A car battery is the simplest battery most people own and the one with the most misleading label. Three numbers decide whether it will work: physical size, capacity and cold cranking current.
Reading the label
| Figure | Meaning | Why it matters |
|---|---|---|
| 12 V | Nominal voltage of six cells in series | Must match the vehicle system |
| Ah | Charge capacity, e.g. 70Ah | Reserve for lights and accessories with the engine off |
| CCA (EN) | Cold cranking amps at minus 18 degrees | The burst needed to turn a cold engine |
| RC | Reserve capacity in minutes | How long it can keep essentials running if the alternator fails |
| Size code, e.g. 096 | Case dimensions and terminal layout | If it does not fit, nothing else matters |
Capacity is not the same as starting power
A 100Ah leisure battery and a 100Ah car battery are not interchangeable. The leisure battery is built for slow, deep discharge. The car battery is built to deliver hundreds of amps for a few seconds and then be immediately recharged.
Fitting a leisure battery to a car gives poor starting current and a short life. Fitting a car battery to a caravan is worse: repeated deep discharge kills it quickly.
Flooded, EFB and AGM
Flooded lead-acid is the traditional battery with removable or sealed caps. EFB is an enhanced flooded battery for basic stop-start systems. AGM is an absorbent glass mat battery, more resistant to cycling and usually required for stronger stop-start and regenerative braking.
Modern cars with stop-start, smart charging or regenerative braking must use the technology specified by the manufacturer. A flooded battery in an AGM car can fail within months and may store less energy than the vehicle expects.
Do not swap battery technology to save money. If the car is specified with AGM, fit AGM. The charging strategy is designed around it, and the battery management system may reject or undercharge the wrong type.
Why a new battery sometimes needs coding
Many cars monitor the battery's age and charge behaviour. Fitting a new battery without telling the car can leave it charging on an old battery's strategy, which shortens the new battery's life.
If the handbook or the replacement battery's fitting notes mention registration or coding, get it done by a garage or with a suitable diagnostic tool.
Fitting safely
Car batteries produce hydrogen while charging, so keep sparks and naked flames away. Disconnect the negative terminal first and reconnect it last, which keeps the spanner away from the bodywork. Wear eye protection if there is any sign of corrosion.
Never disconnect the battery on a modern car while the engine is running, and check the handbook before doing any work that could affect the vehicle's electronics.
Check the date code when buying. A battery that has sat on a shelf for a year is nearer the end of its life than a fresh one, even though it looks new.
The short version
- Match the size code, the Ah rating and the CCA figure.
- Leisure and car batteries are built for completely different jobs.
- AGM-equipped cars must get an AGM replacement.
- Some vehicles need the new battery registered with the car.