Dead Battery or Alternator

When your car won't start or keeps dying, it's either the battery or the alternator — here's how to tell.

The quickest way to distinguish between a dead battery and a bad alternator is a jump start test. If the car starts with a jump and runs fine for days, the battery was just discharged — possibly from lights left on — and may still be good. If the car starts with a jump but dies again within a day, the battery can't hold a charge and needs replacement. If the car dies while running after a jump, the alternator isn't producing enough voltage to keep the engine going.

A mechanic confirms the diagnosis with a multimeter. Battery voltage should read 12.4 to 12.7 volts with the engine off. With the engine running, the alternator should produce 13.5 to 14.5 volts. Anything below 13 volts running means the alternator isn't charging properly. A battery load test reveals whether the battery can deliver its rated cranking amps under load.

Don't guess and replace the wrong part. Ez Mobile Mechanic comes to your Jacksonville location and tests both the battery and alternator on the spot. We're a mobile mechanic in Jacksonville — we bring the testing equipment to you, no tow needed. If something needs replacing, we handle it right there. Call (904) 788-7272.