suspect is a cheap adapter left in the socket.
