How to Tell If It's Your Alternator or Your Battery

Quick answer

If the car starts after a jump and keeps running, the battery is likely bad. If it dies again shortly after a jump, or the battery light comes on while driving, the alternator is likely failing. A running engine should read 13.8–14.7 volts at the battery; below 13 volts points to the alternator.

By the ASE-certified team at On-Site Mobile Mechanics of Dallas · 12 years repairing cars across Dallas–Fort Worth

How to Tell If It's Your Alternator or Your Battery

Signs of a bad battery

Slow crank in the morning
Clicking with dim dash lights
Swollen or leaking case
Battery older than 3 years in Texas

Signs of a bad alternator

Battery warning light while driving
Dimming or flickering headlights
Whining noise from the engine
Car stalls after a jump

How we test both

We load-test the battery and measure charging voltage and ripple in about 10 minutes, then replace only what failed.

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