What does Ah mean on a car battery?

Ampere-hours, the battery's capacity. Fit the capacity the manufacturer specifies. A much larger figure brings little benefit and may not fit.

What is CCA and how much do I need?

Cold cranking amps, the measure of starting power. Match or slightly exceed the figure on the original battery.

Registration tells the car a new battery of the same kind was fitted. Coding tells it the capacity or type has changed.

No. It has to fit the tray and suit the charging system. The right battery is the one that matches the specification.

Written by the Silicon Oasis Car Battery team · Updated 6 October 2026

Ah (ampere-hours) is capacity. CCA (cold cranking amps) is starting power.
AGM and EFB are battery types for start-stop cars.
DIN, JIS and BCI are sizing standards that decide whether a battery physically fits.
State of health, not voltage, is what tells you a battery is worn out.

Voltage (V): The electrical pressure of the battery. Car starter batteries are nominally 12V and read about 12.6V when fully charged and rested.
Ampere-hours (Ah): Capacity: how much charge the battery stores. A higher figure runs the car's electrics for longer with the engine off. Cars commonly take between 35Ah and 110Ah.
Cold cranking amps (CCA): The current a battery can deliver for 30 seconds at −18°C while staying above a set voltage. It is the standard measure of starting power, and testers use it as the reference even in hot climates.
Reserve capacity (RC): The number of minutes a fully charged battery can supply 25 amps before its voltage falls too low. It indicates how long the car could run if the alternator failed.
Date code: A stamp or sticker showing when the battery was made. Batteries age on the shelf, so a recent date matters.
Polarity or terminal layout: Which side the positive terminal is on. The wrong layout means the car's cables will not reach.

Lead-acid battery: The chemistry used by almost every 12V car battery: lead plates in sulphuric acid.
SLI battery: Starting, lighting, ignition: the conventional car battery, designed for a short burst of high current rather than deep discharge.
Flooded or wet-cell battery: A battery with liquid electrolyte. Most modern ones are sealed and maintenance-free.
Maintenance-free (MF): A sealed battery that never needs water added.
EFB (enhanced flooded battery): A strengthened flooded battery made for basic start-stop systems, with about twice the cycle life of a standard one.
AGM (absorbent glass mat): A sealed battery whose acid is held in glass-fibre mats. It handles frequent cycling and deep discharge and is required by many start-stop and premium cars.
VRLA: Valve-regulated lead-acid: the family of sealed batteries, including AGM, that vent gas only through a pressure valve.
Auxiliary battery: A second, smaller 12V battery. In some cars it supports start-stop. In hybrids and electric cars it powers the computers and locks.

DIN / EN size: The European sizing system, used by European cars and many others. Batteries have recessed terminals and a ledge at the base for the clamp.
JIS size: The Japanese Industrial Standard, used by most Japanese and many Korean cars. Codes such as 55D23L describe performance, size and terminal position.
BCI group size: The North American system, used by American cars. A group number such as 65 or 94R defines the case dimensions and terminal layout.
Hold-down: The clamp or bracket that secures the battery. A loose battery is damaged by vibration.
Vent tube: A small hose that carries gas from the battery to the outside. It is essential when the battery is mounted in the boot or cabin.

Alternator: The engine-driven generator that powers the car's electrics and recharges the battery while the engine runs.
Smart charging: A system that varies alternator output to save fuel, charging hardest when the car is slowing down. Charging voltage on these cars is not constant.
Start-stop: A system that switches the engine off when the car is stationary and restarts it automatically. It needs an EFB or AGM battery.
Battery management system (BMS): The electronics that monitor the battery's charge, temperature and age and control how it is charged.
Battery sensor (IBS): A small module on the negative terminal that measures current, voltage and temperature for the BMS.
Battery registration: Telling the BMS that a new battery of the same specification has been fitted, so it resets its ageing record.
Battery coding: Changing the car's settings when the new battery differs in capacity or type from the old one.
DC-DC converter: In hybrids and electric cars, the unit that charges the 12V battery from the high-voltage pack, in place of an alternator.
Memory saver: A small power supply connected during a battery change so the car's settings are not lost.

State of charge (SoC): How full the battery is, as a percentage. It is estimated from resting voltage.
State of health (SoH): How much of its original performance the battery retains. A battery can be fully charged and still have poor health.
Load test: A test that draws a heavy current and watches how far the voltage drops.
Conductance test: The method used by modern handheld testers. It passes a small signal through the battery to estimate cranking ability without discharging it.
Parasitic drain: Current drawn while the car is switched off and locked. A small amount is normal. Too much flattens the battery within days.
Sulfation: Hard lead-sulphate crystals that form on the plates of a battery left discharged. They permanently reduce capacity.
Deep discharge: Running a battery almost completely flat. Each occurrence shortens the life of a starter battery.
Jump start: Starting a car with a flat battery using power from a booster pack or another vehicle.
Booster pack: A portable battery, usually lithium, with clamps and protective electronics, used to jump start a car without a second vehicle.