Guides
Car Battery Glossary
The terms you will see on a battery label or hear from a technician, explained briefly.
- AGM battery
- Absorbent Glass Mat: a sealed lead-acid battery whose acid is held in glass-fibre mats instead of moving freely. It copes with start-stop driving and heavy electrical loads, and it should be replaced with another AGM, never a basic flooded battery.
- Alternator
- The engine-driven generator that recharges the battery and powers the car's electrics while the engine runs. If it undercharges or overcharges, even a new battery fails early, so its output should be checked before the battery is blamed.
- Ampere-hour (Ah)
- A measure of how much energy a battery stores: a 70 Ah battery can in theory deliver 3.5 amps for 20 hours. Fitting a lower figure than the carmaker specifies leaves less margin for short trips and heavy electrical loads.
- Auxiliary battery
- A second, smaller battery fitted to some cars to support start-stop, the gear selector or backup systems while the main battery cranks the engine. When it weakens, the dash often shows an auxiliary battery warning even though the car still starts.
- Battery Management System (BMS)
- The vehicle electronics that monitor the battery's voltage, current and temperature and adjust charging to suit. On cars with a BMS, the system needs to be told when a new battery is fitted, or it may charge it incorrectly.
- Battery registration
- Telling the car's computer, with a diagnostic tool, that a new battery has been fitted and what type and capacity it is. Many modern European cars and some others need it; skipping it can shorten the new battery's life.
- BCI group size
- A North American standard from Battery Council International that codes a battery's case dimensions and terminal layout, such as Group 35 or Group 65. It is common on American vehicles and shows whether a battery will physically fit the tray.
- Calcium battery
- A lead-acid battery whose plate grids are alloyed with calcium instead of antimony. It loses less water and self-discharges more slowly, which helps in heat, but it suffers when left deeply discharged and prefers a slightly higher charging voltage.
- CCA (Cold Cranking Amps)
- The current a 12 V battery can deliver for 30 seconds at minus 18 °C while holding at least 7.2 V. It is the standard measure of starting power; in a hot climate, matching the carmaker's figure matters more than exceeding it.
- Cell
- One of the six compartments inside a 12 V lead-acid battery, each producing about 2.1 V. If a single cell shorts or fails, the battery drops to around 10.5 V and cannot be brought back by charging.
- Charging voltage
- The voltage measured across the battery with the engine running, normally around 13.7 to 14.7 V. Much lower suggests undercharging; higher points to a regulator fault that can overheat the battery. Cars with smart alternators vary it on purpose.
- Corrosion
- The white, green or blue crust that builds up on battery terminals and clamps as acid vapour reacts with the metal. It adds resistance, causing slow cranking and poor charging, and the heat under the bonnet speeds it up.
- Deep discharge
- Draining a battery far below its normal working range, for example by leaving lights on or parking the car for weeks. Each deep discharge does lasting damage to a starter battery, and it is not treated as a manufacturing defect.
- DIN / EN size
- European standards for battery case sizes, terminal position and ratings, with codes such as LN2, LN3 or LN4. Most European cars and many others use them, with a ledge along the base that the hold-down clamp grips.
- EFB battery
- Enhanced Flooded Battery: a strengthened wet battery with tougher plates and a fabric layer that holds the active material in place. Built for entry-level start-stop cars, it handles many more engine starts than a standard flooded battery.
- Electrolyte
- The mix of sulphuric acid and water inside a lead-acid battery that carries the chemical reaction between the plates. High temperatures evaporate the water faster, one reason batteries wear out sooner here than in cooler countries.
- Flooded battery
- The conventional lead-acid battery, with plates sitting in free liquid electrolyte. It suits cars without start-stop and with modest electrical demands, but it handles vibration and repeated deep cycling less well than EFB or AGM types.
- IBS (Intelligent Battery Sensor)
- A small sensor clamped to the negative terminal that measures current, voltage and temperature and reports them to the car. It is how start-stop cars judge battery condition, and a faulty one can look like a failing battery.
- JIS size
- The Japanese Industrial Standard for battery dimensions and performance, with codes such as 55D23L or 80D26R. Most Japanese and Korean cars use these sizes, and the final letter shows which way round the terminals sit.
- Jump start
- Starting a car that has a flat battery by feeding power from a booster pack or another vehicle. It gets you moving but does not repair the battery, so a test afterwards shows whether it will hold charge.
- Load test
- A test that measures how well a battery holds its voltage while delivering a heavy current, as it does when cranking. It exposes weak batteries that show a normal voltage at rest, which a simple voltmeter check would miss.
- Low-voltage battery (EV)
- The 12 V battery in an electric or hybrid car that runs the locks, computers and the contactors that wake the high-voltage pack. If it goes flat the car will not switch on, even with a fully charged traction battery.
- Maintenance-free battery
- A sealed battery that never needs topping up with distilled water during its life. Almost every modern car battery is this type, yet it still depends on clean, tight terminals and a healthy charging system.
- Memory saver
- A small power supply connected to the car, usually through the diagnostic port, while the battery is swapped. It keeps the clock, radio presets and seat settings, and stops some cars asking for a security code afterwards.
- Parasitic drain
- Current that keeps flowing from the battery after the car is switched off and locked. A small draw is normal for alarms and memory, but a faulty module, light or accessory can flatten a healthy battery within days.
- Polarity
- Which terminal is positive and which is negative, and on which side of the battery each one sits. A battery with the opposite layout may not reach the cables, and connecting the leads the wrong way round can damage electronics.
- Reserve capacity (RC)
- The number of minutes a fully charged battery can supply 25 amps before its voltage falls to 10.5 V. It indicates how long the car's essential systems would keep running if the alternator stopped charging.
- Self-discharge
- The slow loss of charge every battery suffers while sitting unused, even when disconnected. Heat speeds it up considerably, so a car parked through a UAE summer can go flat much sooner than one kept somewhere cool.
- Start-stop system
- A fuel-saving feature that switches the engine off when the car is stationary and restarts it as you pull away. It demands an EFB or AGM battery, and it usually disables itself when the battery is weak.
- State of charge (SoC)
- How full a battery is, expressed as a percentage of its capacity. A rested 12 V battery reads about 12.6 V when full and around 12.2 V at half charge; repeated short trips rarely give it time to refill.
- State of health (SoH)
- How much of its original capacity and starting power a battery still has compared with new. A fully charged battery can still be in poor health, which is why a proper tester tells you more than a voltage reading.
- Sulphation
- Hard lead sulphate crystals that form on the plates when a battery sits partly or fully discharged. They block the chemical reaction and cut capacity, and they are a very common cause of early failure in rarely driven cars.
- Terminal
- The lead post or connection point where the car's cables attach to the battery. Terminals must be clean and tight; a loose clamp causes the same slow cranking and flickering electrics as a dying battery.
- Thermal runaway
- A rare fault in which a battery heats up while charging, draws more current as it warms, and heats further. It is usually linked to overcharging or an internal fault; a swollen, hot or sulphur-smelling battery should be checked without delay.
- Trickle charger
- A low-current charger that keeps a battery topped up while a car is parked for long periods. A smart maintainer that switches itself off is safer than a basic one, which can overcharge if left connected indefinitely.
- Voltage drop
- Voltage lost across a cable, connection or switch while current flows through it. Excess drop from corroded terminals or a weak earth strap starves the starter, so the car cranks slowly even with a good battery.
- Voltmeter reading
- The battery voltage shown by a multimeter across the terminals. After resting, about 12.6 V means fully charged, about 12.2 V half charged and below 12.0 V flat. It shows charge level, not the battery's overall condition.
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