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Safety first: preparing to change a car battery

Changing a car battery at home is a satisfying job that can save you a garage bill, but it pays to treat the battery with respect. A 12-volt battery can deliver hundreds of amps, and if it is shorted it can spark, overheat, or even explode. It also contains sulphuric acid, so avoid skin contact and never lean over it while connecting or disconnecting.

Park on level ground, apply the handbrake, switch off the engine and remove the key. On many modern cars, wait at least ten minutes after switching off before touching the battery. This lets control modules shut down and protects sensitive electronics.

Gather your tools before you start:

• Safety glasses and nitrile gloves
• A socket set or insulated spanners
• Terminal brush, hold-down tool and torch
• Memory saver for radio presets
• Rags, petroleum jelly and bicarbonate of soda

Never smoke or create a spark near the battery, and remove rings, watches and metal jewellery. If the battery is under a seat, in the boot, or behind a wheel arch, check you feel confident before starting.

Buying the right battery for your car

Car batteries are not one-size-fits-all. Check the physical size, terminal layout and hold-down type before you buy. The positive and negative posts may be on opposite sides, and cables may not reach if you fit the wrong one.

• Match the original dimensions and terminal positions, including positive left or right.
• Choose same or higher cold cranking amps (CCA) and ampere-hour (Ah) rating. Never go lower than specification.
• Stop-start cars almost certainly need an AGM or EFB battery, not standard lead-acid.
• Check the date code. A battery that has sat for years may already be past its best.

Many UK drivers make short trips, which is hard on batteries in winter. A higher CCA rating can help, but it must fit safely and suit the charging system. If your car has start-stop or smart charging, it may need a diagnostic tool to register the new battery. If that sounds daunting, buy from a garage that can fit and code it.

Disconnecting the old battery safely

Always disconnect the negative terminal first. It is usually marked NEG or -. If you undo positive first and your spanner touches the bodywork, you create a short circuit. Removing negative first breaks that return path, so an accidental touch is far less dangerous.

Loosen the negative clamp, twist gently, lift the cable aside. Now disconnect positive in the same way, and remove any covers.

Next, undo the hold-down clamp. Keep nuts and bolts safe. Batteries are heavy, often 15 to 20 kilograms, so lift with your legs. Use handles if fitted, keep it upright, and never tilt it towards yourself. If stuck, check for a hidden bolt.

Clean the tray and area. Neutralise corrosion with bicarbonate of soda paste, rinse and dry. Clean terminal clamps with a wire brush and check cables for cracked insulation.

Fitting the new battery

Place the new battery in the same orientation as the old one, with the positive and negative posts matching the original layout. Set it down flat and make sure it sits properly in the tray. Refit the hold-down clamp and tighten it enough to stop movement, but do not overtighten and crack the battery case.

Clean the terminals and clamps, then smear a thin layer of petroleum jelly or terminal grease on the posts. Connect the positive terminal first, then the negative terminal. This is the reverse order of removal, and it reduces the chance of a spark if your tool touches the bodywork.

Tighten the clamps firmly, but do not hammer them onto the posts. Give each cable a gentle tug to check it is secure, and make sure the cables are not stretched or trapped. Refit any covers and terminal caps.

Reconnecting, starting and first checks

Double-check that the positive cable is on the positive post and the negative cable is on the negative post. Then start the engine. It may idle a little higher for a few moments while the ECU relearns idle settings.

• The battery warning light should go out once the engine is running.
• Check the lights, radio, clock, central locking and electric windows.
• Some cars need a radio code, a window reset, or a short drive to reset the steering angle sensor.
• Stop-start may not work immediately. It can take a few drive cycles, or it may need battery registration.

If you have a multimeter, check the voltage. A healthy battery should read about 12.4 to 12.7 volts with the engine off. With the engine running, you should see around 13.5 to 14.5 volts. If the running voltage is low, the alternator or belt may need attention.

Aftercare, disposal and when to call a professional

Old batteries are hazardous waste. Take yours to a household waste recycling centre or a local motor factor that accepts used batteries. Never put it in the general rubbish, and keep it upright and away from children and pets until you can dispose of it properly.

Changing a battery is a good DIY job on many older cars, but call a professional if the battery is under a seat, in the boot, inside a wheel arch, or if your car is a hybrid or electric model with high-voltage components. Also seek help if the terminals are badly corroded, the clamp bolts will not budge, or the car needs battery registration. A few minutes of professional advice can prevent an expensive mistake.