Car battery glossary

AGM (Absorbent Glass Mat)

A sealed lead-acid battery in which the acid is soaked into glass-fibre mats between the plates. It handles frequent restarts and deep cycling, so it is fitted to start-stop cars and cars with heavy electrical loads.

Ah (amp-hour)

The battery's capacity, meaning how much charge it can store. A 70 Ah battery can in theory supply 3.5 amps for 20 hours.

Alternator

The engine-driven generator that powers the car's electrics and recharges the battery while the engine is running. A new battery will not survive long behind a failing alternator.

Auxiliary battery

A second, smaller battery that backs up particular systems such as start-stop or the gear selector. It is best known from Mercedes models and has its own warning message.

Battery management system (BMS)

The electronics that monitor the battery and decide how hard the alternator charges it. It has to be told when a new battery goes in.

Battery registration

Telling the car, through a diagnostic tool, that a new battery has been fitted so it resets its charging records. BMW and Mini need it, as do many recent Audi, Volkswagen and Land Rover models.

Battery sensor (IBS)

The Intelligent Battery Sensor, a small unit clamped to the negative terminal that measures current, voltage and temperature. It feeds the battery management system.

BCI group size

The North American size code from Battery Council International, such as 24F, 35 or 65. It fixes the case dimensions and where the terminals sit.

CCA (cold cranking amps)

The current a battery can deliver for 30 seconds at -18°C. It is the standard measure of starting power, and it still matters in a country that never sees frost.

Charging voltage

The voltage measured at the battery with the engine running, usually between about 13.7 and 14.7 volts. Cars with smart charging can read lower at times, but a steady figure well outside that range points to a charging fault.

Coding

Changing the battery type or capacity stored in the car's control unit. It is needed when the new battery differs from the original, for example moving from 80 Ah to 90 Ah or from flooded to AGM.

Deep discharge

Draining a battery far below its normal working range, as happens with lights left on or a car parked for weeks. Every deep discharge costs a starter battery some capacity for good.

DIN / EN size (H5, H6, H7, H8)

The European case-size standard used on most German and many Korean cars. The cases share the same width and get longer as the number rises, with capacity rising to match.

EFB (Enhanced Flooded Battery)

A flooded battery with reinforced plates that tolerates far more restarts than a standard one. It is used on simpler start-stop systems.

Electrolyte

The mix of sulphuric acid and water inside a lead-acid battery that lets current flow between the plates. Heat drives off the water, one reason batteries age faster in the UAE.

Flooded battery

The conventional lead-acid battery, with liquid electrolyte moving freely around the plates. It suits cars without start-stop.

JIS size (e.g. 55D23L)

The Japanese Industrial Standard code found on batteries for Japanese cars. In 55D23L, 55 is the performance rank, D the width and height class, 23 the length in centimetres and L the terminal position.

Jump pack

A portable booster battery that starts a car without needing a second vehicle. Professional units have protection against reversed clamps and voltage spikes.

Jump start

Starting a car with a flat battery by lending it power from a jump pack or another car. It gets the engine running but does not fix a worn-out battery.

Load test

A check of how well the battery holds its voltage while delivering starting current, done with a handheld tester. It separates a battery that is merely flat from one that is finished.

Low-voltage battery (EV)

The small battery in an electric or hybrid car that runs the locks, screens and computers and switches on the high-voltage pack. If it fails, the car will not wake up even with a full main battery.

Maintenance-free

A sealed design that never needs topping up with distilled water. Nearly every car battery sold today is maintenance-free.

Memory saver

A small power supply connected during a battery change so the car keeps its clock, radio presets, window settings and stored adaptations.

Parasitic drain

Current the car draws while it is switched off and locked. A small draw for the alarm and memory is normal, but a faulty module or accessory can flatten a battery overnight.

Polarity

Which side of the battery the positive and negative terminals sit on. Get the layout wrong and the cables will not reach, and connecting a battery the wrong way round can damage the electronics.

Reserve capacity

The number of minutes a fully charged battery can supply 25 amps before its voltage falls to 10.5 volts. It shows roughly how long the car's electrics could run if the alternator stopped.

Resting voltage

The battery's voltage after the car has sat switched off for a few hours. About 12.6 volts or more means fully charged, about 12.2 is roughly half, and anything under 12.0 is close to flat.

Self-discharge

The slow loss of charge in a battery with nothing connected to it. Heat speeds it up, which is why a car left over the summer can come back flat.

Start-stop

A system that switches the engine off when the car is stationary and restarts it as you lift off the brake. It multiplies the number of starts, so it needs an EFB or AGM battery.

Starter motor

The electric motor that turns the engine over to start it. It is the heaviest load the battery ever sees, several hundred amps for a second or two.

State of charge

How full the battery is right now, comparable to a fuel gauge. A low state of charge is cured by charging.

State of health

How much of its original performance the battery still has. A low state of health cannot be charged back and means the battery is due for replacement.

Sulphation

Hard lead sulphate crystals that build up on the plates when a battery sits partly discharged. It cuts capacity and is the usual end for batteries in cars that are rarely driven.

Terminal

The lead posts where the car's cables clamp on. Most are round tapered posts with the positive slightly thicker, while some Japanese cars use thinner posts.

Trickle charger

A small mains charger that feeds a low current to keep a parked battery topped up. Smart versions switch themselves on and off, which is useful for a car stored over the summer near a socket.

Vent tube

A small hose that carries gas from the battery to the outside of the car. It matters when the battery sits in the boot or cabin, and it has to be reconnected after a battery change.

More guides

• How long does a car battery last in the UAE?
• AGM vs EFB vs flooded: which battery does your car need?
• Signs your car battery is dying, and how to tell battery from alternator from starter
• Jump start or replace? How we decide at the car
• Does a new car battery need registration or coding?
• The 12V battery in electric and hybrid cars: why it fails in the UAE and how it is replaced
• Car battery warranty in the UAE: what it covers and how a claim works
• Leaving your car parked while you travel: how to keep the battery alive in Abu Dhabi
• Which car battery brand is best for the UAE?
• What affects car battery price in the UAE?