The basic idea
The battery starts the car, but it can't run it for long. Once the engine is running, the alternator makes the electricity for everything: lights, heater fan, radio and computers. It also recharges the battery for the next start.
How it makes power
The alternator is turned by a belt from the engine (the serpentine belt on most cars), so it only makes power while the engine runs. Inside are four main parts:
- Rotor: a spinning electromagnet. It gets a small amount of power through carbon brushes, which turns it into a magnet.
- Stator: coils of copper wire wrapped around the rotor that don't move. As the magnet spins past them, it creates electricity.
- Rectifier (diodes): the stator makes AC power, but your car runs on DC. The diodes work like one-way valves and turn it into DC.
- Voltage regulator: watches the system voltage and turns the rotor's magnet up or down to keep the output steady. On many newer cars, the engine computer controls this.
How much it puts out
With the engine running, a healthy charging system usually reads roughly 13.5 to 14.5 volts at the battery. With the engine off, a charged battery sits around 12.6. If the running number stays down near 12, the battery isn't being charged. If it's much over 15, it's overcharging, which can cook a battery.
Signs of a charging problem
- Battery or charging warning light on the dash
- Dim or flickering lights, especially at idle
- A battery that keeps dying, even a new one
- Whining or growling from the front of the engine
- Power windows, radio or other electronics acting strange
- A burning rubber or hot electrical smell
Despite its name, the battery light usually means the charging system isn't keeping up, not that the battery itself is bad.
Simple checks you can do
- Belt: with the engine off, look at the belt. If it's cracked, shiny, loose or missing, the alternator can't spin right. A squeal on startup can mean a slipping belt.
- Voltmeter: check the battery with the engine off, then running. The running number should be higher, around 13.5 to 14.5.
- No meter? Our alternator guide has a quick test you can do with just a wrench: try the wrench trick.
- Connections: loose or corroded battery cables can make a good alternator look bad.
It's not always the alternator
A blown fuse, a bad wire, a worn belt or a worn-out battery can all look like a bad alternator. That's why we test the whole charging system before replacing anything.
Related: Signs your alternator is going bad · How a car battery works
Battery light on? Northwoods tests and replaces alternators right where your car sits.