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alternator or battery? how to tell which one has failed

the two get confused constantly, because a failing alternator kills a good battery and then everybody blames the battery.

the difference

the battery starts the engine. the alternator runs everything once the engine is going, and recharges the battery while it does.

so: if the car struggles to start but runs fine once going, suspect the battery. if it starts fine and then dies while driving, suspect the alternator.

the clearer signs of an alternator

  • the battery light, which despite the picture is a charging system warning
  • headlights that dim at idle and brighten when you rev
  • a whine that rises and falls with engine speed
  • a new battery that went flat within weeks
  • the car dying while driving, and not restarting after a boost, or running only until the boost pack comes off
  • a hot smell, or a smell like scorched wiring, from a hard-working alternator

the test that settles it

a voltmeter across the battery terminals. around 12.6 volts with the engine off. between roughly 13.5 and 14.7 with it running. below that it is not charging. well above it, it is overcharging, which is the version that quietly destroys batteries and electronics.

we test the charging system whenever we replace a battery, for exactly that reason: fitting a new battery to a car with a bad alternator gets you a second dead battery and a repeat visit.

belts and pulleys, before you replace anything

an alternator that is not charging is sometimes an alternator that is not being turned. a glazed belt, a failed tensioner or a seized pulley all present the same way and cost far less to fix.

that gets checked first.