Car battery

The starter battery supplies the heavy burst of electricity that turns the engine when you press the button or turn the key. It also feeds lights, locks and control units while the engine is stopped. Once the engine runs, the alternator supplies the electrical load and replenishes what starting took from the battery. This is why a car can start after a jump but fail again when parked: the first start proved that help was available, not that the battery can store enough charge.

The battery in an ordinary petrol or diesel car is separate from a hybrid or electric car's propulsion pack. Many electrified cars still carry a smaller low voltage battery for control systems and accessories. Do not treat a high voltage pack as a DIY version of the ordinary starter battery. The DVSA tells testers to assess accessible propulsion batteries visually and warns them about high voltage components.

Listen to the starting attempt before buying anything. Slow, laboured cranking and lights that dim sharply while the starter turns point towards a supply problem. Repeated clicking with no turn can mean the same thing, but it can also come from a loose connection. A single click with a healthy bright dashboard may lead the mechanic towards the starter instead. The dashboard battery lamp while driving raises a different question: is the alternator charging properly?

Ask the technician to test the battery itself and the car around it. A useful visit starts with the battery label, terminal condition and clamp, then a charge and health test using the correct setting for the battery's technology. VARTA says a conventional battery's open circuit voltage can show charge, but a start-stop battery also needs a health assessment. Its tester instructions call for the battery type and the cold cranking rating printed on the case, because the wrong setting can give a misleading result.

Low charge does not automatically mean worn-out cells. The garage can charge and retest the battery, then check the alternator and any parked electrical drain. Ask which result explains the recommendation.

Use the vehicle's handbook and the label on the fitted battery, then cross-check by registration and exact engine with a reputable supplier. Physical size, terminal position and hold-down arrangement must fit the tray. Electrical capacity and cold cranking specification must suit the car. A battery that drops into the tray but puts its terminals on the wrong side is still the wrong part. The garage should identify the original technology before pricing a substitute.

The choice matters particularly on start-stop cars. VARTA says an AGM should be replaced with AGM, while an EFB can be replaced with EFB or an appropriate AGM upgrade. A conventional battery in an EFB or AGM application may deteriorate rapidly. Ask the supplier to put the specified technology on the quote, not merely 'battery'. If a car has been modified or imported, a registration lookup alone may miss its actual equipment, so compare the label and manufacturer guidance too.

Diesel engines, larger vehicles and cars with substantial electrical equipment can require a different capacity or case size from a small petrol hatchback. A van may have a battery hidden beneath a seat or floor cover, so access becomes part of the labour. A motorcycle has its own battery size and installation guidance; the car price range on this page does not cover a motorcycle.

The repair cost entry shows £30 to £300 across the published UK examples and £169 to £339 for the franchised examples it records. Its labour time is 1 hour. The lower edge is a retailer's starting price for a battery with fitting included, so a larger, AGM or EFB battery will usually sit higher. Ask for an itemised total, including the battery, fitting, VAT, testing, coding where required, and old-battery disposal.

The battery itself changes the bill more than the act of lifting it out. AGM and EFB technology, physical size and cold cranking rating affect the part selected. A boot or under-seat installation may take longer than an exposed battery under the bonnet. Battery management registration can add equipment and time. It tells the car that a fresh unit has been fitted; on cars that require it, skipping that step can spoil charging behaviour or leave start-stop disabled.

On a simple older car, a competent owner may be able to change a conventional battery after checking the handbook. Switch the car off, keep the keys clear of it and identify the terminals before touching a spanner. VARTA's sequence is negative cable off first, then positive. Secure the replacement in its carrier, reconnect positive first and negative last, and confirm the clamp is tight. Keep the battery upright. Never let a metal tool bridge a terminal to the body or the other terminal.

That description is not a substitute for the vehicle's own procedure. Battery sensors, alarm settings, radio codes or window calibration can complicate disconnection. On a start-stop car, VARTA recommends a specialist because the battery management system may need programming. If the battery sits under trim or a seat, do not pull apart unfamiliar wiring to reach it. If there is any sign of leakage, leave the job to a workshop.

The DVSA manual's battery rule is section 4.13. It covers condition and security, not a published capacity or cold cranking threshold. A tired battery that starts the car and sits securely is not failed merely because a garage recommends changing it. If the tester sees a battery loose in its carrier but not likely to fall, the manual calls that a minor defect. A battery likely to fall or short circuit is a major defect, as is a leaking battery.

In Great Britain, GOV.UK lists a battery among items eligible for a free partial retest at the same test centre when you take the car back by the end of the next working day. If the test centre keeps the car for repair and retests it within 10 working days, the partial retest is also free. Take it away and return later within that 10 working day window and a partial retest charge may apply. Beyond the window, expect a full test. Roadworthiness still matters even when an old MOT certificate has time left.

If you think the failure is wrong, discuss it with the test centre before any repair. GOV.UK says to send the DVSA complaint form within 14 working days of a failed test in Great Britain, and to leave the vehicle unchanged until the appeal is considered. A successful appeal refunds the test fee. Do not repair first and then expect the original condition to be proved afterwards.

The section 4.13 table above belongs to the DVSA manual used in England, Scotland and Wales. Northern Ireland's Driver and Vehicle Agency runs its own test system. Its published inspection guidance also tells examiners to check battery condition and security, but you should use the DVA result and DVA booking service rather than assume the Great Britain retest timetable applies.

For a Northern Ireland failure, nidirect says to apply for a retest within 21 days of the original test and complete it within 60 days. The agency says a late application needs a full test fee. If you dispute the result, raise it with the centre manager first, then use DVA's appeal route before work changes the vehicle. These are Northern Ireland procedures, separate from the DVSA deadlines above.