What does the battery warning light mean?
The battery warning light means the car's charging system is not working correctly — not necessarily that the battery itself is flat. The most common cause is a failing alternator. When this light comes on while driving, the battery is running purely on its reserve charge. You typically have 20–50 minutes of driving before the car loses power. Finish your journey safely and have it checked immediately.
Likely causes
Failing or failed alternator
Most common cause — alternator no longer charges the battery
Worn or broken alternator belt
The belt drives the alternator — check for squealing noise
Battery failure
An old or failed battery can trigger the warning even with a working alternator
Loose or corroded battery terminals
Poor connection causes charging system errors
Wiring fault in charging circuit
Less common but worth checking if alternator and battery are fine
Repairs you may need
Typical cost range: £60–£450 — Battery replacement £80–200. Alternator replacement £180–450. Diagnostic first recommended.
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