How to change a car battery

Haynes How-To: change a car battery

Go flat out


What is a 12V car battery?

All cars have batteries. The ones in petrol and diesel cars are used to run the 12V electrical system (lights, stereo, heating, etc.) and generally use lead-acid chemistry. These batteries are designed to last for many years but they aren’t as energy dense as the lithium-ion batteries found in mobile phones, nor do they like to be fully drained and then fully recharged. For this reason they’re kept topped up via an alternator (or dynamo in classic cars).

Running your 12V battery down completely — for example by leaving the interior light switched on —  is actually quite damaging to its long-term health. You can revive it via jump leads, but its efficiency will have been reduced.

If you find your electric system is having problems it may be down to a sluggish battery, and it will need replacing. Fortunately, this is an easy task and only requires basic tools. In most cases this procedure will take only half and hour or so.

How will I know when to replace the battery?

It’ll be obvious when your battery is completely flat — the central locking may not work and you won’t be able to start the engine — but knowing when a battery is reaching the end of it life is trickier.

If you come to the car and find the battery is flat, you should still be able to start it up. Find the battery (usually under the bonnet, but check the car’s manual if you’re not sure) and use jump leads or a battery trickle-charger to send electrical charge from an external device directly into the 12V system (there should be instructions with the device).

You may also “bump start” a manual car. You do this by switching the ignition to “on”, depress the clutch and engage first gear, then have someone push the car forwards. Once you have sufficient motion, step off the clutch; this connects the moving wheels with the engine, turning it over and hopefully firing it up.

Once it’s started, take it for at least a 20-minute run to charge the battery. When you get back, turn the engine off and restart it the next morning. If it’s flat again you’ll know there is a problem somewhere.

The first thing to do is check the battery leads are secure and clean.

Next, check the battery’s state of charge by either looking at the indicator eye (not present on all batteries), sampling the electrolyte in a battery hydrometer (not possible on sealed batteries) or perform a battery load or drain test with a suitable meter.

If it’s low, you should try replacing the battery with a new one.

Why change your battery?

Aside from the obvious inconvenience of being stranded somewhere without leads or someone to give you a bump start, a dead battery may require you to reset a car’s systems, such as the throttle position sensor, audio system, clock and more. This is undoubtedly time-consuming and Haynes strongly recommends fitting a new battery as soon as possible.

How to change your 12V battery

Work time

  • Around 15-30 mins

Tools required

  • Socket set

Parts that you may need

  • New battery (£50-£200)
  • Battery tray
  • Battery retainer

Changing a battery: step-by-step

  1. Undo the clamp nut and remove the cable from the negative terminal first
  2. Do the same on the positive terminal
  3. Undo the battery hold-down clamp or bracket and lift out the battery. Be careful: it’s heavy
  4. Examine the battery tray and clean it if necessary
  5. Install the new battery, fitting the hold-down clamp(s) and securing the positive cable before the negative cable

Please note: all cars are different so if it is time to change your battery, you may want to look up instructions for your specific car at Haynes OnDemand. There may even be a video tutorial.