Car battery glossary

The terms you will hear from a technician or see on a battery label, in one or two sentences each.

AGM (absorbent glass mat): A sealed lead-acid battery in which the acid is absorbed in glass-fibre mats. It tolerates deep cycling, vibration and heat better than a flooded battery and is the standard type in start-stop Audis.
EFB (enhanced flooded battery): A strengthened flooded battery built for basic start-stop systems. It sits between a standard battery and AGM in durability and can be replaced by AGM.
Ah (ampere-hours): The battery's capacity: how much current it can supply over time. Audi batteries range from about 59 Ah in small cars to 110 Ah in large ones.
CCA (cold cranking amps): The current a battery can deliver for starting at low temperature. Used as the reference figure when a tester measures battery health.
BEM code: Battery energy management code. A string on original Audi and Volkswagen battery labels combining part number, maker and serial, entered into the car when the battery is replaced.
Battery coding (registration, adaptation): Telling the car's energy management that a new battery is fitted, with its capacity and technology, so charging and start-stop are managed correctly.
Energy management: The control system that monitors battery charge and health, adjusts alternator output and switches off comfort features when the battery is low.
Battery monitor sensor (J367): A sensor on the negative battery terminal that measures current, voltage and temperature for the energy management.
DIN size (H5, H6, H7, H8, H9): European case sizes. The number rises with length and capacity. The size decides whether a battery fits the tray and hold-down clamp.
State of charge: How full the battery is now, shown by resting voltage. About 12.7 volts is full, 12.2 volts is roughly half.
State of health: How much of its original capacity and cranking power a battery still has. Low health means replacement, however full it is.
Parasitic drain: Current the car draws while parked. A small draw is normal. A larger one, from a module that stays awake or an accessory, flattens batteries.
Memory keeper: A small power supply connected through the diagnostic port during a battery change so the car keeps its settings.
Start-stop: A system that switches the engine off when the car is stationary and restarts it on demand. It needs an AGM or EFB battery and is disabled by the car when the battery is weak.
Jump posts: Dedicated positive and earth points in the engine bay for boosting, used on cars whose battery is in the boot or under a seat.
48V mild hybrid: A system on larger recent Audis with a separate 48V lithium-ion battery and belt starter-generator. It works alongside, not instead of, the 12V battery.
Alternator: The engine-driven generator that charges the battery while driving. A failing alternator produces the same symptoms as a failing battery.
Vent tube: A small hose that carries gases from a battery mounted inside the cabin or boot to the outside. It must be reconnected after replacement.