Car battery glossary
32 terms you will hear from a technician or read on a battery label, each defined in a sentence or two.
- Card
- Cash
- 12V lithium battery
- A 12V lithium battery is a low-voltage lithium-ion battery used in place of lead-acid in some EVs and performance cars. It is lighter and longer-lived but has its own management electronics and cannot simply be swapped for a lead-acid battery.
- AGM (Absorbent Glass Mat)
- AGM is a sealed lead-acid battery in which the electrolyte is absorbed in glass-fibre mats between the plates. It tolerates repeated cycling and recharges quickly, so it is fitted to start-stop cars with regenerative braking and most premium models.
- Ah (ampere-hour)
- Ampere-hour (Ah) is the unit of battery capacity, showing how much current a battery can supply over time. A 70 Ah battery can in theory deliver 3.5 amps for 20 hours. A replacement should match the capacity the car maker specifies.
- Alternator
- The alternator is the engine-driven generator that powers the electrics and recharges the battery while the engine runs. A healthy one gives roughly 13.8 to 14.4 V at the battery. If it fails, the car runs only until the battery is empty.
- Auxiliary battery
- An auxiliary battery is a second, smaller 12V battery that supports specific systems such as start-stop restarts or comfort electronics. Some Mercedes-Benz, Volvo and Jeep models have one, and it can fail separately from the main battery.
- Battery coding
- Battery coding updates the control unit with the new battery specification, such as its capacity or technology, when that differs from the old one. It goes a step beyond registration, which only records that a like-for-like battery was fitted.
- Battery date code
- A battery date code is the stamp or sticker showing when a battery was made or shipped. Formats differ between manufacturers. It matters because batteries slowly lose charge in storage, so a recently produced battery is preferable.
- Battery registration
- Battery registration tells the car that a new battery has been fitted, so it clears the stored ageing data and charges accordingly. BMW, Mini, Audi, Volkswagen, Porsche, Land Rover, Volvo and many newer Ford and Mercedes-Benz models need it.
- BCI group size
- BCI group size is the Battery Council International numbering for battery case dimensions and terminal placement, such as Group 24, 35 or 65. American cars use it, and the group number must match for the battery to fit the tray.
- BMS (battery management system)
- A battery management system (BMS) is the software and sensors that track the charge, health and temperature of the 12V battery and adjust alternator output to suit. After a battery change it needs resetting so it stops treating the battery as aged.
- Booster pack
- A booster pack is a portable battery unit with clamps, used to jump-start a car without a second vehicle. Professional units include protection against reversed polarity and voltage spikes, which matters on cars with sensitive electronics.
- CCA (cold cranking amps)
- Cold cranking amps (CCA) is the current a 12V battery can deliver for 30 seconds at minus 18 degrees Celsius while staying above 7.2 V. It indicates starting power, and battery testers compare a measured value against this rating.
- Charging voltage
- Charging voltage is the voltage measured at the battery with the engine running. Roughly 13.8 to 14.4 V shows the alternator is charging. Cars with smart charging vary it deliberately, so readings are judged against the model.
- Conductance test
- A conductance test sends a small alternating signal through the battery and measures how easily current passes. The tester turns this into an estimate of cranking ability and compares it with the rated CCA, without discharging the battery.
- Deep discharge
- Deep discharge is when a battery is drained far below its normal working range, to the point where it cannot start the car. Starter batteries are not built for it, and each deep discharge permanently removes some capacity.
- DIN size
- DIN size is the European battery case standard, now defined under EN. Cases run from L1 to L6, also called H4 to H9, and are often named by capacity, such as DIN 70. European cars use these sizes.
- EFB (Enhanced Flooded Battery)
- EFB is a flooded lead-acid battery with reinforced plates that withstand more charge cycles than a conventional battery. It is used in entry-level start-stop cars and sits between a standard flooded battery and an AGM.
- Flooded battery
- A flooded battery is the conventional lead-acid car battery, with its plates immersed in liquid electrolyte. It suits cars without start-stop. It is the most common type but copes poorly with deep discharge and frequent restarts.
- IBS (intelligent battery sensor)
- An intelligent battery sensor (IBS) is a small module on the negative terminal that measures current, voltage and temperature and reports them to the energy management. Connecting a booster or charger past it gives the car false readings.
- JIS size code
- A JIS size code is the Japanese Industrial Standard label for a battery, such as 55D23L. Its characters give the performance rank, the case width and height, the length in centimetres and the terminal position. Japanese and Korean cars mostly use JIS sizes.
- Jump start
- A jump start lends power from a booster pack or another battery to start a car whose own battery is flat. It does not repair or recharge the battery, so the battery should be tested once the engine is running.
- Load test
- A load test draws a heavy current from the battery for several seconds while the voltage is watched. A healthy 12V battery stays at 9.6 V or above. A lower reading means it can no longer deliver starting power.
- Maintenance-free battery
- A maintenance-free battery is a sealed lead-acid battery that does not need topping up with distilled water. Its plate alloys keep water loss low over its service life. Most car batteries sold today are of this kind.
- Memory saver
- A memory saver is a small power supply connected to the car, usually through the OBD port, while the battery is removed. It keeps the control units powered so radio presets, the clock and window or seat settings are not lost.
- Parasitic drain
- Parasitic drain is current drawn from the battery while the car is switched off. A small draw for alarms and memory is normal. A faulty module, relay or accessory can draw far more and flatten a healthy battery within days.
- Reserve capacity (RC)
- Reserve capacity (RC) is the number of minutes a fully charged battery can supply 25 amps before its voltage drops to 10.5 V. It shows how long the electrics can keep running if the alternator stops charging.
- Resting voltage
- Resting voltage is the battery voltage with the engine off and no load, after it has settled for a few hours. About 12.6 to 12.8 V indicates a full charge, and below 12.4 V the battery should be recharged.
- Self-discharge
- Self-discharge is the slow loss of charge inside a battery even when nothing is connected to it. Heat speeds it up, so a car parked through a Gulf summer loses charge faster than one stored somewhere cool.
- Start-stop system
- A start-stop system switches the engine off when the car is stationary and restarts it when the driver moves off, to save fuel. It asks the battery for many more starts, so these cars need an EFB or AGM.
- Starter motor
- The starter motor is the electric motor that turns the engine over until it fires. It briefly draws one of the largest currents on the car. A single heavy click with a good battery usually points to a starter fault.
- Sulfation
- Sulfation is the build-up of hard lead sulfate crystals on the plates when a battery sits partly or fully discharged. The crystals cut capacity and resist charging. It is a main reason batteries left flat for long cannot be recovered.
- Terminal layout
- Terminal layout describes which side the positive and negative posts sit on, and what type they are. With a reversed layout the cables may not reach or could be connected the wrong way round, so it must match the original.
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