To tell whether the alternator or the battery is the problem, look at when it fails. A battery problem usually shows up as a slow crank or no start, especially after the car sits, and the car runs normally once started. An alternator problem usually shows up while driving: a battery warning light, dimming lights and electrical faults, and eventually the car dies while running. A voltmeter settles it in a few minutes.
Alternator or battery: how to tell
| Symptom | More likely |
|---|---|
| Slow crank or clicking after sitting overnight, runs fine once started | Battery; see car battery dies overnight |
| Starts with a jump and keeps running, then will not restart later | Battery not holding charge, or a drain |
| Starts with a jump, then dies soon after | Alternator not charging |
| Battery light on while driving, lights dimming | Alternator or its belt; see battery light on |
| Whining or grinding from the front of the engine | Alternator bearings |
Test with a voltmeter
- Engine off: a fully charged 12-volt battery reads about 12.6 volts; around 12.2 volts is roughly half charged.
- Engine running: charging voltage is usually around 13.5 to 14.5 volts; check your vehicle’s specification, as some modern systems vary it.
- Load: turn on headlights and the blower; voltage should stay in the charging range.
Low running voltage points to the alternator, belt or wiring; good running voltage with a battery that will not hold a charge points to the battery or a drain. Many parts stores will load-test both.
Don’t disconnect the battery while running
An old trick was to pull a battery cable with the engine running to see if it stalls. On modern vehicles, that can cause voltage spikes that damage electronics. Use a voltmeter instead.
When both are the problem
A failing alternator can repeatedly drain a battery, and a battery that has been run flat many times may be damaged even after the alternator is fixed. If you replace the alternator, have the battery tested too, and fully charge it with a charger rather than relying on the alternator to recover a deeply flat battery.
How old is the battery?
Batteries commonly last a few years, less in hot climates. An old battery is the more likely culprit when symptoms appear on cold mornings; see how long car batteries last.
Check the simple things first
- Corroded or loose battery terminals
- A slipping or worn serpentine belt; see serpentine belt
- Bad grounds; see bad ground symptoms
Related
See bad alternator symptoms, how long car batteries last, how to jump start a car and codes P0562 and P0563.
Trouble codes: P0560 System Voltage · P0603 Control Module KAM Error · P0685 ECM Power Relay Circuit · P0620 Generator Control Circuit · P0621 Generator Lamp Circuit · P0622 Generator Field Circuit · all OBD2 codes.
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