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What Does an Alternator Do in a Car?
It is the car's own power station: while the engine runs, the alternator feeds every electrical system and puts back the charge the starter motor took out. Here is how it works — and how it fails.
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Every electrical thing in a moving car — headlights, ignition, fuel pump, heater fan, heated screens, the ECU itself — runs on power the alternator generates. The battery's real jobs are narrower than most people think: it starts the engine and acts as a buffer. Once the engine is turning, the alternator takes over the electrical load and tops the battery back up, which is why a car can drive for hours without its battery going flat.
Understanding that split of labour explains most charging faults. A battery that keeps going flat despite being new usually points at the alternator or its drive belt, not the battery. A car that runs fine until the battery warning light appears, then slowly loses its electrics one by one, is running on stored charge alone. This guide covers what the alternator does, the voltage it should produce, and the order in which things fail when it stops doing it.
How an alternator generates power
The alternator is belt-driven from the engine's crankshaft. Inside, a spinning rotor creates a rotating magnetic field within fixed coils of wire (the stator), generating alternating current. A set of diodes — the rectifier pack — converts that AC into the direct current the car's systems use, and a voltage regulator holds the output steady regardless of engine speed or electrical load.
Output is substantial: modern alternators produce anywhere from around 70 to over 150 amps depending on the car, because heated windscreens, seats, steering and fans all draw heavily at once. The regulator targets a charging voltage of roughly 13.8 to 14.4 volts on a conventional car — measurably higher than the battery's resting 12.6 volts, which is exactly what pushes charge back into it.
Smart charging on modern cars
Many newer cars vary the alternator's output deliberately. The engine computer raises voltage when the battery needs charge or the car is slowing down, and lowers it during acceleration to save fuel — so a reading that moves between about 12.5 and 15 volts can be normal behaviour rather than a fault on those models. Stop-start cars go further, pairing the alternator with AGM or EFB batteries built for constant cycling, as explained in our guide to stop-start batteries.
That sophistication is why charging problems on modern cars need proper testing rather than a single voltage glance. A smart system can mask a weakening alternator for months, and a failing battery can make a healthy alternator look guilty. Measured readings under load, at the battery and at the alternator itself, separate the two.
What happens when an alternator starts to fail
Failure is usually gradual. Worn brushes, a tired regulator or failing diodes reduce output first under heavy load, so early clues appear on cold, wet nights: lights dimming at idle, a slow heater fan, warning lights flickering. The battery warning light is the dashboard's own report — it compares alternator output against battery voltage and lights up when the alternator falls behind.
Left alone, the car increasingly runs from the battery alone. Systems shut down in a rough priority order, the engine eventually cuts out, and the car will not restart — a sequence covered in detail in our guide to the signs of a failing alternator. At that point the battery is usually damaged by deep discharge too, turning one repair into two. Testing at the first warning light is the cheap moment to act.
Related guides
Guide
Stop-Start Batteries Explained: AGM vs EFB
Stop-start cars need special AGM or EFB batteries — fitting a standard one causes real problems. What the two types are, how they differ, and why registration matters.
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How Much Does a Car Electrician Cost in the UK?
What common car electrical jobs cost across the UK — batteries, alternators, starter motors, wiring and fault-finding — and what drives the price.
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Signs of a Failing Alternator: Symptoms to Spot
Dim lights, a battery warning light, strange noises and a car that dies after a jump start — the classic signs your alternator is failing, explained.
Read the guide →Frequently asked questions
Can a car run without an alternator?
Only briefly, on whatever charge remains in the battery. The engine, fuel pump and electronics all draw power continuously, so a healthy battery alone typically lasts somewhere between a few minutes and half an hour of driving before systems shut down and the engine cuts out — it is not a way to get home reliably.
What voltage should an alternator charge at?
On a conventional car, roughly 13.8 to 14.4 volts measured at the battery with the engine running. Readings near resting battery voltage (12.6 or below) suggest little or no charge, while sustained readings well above 15 volts point to a regulator fault. Smart-charge cars vary voltage deliberately, so readings need interpreting against the model.
Does the alternator charge the battery while idling?
Yes — the alternator produces charge at idle, though output is lower than at driving speeds. A short idle rarely recovers a deeply flattened battery; charging properly needs either a decent run or a mains charger. If the battery keeps going flat despite regular use, the charging system itself needs testing.
What is the difference between an alternator and a dynamo?
Both generate electricity from engine rotation, but alternators produce alternating current converted to DC by diodes, giving strong output even at idle. Dynamos generated DC directly, performed poorly at low revs, and disappeared from cars decades ago. Classic cars aside, every modern vehicle uses an alternator.
Will a bad alternator damage the battery?
Yes, in both directions of failure. Undercharging leaves the battery partially flat for long periods, which sulphates its plates and shortens its life sharply. Overcharging from a failed regulator boils off electrolyte and warps plates. That is why a charging fault found early often saves the battery as well as the alternator.
How long does an alternator usually last?
Most alternators last somewhere between 80,000 and 150,000 miles, though heat, oil leaks onto the unit, a slipping belt and heavy electrical loads shorten that. Because wear is gradual, output testing at the first symptom — dim lights, a warning light, slow cranking — catches failure before it strands the car.
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