How do I tell if my car alternator is bad?

A bad alternator usually shows up as dim or flickering lights, a battery that keeps dying, sluggish accessories, a battery/ALT warning light, or whining/growling noises that change with engine speed.

Quick check with a multimeter:

1. Measure the battery with the engine off — about 12.6V is typical for a charged battery.
2. Start the engine and measure again — the voltage should rise if the alternator is charging.
3. Turn on headlights, AC, and radio — if voltage drops a lot and stays low, the charging system may be failing.

Other clue: If the car starts with a jump but soon loses power again, the alternator may be the issue.

Don’t disconnect the battery cable while the engine is running; that can damage modern electronics. If you’re still unsure, have the battery and charging system tested at a parts store or repair shop.