Battery types
- 12V starter battery (SLI)
- The standard car battery. SLI stands for starting, lighting and ignition. It is a lead-acid battery made of six 2-volt cells, built to deliver a short, high-current burst to start the engine.
- Flooded (conventional) battery
- A lead-acid battery with liquid electrolyte. It suits cars without start-stop. Most modern flooded batteries are sealed and maintenance-free.
- SMF (sealed maintenance-free)
- A sealed flooded battery. Calcium grids and a lid with pressure-relief vents cut water loss so much that you never need to top it up. It is still a flooded battery, not an AGM.
- Calcium-calcium (Ca/Ca)
- A battery whose positive and negative grids are both made of a lead-calcium alloy. Calcium reduces gassing and water loss, which is what makes sealed, maintenance-free batteries possible.
- EFB (enhanced flooded battery)
- A flooded battery with design improvements that let it handle much more frequent charging and discharging. Used in cars with simpler start-stop systems. It can be upgraded to AGM but not downgraded to a conventional battery. AGM vs EFB
- AGM (absorbent glass mat)
- A sealed battery in which the electrolyte is held in glass-fibre mats and the gases recombine inside, so it never needs topping up. Built for advanced start-stop, brake energy recovery and high electrical loads. An AGM battery must be replaced with AGM. AGM vs EFB
- VRLA (valve-regulated lead-acid)
- A sealed lead-acid design with a valve that releases gas only under pressure. AGM car batteries are a type of VRLA battery.
- Lithium 12V battery (LFP)
- A lightweight low-voltage battery using lithium chemistry, often lithium iron phosphate (LFP). Fitted from the factory to some electric cars, such as newer Teslas and BYDs, and to some performance models. It is not simply swapped for a lead-acid battery.
- Auxiliary battery (hybrid 12V battery)
- The 12V battery in a hybrid, separate from the high-voltage drive battery. It powers the electronics and starts the hybrid system. Toyota-type hybrid auxiliary batteries have JIS codes starting with S and are not interchangeable with ordinary batteries. Toyota hybrid batteries
Ratings and sizes
- Ah (amp-hours, capacity)
- How much charge the battery stores: current multiplied by hours. A battery that delivers 5 amps for 20 hours is a 100 Ah battery. A replacement should have at least the original capacity.
- CCA (cold cranking amps)
- The current a new, fully charged battery can deliver at −18°C for 30 seconds while staying at or above 7.2 volts (BCI definition). The European EN standard measures it slightly differently, so figures are similar but not always identical. Choose at least the CCA your car was built with.
- RC (reserve capacity)
- The number of minutes a battery can supply a steady 25 amps at about 27°C before its voltage falls to 10.5 volts. A rough guide to how long the car could run its electrics if the alternator failed.
- Group size
- A standard size class that fixes the battery's maximum dimensions, terminal arrangement and hold-down features. Different systems exist: BCI numbers (such as 48), European DIN/EN codes (such as H6 or LN3, which match BCI 48 at 278 × 175 × 190 mm) and Japanese JIS codes.
- JIS battery code (for example 80D26L)
- The Japanese sizing code used on many Asian cars. In 80D26L, 80 is the performance rank, D the width and height class, 26 the length in centimetres, and L or R the terminal position. Makers describe L and R differently, so the safe rule is to match the old battery's layout.
- Terminal layout (polarity)
- Which side the positive and negative terminals sit on. The wrong layout means the cables won't reach, or reach the wrong terminal, so it must match the original.
- Hold-down
- The clamp or bracket that fixes the battery in its tray. Batteries have a ledge or recess designed for a specific hold-down type, which is one reason size matters.
Testing and health
- Resting (open-circuit) voltage
- The battery's voltage with the engine off and nothing running, measured a few hours after driving or charging. Varta puts a full battery at about 12.8 volts and recommends recharging below 12.4 volts.
- State of charge (SoC)
- How full the battery is right now. A flat battery has a low state of charge but may be perfectly healthy once recharged.
- State of health (SoH)
- How much of its original capability the battery has left. An old battery can be fully charged yet have poor health, which is why voltage alone doesn't tell you whether to replace it.
- Conductance test
- An electronic battery test that estimates health by comparing the battery's measured performance with its rated CCA. GS Yuasa notes a new battery typically reads 10 to 15% above its rating. Our battery test
- Load test
- A test that draws a heavy current, typically half the CCA rating, for 15 seconds. GS Yuasa's guidance is that a battery holding steadily above 9.6 volts passes. It should only be done on a well-charged battery.
- Sulphation
- The build-up of lead sulphate crystals on the plates when a battery sits discharged. Long periods flat, such as weeks parked during summer travel, can cause permanent capacity loss. Summer travel tips
- Date code
- The manufacturing date marked on the battery, in a format that varies by maker. Batteries slowly discharge on the shelf; Varta, for example, says its batteries should be fitted within 15 months of manufacture and above 12.2 volts.
Car systems
- Alternator (charging system)
- The engine-driven generator that recharges the battery and powers the electrics while the engine runs. With the engine running, most systems read about 13.7 to 14.7 volts at the battery. Battery or alternator?
- Smart alternator
- An alternator whose output the car varies deliberately, charging harder when braking and less when accelerating to save fuel. It means a single voltage reading isn't a reliable test.
- Start-stop system
- A system that switches the engine off when the car stops and restarts it when you move off. The battery works much harder, so start-stop cars need EFB or AGM batteries.
- Battery sensor (IBS)
- A sensor on the battery terminal that measures current, voltage and temperature so the car can manage charging. Many start-stop cars have one.
- Battery registration (coding)
- Telling the car's energy management system that a new battery has been fitted, using a diagnostic tool. Makes such as BMW, Mini, Audi, Volkswagen, Land Rover and Jaguar expect it. Volkswagen Group cars call the system BEM. BMW battery notes
- Parasitic drain (standby current)
- The small current a parked, locked car uses to keep the alarm, keyless entry and control units ready. ADAC measured 5 to 40 milliamps on properly locked cars; a fault or a door left ajar can make it much higher.
- Jump-start points
- Terminals under the bonnet that let you jump start a car whose battery is in the boot, under a seat or hard to reach. Carmakers such as Mercedes-Benz tell owners to use only these points. Jump starting safely
Tools and care
- Jump starter (booster pack)
- A portable battery pack with clamps that can start a car without a second vehicle. Useful in basement car parks where cars can't park nose-to-nose.
- Memory saver
- A small power supply connected while the battery is disconnected, so the car keeps settings such as radio presets and the clock.
- Smart charger / battery maintainer
- A charger that monitors the battery and keeps it topped up without overcharging. The best protection for a car left parked for weeks.
- Battery recycling
- Used car batteries contain lead and sulphuric acid. In Dubai they must go to licensed collectors and never into general waste. We take the old battery away for recycling when we fit a new one.
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