What the battery does
In a gasoline or diesel car, the battery supplies a large burst of current to the starter motor. Once the engine runs, the alternator takes over, powering the electrics and recharging the battery. In hybrids and electric cars, a separate high-voltage battery drives the car, while a 12-volt battery still runs the lights, locks, screens and control computers.
Reading the label
| Term | What it means |
|---|---|
| Group size | A standard code for the case dimensions and terminal layout. The battery has to fit the tray and hold-down, and the cables have to reach. |
| Terminal position | Which side the positive and negative posts are on. Some group sizes come in mirror versions. |
| CCA (cold cranking amps) | How much current the battery can deliver in freezing conditions. Your manual or the old battery shows the minimum for your car. |
| Reserve capacity | Roughly how long the battery can run essential electrics if the charging system fails. |
| Type | Flooded lead-acid, enhanced flooded (EFB) or AGM. The car may require a specific type. |
| Date code | When the battery was made. A battery loses charge slowly while it sits on a shelf. |
Battery types and start-stop cars
Conventional flooded batteries are common in older cars. Cars with automatic start-stop systems, which shut the engine off at traffic lights, cycle the battery much harder and usually need an AGM or enhanced flooded battery. Fitting a conventional battery in a car designed for AGM can shorten the battery's life and cause start-stop or electrical faults. Use the type the vehicle maker specifies.
On some newer cars, a replacement battery has to be registered with the car's computer so the charging system treats it as new. A shop with the right scan tool can do this. Your manual or a dealer can tell you whether your car needs it.
Signs of a weak battery
- The engine cranks slowly, especially on cold mornings.
- A clicking sound and no start.
- Dim headlights or interior lights at startup.
- A battery or charging warning light. This can also point to an alternator problem.
- White or blue-green crust on the terminals.
- A swollen or cracked case, which means the battery should be replaced and handled with care.
Batteries often last somewhere around three to five years, though heat, short trips and long periods of sitting can shorten that. Many parts stores and repair shops will test a battery and the charging system for you.
Handling a battery safely
Car batteries contain sulfuric acid and can give off flammable hydrogen gas, especially while charging. They are also heavy. If you change one yourself:
- Wear eye protection and gloves, and remove metal jewelry.
- Turn the car off and remove the key or move the fob away.
- Disconnect the negative (black, minus) cable first, then the positive.
- Lift the battery straight up and keep it upright.
- Fit the new battery and secure the hold-down, then connect positive first and negative last.
- Take the old battery to a retailer or recycling center. Lead-acid batteries are widely recycled.
Some cars lose radio presets, window settings or clock time when the battery is disconnected. The manual explains any reset steps.
High voltage. Never touch the high-voltage battery or orange cables in a hybrid or electric car. Those systems carry dangerous voltages and are for trained technicians only.
Jump-starting
If you jump-start a car, follow the steps in the owner's manual for that car. Many vehicles have specific jump points under the hood that are separate from the battery. If the battery is frozen, cracked or leaking, don't jump it. Call for roadside help instead.