Weak alternator or weak battery — how to tell them apart?
The car stalls or starts poorly, but what’s to blame — the battery or the alternator? Find out how to tell them apart quickly by symptoms and a measurement.
Weak alternator or weak battery — how to tell them apart?
4 min readStarting and power problems are easily blamed on the battery, but the alternator can be at fault — the part that charges the battery while driving. Telling them apart saves money and nerves.
Symptoms of a weak battery
- A heavy start, especially after a longer stop
- Problems get worse in the cold
- After a jump-start the car drives normally
- The battery is already a few years old
Symptoms of a weak alternator
- The battery warning light on while driving
- Dimming lights under heavier load
- The car stalls while driving or the electrics weaken
- Even a new battery goes flat quickly
A simple measurement test
Measure the voltage at the terminals: at rest a healthy battery shows about 12.6 V, and after starting the engine the voltage should rise to 13.8–14.4 V. No such rise points to the alternator, not the battery.
Rule of thumb: if the voltage doesn’t rise after starting, the alternator is to blame. If it rises but the car still starts poorly — the battery.
The battery light isn’t about the battery
Despite the name, the light with the battery symbol signals a charging problem, so most often the alternator. We cover its meaning and other alternator symptoms separately.
When replacing the battery we check the charging voltage, so we don’t replace a good battery when the real problem is the alternator.
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