Car battery glossary

The 31 terms you are most likely to meet on a battery label, in a quote or on a test printout, each explained in a sentence or two.

AGM batteryAbsorbent glass mat

A sealed lead-acid battery in which the acid is soaked into glass-fibre mats between the plates. It delivers high current, accepts charge quickly and survives repeated draining, which is why start-stop and heavily equipped cars use it. It is the most expensive of the common types.AGM and start-stop battery replacement

Alternator

The engine-driven generator that powers the car's electrics and recharges the battery while the engine runs. A failing alternator produces the same symptoms as a failing battery, because the battery is drained and never refilled.Battery, alternator or starter: telling them apart

Amp-hour (Ah)Capacity

The unit for how much charge a battery stores. A 70 Ah battery can in principle supply 3.5 amps for 20 hours. Bigger engines and cars with more electronics need more amp-hours.How to read a car battery label

Auxiliary battery

A small second 12-volt battery fitted to some cars, many Mercedes-Benz models among them, to support start-stop and the electronics while the main battery handles starting. It has its own dashboard warning and is replaced separately.

Battery coding

Changing a car's stored battery settings after fitting a battery of a different capacity or type, for example AGM in place of EFB. It is done with a diagnostic tool and goes one step beyond registration.Battery registration and coding explained

Battery management system (BMS)Energy management

The software and sensors a modern car uses to monitor its battery's charge, health and temperature and to control how it is charged. It also decides when start-stop is allowed and when comfort features are switched off to protect a weak battery.

Battery recycling

Used lead-acid batteries are classed as hazardous waste because of the lead and acid they contain, and nearly all of that material can be recovered. In Dubai the transport and recycling of used batteries is regulated by Dubai Municipality. We take the old battery away at no charge and pass it to a licensed recycler.

Battery registration

Telling a car's computer that a new battery has been fitted, so that it clears the old battery's history and charges the new one correctly. BMW, Mini, Audi, Volkswagen, Jaguar and Land Rover models commonly require it.Battery registration and coding explained

Booster packJump pack, jump starter

A portable battery with clamps that supplies the burst of current needed to start a car with a flat battery. It does away with the need for a second car and jump leads, which makes it the practical tool in a basement car park.Car jump start service in Al Nahda

Cold cranking amps (CCA)

The current a battery can deliver for 30 seconds at minus 18°C while holding at least 7.2 volts. It is the standard measure of starting strength, and a battery tester judges a battery's health by comparing what it can deliver now with the CCA printed on its label.

Date code

The marking that shows when a battery was manufactured. Formats differ from brand to brand: some print the month and year, others stamp a short code. Batteries age on the shelf, so stock less than six months old is ideal.

Deep discharge

Running a battery down until it is almost or completely empty. A starting battery is not built for it, and each deep discharge removes some of its capacity for good, more so if it is left flat in the heat.Leaving a car parked while you travel

DIN size

The European family of battery cases. A DIN battery is low, has terminals recessed below its top surface and is held by a clamp gripping a lip at its base. Sizes are written as DIN55, DIN66 and so on, or as L2, L3 and L4.

EFB batteryEnhanced flooded battery

A flooded lead-acid battery with reinforced plates, built to survive far more engine restarts than a conventional one. Car makers fit it to simpler start-stop systems. It costs more than a flooded battery and less than AGM.AGM vs EFB vs flooded batteries

Electrolyte

The mixture of sulphuric acid and water inside a lead-acid battery, in which the chemical reaction that stores and releases energy takes place. Heat slowly drives water out of it, which is one reason batteries have shorter lives in hot climates.

Flooded batteryConventional or wet battery

The long-established car battery design, with lead plates standing in liquid acid. It is the least expensive type and suits cars without start-stop. Nearly all those sold today are sealed and maintenance-free.

Gel battery

A sealed battery whose acid is set into a gel. It is made for slow, deep discharge, such as running a fridge or a winch from a second battery, and is not intended for starting engines.

JIS size

The Japanese family of battery cases, used by most Japanese cars and many older Korean ones. A JIS battery is taller than a DIN one, its posts stand above the top surface and it is held by a frame across the top. Sizes are written as codes such as 55D23L.How to read a car battery label

Jump startBoost

Starting a car that has a flat battery by lending it current from a booster pack or from another car's battery. It gets the engine running but does not repair a battery that can no longer hold a charge.How to jump start a car safely

Lead-acid battery

The type of rechargeable battery used to start almost every petrol and diesel car, and to run the 12-volt systems of most hybrids. Flooded, EFB, AGM and gel batteries are all lead-acid designs.

Maintenance chargerTrickle charger, smart charger

A mains-powered charger that keeps a parked car's battery topped up without overcharging it. It is the reliable way to preserve a battery through a long absence, provided the parking bay has a socket.

Maintenance-free battery

A sealed battery that never needs topping up with water. The term describes the casing and says nothing about how long the battery will last.

Memory keeperMemory saver

A small power supply connected to the car while its battery is out, so that the radio, clock, seat and window settings are not lost during a replacement.

Parasitic drainStandby drain

The current a car uses while parked and locked, for the alarm, immobiliser, keyless entry and clock. A small drain is normal. A faulty module or a badly wired accessory can raise it enough to flatten a healthy battery overnight.

Reserve capacity (RC)

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

Resting voltageOpen-circuit voltage

A battery's voltage with the engine off, measured after the car has stood for a few hours. About 12.6 volts means fully charged, 12.2 volts about half and 12.0 volts or less flat. It shows how full a battery is, not how strong it is.Free car battery testing in Al Nahda

Start-stopIdle stop, auto stop-start

A system that switches the engine off when the car is stationary and restarts it as the driver moves off, to save fuel. It multiplies the number of starts a battery must deliver, so these cars need an EFB or AGM battery.

State of charge

How full a battery is, as a percentage. It changes from hour to hour with use and charging, and a low reading can be put right by recharging.

State of health

How much of its original strength a battery still has, as a percentage. It falls with age and heat and does not come back. A battery tester estimates it from the starting current the battery can still deliver.

Sulphation

The build-up of hard lead sulphate crystals on a battery's plates when it is left partly or fully discharged. It reduces capacity, and once well established it cannot be reversed by normal charging.

TerminalPost

One of the two connection points on a battery, marked positive (+) and negative (−). Their position and diameter differ between battery sizes, and a white, blue or green crust on them is corrosion that restricts the flow of current.