Alternator Guide
Last updated: 24 April 2026
The alternator converts mechanical energy from the engine into electrical energy to charge the battery and power the vehicle's electrical systems while running. A failing alternator leads to a flat battery, dim lights, and eventually a vehicle that won't start. Modern alternators are compact, efficient, and typically last 100,000-150,000 miles. We stock alternators from Bosch, Valeo, Denso, and other leading brands.
Signs of alternator failure
Alternator problems often mimic battery issues. A simple voltage test while the engine is running distinguishes the two: 13.5-14.5V indicates a healthy alternator; below 13V suggests failure.
- Battery warning light on the dashboard (most common early sign)
- Dim or flickering headlights and interior lights
- Electrical accessories (heated seats, radio) cutting out intermittently
- Battery goes flat despite being recently charged or replaced
- Whining or grinding noise from the alternator pulley area
Choosing the right alternator
Alternator output (measured in amps) must meet your vehicle's electrical demand. Vehicles with start-stop, heated windscreens, or powerful audio systems require higher-output units.
- Match the amperage rating to the original specification or higher
- Check the pulley type: free-wheel (overrunning) pulleys reduce belt vibration
- Ensure the plug connector and mounting points match your vehicle
- Remanufactured alternators offer OE quality at a lower price point
- Always replace the drive belt and tensioner if worn — a slipping belt reduces output
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Frequently Asked Questions
How long does an alternator last?
Typically 100,000-150,000 miles. Heavy electrical loads (heated seats, powerful audio systems) and short trips can reduce alternator life.
What are the signs of a failing alternator?
Dim headlights, battery warning light, electrical accessories cutting out, difficulty starting, and a dead battery despite being recently charged.
Alternator vs battery — how do I tell which has failed?
If the battery is new and keeps going flat, suspect the alternator. A voltage test while the engine is running should show 13.5-14.5V. Below 13V indicates alternator failure.
How much does alternator replacement cost?
The price of an alternator depends on your exact vehicle and the brand you choose. Enter your registration above to see parts confirmed to fit, then compare prices from the brands we stock on this page. Free UK delivery on orders over £99.
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