Guides
Car battery glossary
Plain-language definitions of the terms you will see on a battery label or hear from a technician.
- AGM (Absorbent Glass Mat)
- A sealed lead-acid battery in which the acid is absorbed in glass fibre mats between the plates. It tolerates repeated discharging and recharges quickly, which is why cars with stop/start and heavy electrical loads require it.
- Alternator
- The engine-driven generator that powers the car and recharges the 12V battery while the engine is running. A failed alternator leaves the car running on battery power alone until the battery is flat.
- Amp hour (Ah)
- The unit of battery capacity, describing how much current a battery can supply over time. The replacement should match the capacity the car was designed for.
- Auxiliary battery
- A second, smaller 12V battery fitted to some cars to support stop/start or keep certain systems powered during an engine restart. Whether a car has one depends on the model, engine and year.
- Battery registration (BMS reset)
- The diagnostic step that tells the car a new battery has been fitted, so charging is adjusted for a fresh battery. Skipping it can cause stop/start faults and repeat battery warnings.
- BMS (Battery Monitoring System)
- The system that measures battery current, voltage and temperature through a sensor on the battery terminal, and uses the readings to control charging and features such as stop/start.
- CCA (Cold Cranking Amps)
- A rating of how much current a battery can deliver to start an engine, measured at low temperature under a standard test. A higher figure means more starting power.
- Charging voltage
- The voltage measured at the battery with the engine running, normally around 13.7 to 14.7V. A reading well outside that range points to a charging system fault.
- Deep discharge
- Draining a battery far below its normal working level, for example by leaving lights on or parking for weeks. Each deep discharge causes lasting damage to a starter battery.
- DIN size
- The European standard for battery case dimensions and terminal layout, used on most European cars. Japanese and Korean cars more often use JIS sizes.
- EFB (Enhanced Flooded Battery)
- A flooded battery with reinforced plates that withstands more charge cycles than a standard one. It is fitted to cars with basic stop/start systems.
- Electrolyte
- The mixture of sulphuric acid and water inside a lead-acid battery that reacts with the plates to store and release energy. Heat drives water out of it over time.
- Flooded battery
- The conventional lead-acid battery, with plates immersed in free liquid electrolyte. It suits cars without stop/start and is not a substitute for AGM or EFB.
- H7 / H8 / H9 sizes
- Common European battery case sizes, also written L4, L5 and L6, that share the same width and height and increase in length. Larger cases generally hold more capacity.
- Jump start
- Starting a car with a flat battery by supplying power from a jump pack or another vehicle. On modern cars the connections go to the designated jump points shown in the owner's handbook.
- Lead-acid battery
- The battery chemistry used for 12V car batteries, based on lead plates and sulphuric acid. Flooded, EFB and AGM are all lead-acid types.
- Load test
- A test that checks whether a battery can hold its voltage while delivering a heavy current. It reveals weak batteries that still show a normal voltage at rest.
- Maintenance-free
- A battery that is sealed and never needs topping up with water. It still ages and still needs testing.
- MHEV (48V mild hybrid)
- A car with a 48V lithium pack and starter-generator that assist the engine and recover braking energy. It still has a separate 12V battery for the normal electrics.
- Parasitic drain
- Current drawn from the battery after the car is switched off and locked. A small draw is normal, but a fault can flatten a healthy battery overnight.
- Reserve capacity (RC)
- The number of minutes a fully charged battery can supply a set current before its voltage drops too low. It indicates how long the car could run its essentials if charging stopped.
- Resting voltage
- Battery voltage measured with the engine off after the car has stood for a few hours. About 12.6V is healthy and under about 12.2V is low.
- Smart charging
- A charging strategy in which the car varies alternator output according to driving conditions and battery state, to save fuel. It makes a single voltage reading less conclusive.
- Start-stop
- A fuel-saving system that switches the engine off when the car is stationary and restarts it when you move off. It needs an AGM or EFB battery and is disabled automatically when the battery is weak.
- State of charge (SoC)
- How full a battery is at a given moment, expressed as a percentage. A low state of charge can be corrected by charging.
- State of health (SoH)
- How much of its original capacity and power a battery still has, expressed as a percentage. A low state of health cannot be charged back and means replacement.
- Sulphation
- The build-up of hard lead sulphate crystals on the plates of a battery left undercharged. It reduces capacity and is a common cause of early failure.
- Terminal corrosion
- White, green or blue crust on the battery posts and clamps. It adds resistance, which can cause slow cranking and poor charging.
- Trickle charger
- A low-current charger that keeps a parked car battery topped up. A smart type that suits the battery technology should be used, especially for AGM.
- VIN (Vehicle Identification Number)
- The unique 17-character code that identifies a car. It is used to confirm the exact battery specification for a given engine and model year.
- VRLA (Valve Regulated Lead Acid)
- A sealed lead-acid battery that releases gas only through pressure valves. AGM is the VRLA type used in cars.
Battery types
AGM, EFB or flooded: which battery your car needs
The technology has to match what the car was built with. This is the short version.
| Type | How it works | Fitted to | Good to know |
|---|---|---|---|
| AGM Absorbent Glass Mat | Electrolyte is held in glass-fibre mats, so the battery is sealed, vibration resistant and copes with deep, repeated discharges. | Range Rover Velar, Sport, Evoque and most premium cars with start-stop or heavy electronics. | Must be replaced with AGM. The car should be told a new battery is fitted. |
| EFB Enhanced Flooded Battery | A reinforced flooded design that handles more start cycles than a standard battery. | Entry-level start-stop cars, many Japanese and Korean models. | Can be upgraded to AGM, never downgraded to standard flooded. |
| Flooded Maintenance-free lead-acid | The conventional sealed calcium battery. Simple, proven and the most affordable. | Older cars and models without start-stop, including earlier Land Rovers. | Most affected by heat. Test yearly after the second summer. |
| 48V lithium Mild-hybrid pack | A separate lithium-ion pack that powers the mild-hybrid system. It works alongside the 12V battery, not instead of it. | Mild-hybrid Range Rover and Velar engines, and other MHEV cars. | Not a roadside item. We replace the 12V battery and leave the 48V system alone. |
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