Car battery terms explained: a plain-language glossary
Short definitions of the car battery terms you will see on a label or a quote: Ah, CCA, AGM, EFB, DIN and JIS sizes, battery registration and more.
A car battery label tells you four things that matter for a replacement: capacity in amp-hours (Ah), cold cranking amps (CCA), the technology (standard, EFB or AGM) and the case size standard (DIN, JIS or BCI). The terms below explain those and the other words you are likely to hear when a technician tests or replaces your battery.
- 12-volt battery
- The low-voltage battery that starts a petrol or diesel engine and powers the car's electronics. Hybrids and electric cars also have one, separate from the high-voltage traction battery.
- AGM (Absorbent Glass Mat)
- A sealed lead-acid battery in which the electrolyte is held in glass-fibre mats. It withstands many more charge cycles than a standard battery and is required on many start-stop cars.
- Alternator
- The engine-driven generator that recharges the battery and supplies the car's electrical systems while the engine runs.
- Amp-hour (Ah)
- The battery's capacity: how much current it can supply over time. A replacement should match the original rating or be close to it.
- Auxiliary battery
- A second, smaller battery fitted to some cars to support start-stop, electronics or gear selection. It can fail separately from the main battery.
- Battery management system (BMS)
- Vehicle software that monitors the battery and controls how it is charged. It usually works with a sensor on the negative terminal.
- Battery registration
- Telling the car's battery management system that a new battery has been fitted, with its capacity and type, so charging is adjusted correctly.
- BCI group size
- The North American standard for battery case dimensions and terminal layout, common on American vehicles.
- Cold cranking amps (CCA)
- The current a battery can deliver to start the engine, measured under a standard cold test. Higher is stronger; meet or exceed the car's original figure.
- DIN size
- The European standard for battery case sizes, used by most European cars.
- EFB (Enhanced Flooded Battery)
- A strengthened version of the conventional flooded battery, designed for entry-level start-stop systems.
- Intelligent battery sensor (IBS)
- A sensor on the negative terminal that measures current, voltage and temperature for the battery management system.
- JIS size
- The Japanese Industrial Standard for battery sizes, used by most Japanese and many Korean cars. Codes look like 55D23L.
- Jump-start
- Starting a car with a discharged battery using power from a jump pack or another vehicle. It does not recharge or repair the battery.
- Maintenance-free (SMF)
- A sealed battery that does not need water added during its life. Most modern car batteries are of this type.
- Parasitic drain
- Current drawn from the battery while the car is parked and switched off. A small amount is normal; a fault or accessory can make it large enough to flatten the battery.
- Polarity / terminal layout
- Which side the positive and negative posts sit on. A battery with the wrong layout may not reach the car's cables safely.
- Reserve capacity (RC)
- How many minutes a fully charged battery can run essential loads if the charging system fails.
- Start-stop system
- A system that switches the engine off when the car is stationary and restarts it when you move off. It needs an EFB or AGM battery.
- State of charge (SoC)
- How full the battery is at the moment, as a percentage. A flat battery has a low state of charge but may still be healthy.
- State of health (SoH)
- How much of its original capacity and cranking ability the battery still has. A battery tester estimates this to decide whether replacement is due.
- Sulphation
- Hard lead-sulphate crystals that form on the plates when a battery sits discharged. It reduces capacity and is a common cause of early failure.
Quick answers
If your situation is different, tell us what happened when you last tried to start the car.
What do the numbers on a car battery mean?
The main ones are capacity in amp-hours (Ah), cold cranking amps (CCA) and a size code such as a DIN or JIS reference. Together with the technology (standard, EFB or AGM) they define which replacement fits.
Is a higher Ah battery always better?
Not necessarily. A slightly higher rating is usually acceptable if the battery physically fits, but cars with battery management may need the new capacity registered. Stay close to the original specification unless a technician advises otherwise.
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