Two parts, two jobs
The battery's main job is starting the engine. Once it's running, the alternator takes over, powering everything electrical and recharging the battery. If the battery fails, the car won't start but will usually run fine after a jump. If the alternator fails, the car runs on battery power alone until the battery is drained, then stalls, and it'll keep killing batteries until the alternator is fixed.
Since a new battery won't fix a bad alternator and a new alternator won't fix a dead battery, knowing which one failed is worth a few minutes of testing.
+ 01 Clues
Symptoms that point each way
- Won't start, but runs fine after a jump: probably the battery
- Starts after a jump, dies again within minutes or miles: probably the alternator
- Battery warning light while driving: charging system, often the alternator
- Lights brighten and dim with engine speed: alternator
- Slow crank on cold mornings, fine once running: battery
- Brand-new battery dead within weeks: alternator or a parasitic drain
+ 02 The test
How the tech tells for sure
The tech load-tests the battery, then measures charging voltage with the engine running, at idle and with loads switched on, and checks for ripple from failing diodes. Healthy charging voltage is usually around 13.7 to 14.7 volts. If the battery is bad, it's replaced on the spot. If the alternator is bad, you'll know before spending anything, and the replacement is handled by Mobile Auto Repair.
Questions about battery or alternator
Can a bad alternator ruin a new battery?
Yes. An alternator that undercharges leaves the battery constantly drained, which damages it. One that overcharges can cook it.
Can I test the alternator myself?
With a multimeter you can check charging voltage at the battery with the engine running. It's a useful first check, but a proper test under load is more reliable.
Is the old trick of disconnecting the battery while running safe?
No. On modern cars, pulling a battery cable with the engine running can damage electronics. Use a meter or have it tested.