Symptoms of a faulty alternator

The alternator charges the battery and powers the electronics while driving. When it fails, the car starts “living off the battery”, which quickly runs out. The symptoms are often confused with a dying battery, so it’s worth knowing them.

The most common symptoms

• The battery warning light on while driving
• Dimming or pulsing lights, especially under load
• Even a new battery goes flat quickly
• A squeal or whine from the alternator bearing
• The car stalls while driving or all the electrics weaken

How to confirm the diagnosis

The key is measuring the charging voltage: after starting the engine it should be about 13.8–14.4 V. A value close to the resting voltage (about 12.6 V or lower) means the alternator isn’t charging.

If a new battery dies after a few days, don’t replace it a second time — first check whether the alternator is charging at all.

Don’t confuse it with the battery

The symptoms of an alternator and a worn battery can be similar but differ in the details. A quick comparison lets you avoid replacing a good battery when the charging system is at fault.

What to do

If you suspect the alternator, don’t put off the diagnosis — driving “off the battery” ends in an immobilised car. When replacing the battery we check the charging voltage, and we carry out diagnostics as part of our other mobile services.