Car battery glossary

AGM (absorbent glass mat)

A sealed lead-acid battery in which the acid is held in glass-fibre mats between the plates. It tolerates repeated deep discharges and accepts charge quickly, which is why start-stop, German and luxury cars use it. Read more →

Alternator

The engine-driven generator that powers the car's electrics and recharges the battery while you drive. If it fails, the car runs on the battery alone until the battery is flat. Read more →

Amp-hour (Ah)

The unit of battery capacity: how much current a battery can supply over time. A 70Ah battery can in theory deliver 3.5 amps for 20 hours. Read more →

Battery registration

Telling a car's energy management system that a new battery has been fitted, so it resets its record of battery age and charges the new one correctly. Required on many BMW, Audi, Volkswagen and other European models. Read more →

BMS (battery management system)

The electronics that monitor a car battery's voltage, current and temperature and adjust charging to match its condition. Read more →

Booster pack

A portable power unit used to jump start a car without a second vehicle. Professional units have fused, protected leads that are gentler on modern electronics than ordinary jump leads. Read more →

CCA (cold cranking amps)

The current a battery can deliver for 30 seconds at −18°C while staying above a set voltage. It measures starting power and should meet or exceed the car maker's figure. Read more →

Charging system

The alternator, voltage regulator, drive belt and wiring that keep the battery charged. A fault here drains even a brand-new battery. Read more →

Conductance test

A fast electronic test that estimates how much current a battery can still deliver without discharging it. It is what a handheld battery tester performs. Read more →

Date code

The manufacturing date stamped or stickered on a battery case. Batteries age on the shelf, so fresher stock lasts longer once fitted.

Deep discharge

Running a battery down to a low state of charge. Starter batteries are damaged by it; AGM and gel batteries tolerate it far better.

DIN battery size

The European case standard used by BMW, Mercedes-Benz, Audi, Volkswagen and many others. DIN batteries are low and long with recessed terminals and a clamping lip along the base. Read more →

EFB (enhanced flooded battery)

A flooded battery with reinforced plates that lasts roughly twice the charge cycles of a conventional one. Fitted to cars with basic start-stop systems. Read more →

Electrolyte

The mix of sulphuric acid and water inside a lead-acid battery that carries current between the plates. Heat evaporates the water over time.

Flooded battery (SLI)

The conventional car battery, with plates immersed in liquid electrolyte. SLI stands for starting, lighting and ignition. It suits cars without start-stop. Read more →

Gel battery

A sealed battery whose electrolyte is thickened into a gel. Built for slow, deep discharges — auxiliary and off-road setups — rather than for starting engines.

Grid corrosion

The gradual breakdown of the lead grids that hold a battery's active material. It is the main way batteries wear out, and heat speeds it up sharply. Read more →

IBS (intelligent battery sensor)

A small module on the negative battery terminal that reports current, voltage and temperature to the car. Cars fitted with one usually need battery registration. Read more →

JIS battery size

The Japanese case standard used on most Toyota, Nissan, Honda, Lexus and Mitsubishi cars. A code such as 55D23L gives performance rank, case size, length and terminal side. Read more →

Jump start

Starting a car with a flat battery by borrowing current from a booster pack or another vehicle. It gets the engine running but does not repair a worn battery. Read more →

Load test

A test that draws a heavy current from the battery and watches how far the voltage drops, showing whether it can still crank the engine. Read more →

Maintenance-free battery

A battery sealed so that it never needs topping up with water. Almost every car battery sold today is this type.

Parasitic drain

Current drawn from the battery while the car is switched off. A small draw is normal; a faulty module, light or accessory can flatten a battery overnight. Read more →

Reserve capacity (RC)

The number of minutes a fully charged battery can supply 25 amps before dropping to 10.5 volts — how long it could run the car's essentials if the alternator failed. Read more →

Resting voltage

The voltage across a battery after the car has been off for several hours. About 12.6 volts is fully charged, about 12.2 is half, and below 12.0 is effectively flat. Read more →

Self-discharge

The charge a battery loses on its own when nothing is connected. It rises with temperature, which is why parked cars go flat faster in summer.

Start-stop system

A fuel-saving system that switches the engine off at a standstill and restarts it when you release the brake. It needs an EFB or AGM battery. Read more →

State of charge (SoC)

How full a battery is, as a percentage. It changes from hour to hour and is recovered by charging. Read more →

State of health (SoH)

How much of its original capacity a battery still has. Unlike state of charge, it only goes down, and charging does not bring it back. Read more →

Sulfation

Hard lead-sulphate crystals that form on the plates when a battery sits partly discharged. They block the active surface and permanently cut capacity.

Terminal corrosion

The white, blue or green crust that builds up on battery posts and clamps. It adds resistance and can stop a car starting even when the battery is healthy.

Trickle charger

A low-current charger that keeps a parked car's battery topped up. Worth having if the car regularly stands for weeks at a time.

Last reviewed 6 October 2026 by the 800BATTERIES team.