Change Car Battery
The battery is one of the most important wear parts on your car. Without it, there is no start, no electrical equipment, no journey. Understanding how it works, knowing how to spot the signs of a tired battery and choosing the right replacement will save you a lot of unpleasant surprises.
What does the battery do?
The battery has two essential jobs: powering the starter motor to crank the engine, and supplying the electrical equipment when the engine is off (lighting, radio, keeping the ECUs in standby).
Once the engine is running, the alternator takes over and supplies the energy the equipment needs. It also recharges the battery by converting the engine's mechanical energy into electrical energy stored chemically in the battery.
The three types of battery
Which type does your vehicle use?
Depending on the age and equipment level of your car (in particular whether or not it has a stop-start system), it will be fitted with one of these three types:
Flooded (standard) battery
- For older vehicles without stop-start
- Lead/antimony grids
- May need a regular check of the electrolyte level
- Top up with deionised water if needed
With maintenance
EFB battery
- An improved flooded battery (also called AFB or ECM)
- Lead/calcium grids, labyrinth lid
- Good resistance to charge/discharge cycling
- Suited to vehicles with simple stop-start
Maintenance-free
AGM battery
- A sealed VRLA battery with glass-fibre separators
- Very good resistance to vibration and cycling
- Suited to vehicles with stop-start and brake energy recovery
- Top-of-the-range technology
Maintenance-free
When should you change the battery?
The battery is a wear part: its replacement is rarely set out in the manufacturer's service schedule. It is its condition that should guide your decision. Its average life is 5 years, but this depends on many factors: how often it is discharged and recharged, the depth of discharge, extreme temperatures, and the general state of the vehicle's electrical system.
Change your battery at the first signs of weakness, in particular if starting becomes hesitant or slow.
How to choose the right battery
The specifications to match
Voltage and capacity
Match the original voltage (12 V on almost all vehicles), the capacity in amp-hours (Ah) and the cranking current in amps (A). You can fit a battery with a slightly higher capacity or cranking current without any problem.
Dimensions and terminal positions
The dimensions and the position of the + and - terminals must match your original battery exactly, so it fits the tray and the clamps without modification.
Keep to the original type
On a vehicle with an AGM battery, the replacement must be another AGM. On a vehicle with an EFB battery, you can fit an EFB or an AGM (better durability), but you will then need to register the new specification in the BMS with a diagnostic tool.
Where to buy?
From a motor factor, allow from £55 for a flooded battery to £250 for a top-of-the-range AGM. Online shops offer keen prices: stick to known brands and UK-based sellers to avoid counterfeits.
How to change the battery yourself
Changing a battery is rated difficulty 2/5: technically accessible, but it calls for a few precautions and two special tools: a memory-saver device and a multimeter.
The key steps of the replacement
Before you start
- Switch off the ignition and all electrical equipment. On versions with a keyless card, keep the card away from the vehicle.
- Connect the memory-saver device (to the cables, the accessory socket or the diagnostic socket) to avoid losing the settings.
- Close the doors and wait five minutes.
- Wear gloves and eye protection against the sulphuric acid.
Take out the old battery
- Move aside anything blocking access (plastic cover, fuse box, ECU).
- Disconnect the negative terminal first (the one connected to the bodywork), then the positive terminal, which you should insulate. Exception: if the positive terminal has a quick-release connector, start with that.
- Remove the clamp bolt and bracket, then lift out the battery.
Install the new battery
- Clean the tray and the terminals (old grease, sulphate).
- Position the new battery correctly and secure it lightly. Connect the vent tube if fitted.
- Reconnect the positive terminal then the negative terminal, tightening lightly.
After the replacement
- Switch on the ignition without starting and wait a few minutes for the systems to reset.
- Start the engine and check the equipment works. Re-initialise the one-touch windows and re-tune the radio stations.
- If any warning lights stay on (ESP, power steering), turn the wheel from lock to lock and drive for a few minutes.
- On stop-start vehicles from certain makes (Audi, BMW and others), re-register the battery in the BMS with a diagnostic tool.
Keeping the vehicle's settings
Disconnecting the battery resets the radio, the clock, the window settings and sometimes some engine parameters. To avoid this, use a memory-saver device that keeps 12 V in the circuit during the job. Allow from £10; choose a model protected against reverse connection and short circuits.
