Should I replace the battery or the alternator first?
Test the battery at rest
With the engine off and the car sitting for a while, a healthy 12-volt battery usually reads a little above 12.4 volts, with about 12.6 volts or higher when fully charged. A reading well below that suggests a weak or discharged battery. Surface charge from recent driving can skew the number, so let the car rest first.
A load test at an auto parts store or a shop can show whether the battery holds up under current draw. A battery that passes a resting test can still fail under load.
Test the charging system running
With the engine running, typical charging voltage at the battery falls around 13.5 to 14.7 volts. If the reading stays near battery voltage or drops below 13 volts with lights and accessories on, the alternator may not be keeping up. Check the serpentine belt condition and the battery terminals before buying parts. Clean the terminals first, because corrosion can produce misleading voltage readings and send you after the wrong part. Retest once the cleaning is done.
Deciding which to replace
If the battery fails a load test but the charging voltage is normal, replace the battery. If charging voltage is low and the battery recovers after a jump start, the alternator is the likely problem. Many shops will test both at no charge, which can save money.
Common mistakes
- Replacing a battery that is only discharged because the alternator is not charging it.
- Testing voltage immediately after a long drive and misreading the result.
