If the car won't start but runs fine after a jump and keeps running, the battery is the likely problem. If the car starts after a jump but dies again soon after, or the battery warning light comes on while driving, the alternator probably isn't charging. A quick voltage check with the engine off and running confirms which one it is.
What each part does
- The battery stores power to start the engine and run electronics when the engine is off.
- The alternator makes power while the engine runs, powering the car and recharging the battery.
A bad alternator drains the battery, so it often looks like a battery problem at first.
Signs it's the battery
- Slow cranking or rapid clicking when you try to start
- The car starts fine after a jump and stays running
- The battery is three to five years old or more
- Corrosion on the terminals, a swollen case or a rotten-egg smell
- It fails on the first cold morning
Signs it's the alternator
- The battery warning light comes on while driving
- Headlights and dash lights dim, then brighten with engine speed
- Electrical things act up: weak blower, flickering radio
- The car died while driving
- A new battery goes flat within days
- A whining or grinding noise from the front of the engine
- A burning rubber or electrical smell
The voltage check
With a simple multimeter:
Engine off
a healthy, fully charged battery reads about 12.6 volts. Around 12.2 is partly discharged; below 12 is low.
Engine running
the reading should rise to roughly 13.7 to 14.7 volts. That means the alternator is charging.
If it stays near 12 volts with the engine running
, or drops, the alternator likely isn't doing its job.
A mobile mechanic will also load-test the battery, which shows whether it can hold up under the heavy draw of starting even if the resting voltage looks fine.
Why it matters which one you replace
Replacing the battery when the alternator is bad means the new battery dies too. Replacing the alternator when the battery is bad wastes a few hundred dollars. Test first.
What replacement involves
- Battery: usually quick. See our guide to mobile battery replacement.
- Alternator: more labor, depending on where it sits in the engine. See mobile alternator replacement.
Both are standard mobile mechanic jobs done in a driveway or parking lot.
Frequently asked questions
Can a bad battery damage the alternator?
A badly failing battery can make the alternator work harder. It's one more reason to test both.
How long does an alternator last?
Often many years, sometimes the life of the car, but they do wear out.
Can I drive with a bad alternator?
Only briefly. The car runs on battery power until it's drained, then stalls, sometimes in traffic. Get it checked soon.
Still not sure?
Our guide to why your car won't start covers the other causes. Or have a mobile mechanic test both in your driveway: find one on the locations page.
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