The alternator is the component that charges your car's battery and powers the electrical systems while the engine is running. Without a functioning alternator, your car will run on battery power alone — and eventually the battery will drain, the engine will stop and you will be stranded. In 2026, understanding the signs of alternator failure, the replacement costs involved and what to check before condemning the alternator could save you an unnecessary and expensive replacement.

How the Alternator Works

The alternator is a generator driven by the engine via a belt — typically the same serpentine belt that drives other ancillaries such as the power steering pump and air conditioning compressor. As the engine runs, the alternator generates electricity, which is used to charge the battery and power the car's electrical systems. The voltage regulator — often integrated into the alternator on modern cars — controls the charging voltage to prevent overcharging.

A healthy alternator produces around 13.8 to 14.4 volts when running. Below 13 volts, the battery will not be charged properly. Above 15 volts, the battery will be overcharged, which damages it.

Signs of Alternator Problems

Battery Warning Light

The battery warning light — shaped like a battery — illuminates when the alternator is not charging correctly. Unlike most warning lights, this one is informative rather than indicating an immediate danger: the car can run for a while on battery power alone, but will eventually stop. Drive directly to the nearest garage rather than continuing your journey unnecessarily.

Dim or Flickering Lights

As the alternator output drops, the electrical systems receive less power. Headlights dim, dashboard lights flicker and electrical accessories operate more slowly. The dimming becomes more pronounced as engine RPM drops, such as at idle.

Electrical Failures

A failing alternator may struggle to power all the car's electrical systems simultaneously. You may notice power windows moving more slowly, the infotainment system resetting, or central locking becoming erratic. These symptoms often appear before the battery warning light.

Battery Keeps Going Flat

If the battery keeps going flat despite being relatively new, the alternator may not be charging it properly. Before replacing the alternator, check the battery condition — a failing battery may appear to be a charging problem when it is actually a battery failure.

Whining or Grinding Noise

Mechanical problems with the alternator — worn bearings or a failing internal component — can produce whining, grinding or growling noises. A whining noise may indicate a slipping belt or bearing problem. A grinding noise suggests a seized bearing and requires immediate attention.

Alternator Replacement Cost 2026

  • Remanufactured alternator: GBP 80 to GBP 200 for common models
  • OEM quality replacement: GBP 150 to GBP 350
  • Main dealer replacement: GBP 300 to GBP 800 including parts and labour
  • Labour: 1 to 3 hours depending on vehicle accessibility

What to Check Before Replacing the Alternator

  • Battery condition: Test the battery with a multimeter or battery tester
  • Alternator output: Measure voltage at the battery terminals with the engine running — should be 13.8-14.4V
  • Belt condition: Check the drive belt for cracks, glazing and correct tension
  • Electrical connections: Check the alternator wiring connectors for corrosion and secure connection
  • Fuses: Check the fuse for the charging system