The alternator is what keeps the battery charged while the engine runs. When it starts failing, the battery gets blamed, replaced, and then dies again, which is how one alternator fault turns into two battery purchases.
The clearest sign: a new battery that dies
If you fitted a battery recently and the car is flat again within days or weeks, stop buying batteries. Something is either draining it while the car sits or failing to charge it while you drive, and the alternator is the first place to look.
The battery warning light
The light shaped like a battery on the dashboard is not a battery light in any useful sense. It is a charging system light. It comes on when the system voltage is outside the expected range while the engine is running, which usually means the alternator, the belt driving it, or the wiring between them.
If it comes on while you are driving, you are running on battery alone. You have a limited window before the car stops, so head somewhere you can safely stop rather than somewhere far away.
Lights that dim and brighten with the engine
Headlights that go dim at idle and brighten when you rev, or dashboard lighting that pulses, point at unsteady output. So does a stereo that cuts out or a screen that resets while driving.
Noise and smell
A failing alternator bearing whines or grinds, and the pitch tracks engine speed. A slipping belt squeals, usually worst on a cold start or when you turn the wheel to full lock. A hot electrical smell is worth taking seriously and worth stopping for.
How it differs from a dying battery
A worn out battery struggles at the moment of starting and is fine once running. A failing alternator does the opposite: the car may start normally and then develop problems as you drive, because the battery is slowly being drained instead of topped up.
That difference is the quickest self diagnosis available to you. Hard to start, fine to drive, is usually the battery. Starts fine, gets worse as you drive, is usually charging.
What we can and cannot do
We test alternator output at idle and under load as part of every call, so we can tell you whether this is your problem and show you the numbers. Replacing an alternator is workshop work, and we will say so plainly rather than sell you a battery that will be flat again by Thursday.