Battery or alternator? How to tell which one failed.

It's probably the battery if...

• The car starts fine after a jump and keeps running
• It's slow to crank on cold mornings
• The battery is four or more years old
• The problem started gradually

It's probably the alternator if...

• The car dies while driving
• The battery light is on with the engine running
• Headlights dim at idle and brighten when you rev
• A brand-new battery went dead within days

The voltage test

With a basic multimeter, a healthy battery reads about 12.6 volts with the engine off. With the engine running, a working alternator should bring that up to roughly 13.8–14.5 volts. If it stays near 12 volts while running, the alternator likely isn't charging.

Don't forget the simple stuff

A loose belt, corroded terminals or a bad ground wire can cause the same symptoms. That's why we test the whole charging system before recommending a part.