How a Car Battery Works

What the battery does

Your battery has one big job: give the starter enough power to crank the engine. It also runs the lights, locks and radio while the engine is off, and keeps the car's computers powered. Once the engine is running, the alternator takes over and recharges the battery for next time.

What's inside

A regular car battery is a box with six cells inside. Each cell has lead plates sitting in acid and makes about 2.1 volts. Six cells together give you about 12.6 volts when it's fully charged.

When you use power, a chemical reaction in those cells turns stored energy into electricity. When the alternator charges it, the reaction runs the other way and stores energy again.

The numbers on the label

• Volts: almost every car uses a 12-volt battery.
• CCA (cold cranking amps): how much power the battery can push out at 0°F. More CCA means a stronger start in the cold, which matters up here.
• Group size: the battery's physical size and where the posts are, so it fits your car.

Why batteries die in the cold

Cold slows down the chemical reaction inside the battery. According to AAA, a battery loses about 35% of its strength at 32°F and about 60% at 0°F. At the same time, a cold engine can take up to twice as much power to crank. That's why a battery that seemed fine in October can leave you stuck in January.

How long they last

AAA puts the average battery life at three to five years, and closer to three in cars with lots of electronics. If yours is past three years old, it's worth testing before winter.

Signs your battery is getting weak

• The engine cranks slower than normal, especially on cold mornings
• Rapid clicking when you try to start
• Dim or yellowish headlights
• The radio or clock resets, or the dash flickers when you start it
• You've needed a jump more than once lately
• White or green crust (corrosion) on the terminals
• A swollen, cracked or leaking case, or a rotten-egg smell

Simple checks you can do

• Look at it: check for corrosion, loose cables or a bulging case. Never jump a battery that's swollen, cracked or leaking.
• Headlight test: turn the headlights on with the engine off. Bright and white is a good sign. Dim or yellow means the battery is low.
• Voltmeter: with the car sitting for a few hours, 12.6 volts or a bit more is fully charged, about 12.4 is around 75%, about 12.2 is half, and 12.0 or lower is nearly dead.
• Load test: a battery can show good voltage and still fail when it has to do real work. A load test checks it under strain, and it's the best way to know for sure.

Battery keeps dying?

If a good battery keeps going dead, something else is usually to blame: an alternator that isn't charging, or something staying on and draining it while the car sits (called a parasitic drain). Lots of short trips can also keep a battery from ever fully recharging.

Related: How your alternator works · How to jump-start a car