The jump-start test
Jump the car and remove the cables. If it dies right away or within minutes, the alternator isn't charging. If it keeps running and restarts later fine, the battery may just be weak or drained.
Voltage readings
- 12.6 V engine off — fully charged battery
- 12.2 V or less — discharged or weak
- 13.8–14.7 V engine running — alternator charging normally
- Under 13.5 V running — weak alternator
Symptom comparison
Battery: slow cranking, clicking, dies after sitting, swollen case. Alternator: dimming lights while driving, battery warning light, whining noise, electrical accessories acting up.
Sometimes it's both
A failing alternator can overwork and kill a new battery, and a dead battery can strain an alternator. Always test both.