There’s a particular sort of dread that sets in when you turn the key, or press the starter button, and your car doesn’t start. Slow cranking, a few pathetic clicks and flickering lights on the dashboard followed by silence. You’ve fallen prey to a dead car battery, and while changing it might sound like a job for a garage, in reality, it’s a relatively simple DIY task.
I’ve done this countless times, often on tired old ‘classics’ that have been unused for too long or on more modern cars that have quietly killed their batteries before their time thanks to poor weather or a persistent current drain from an immobiliser or alarm. Whatever the cause, replacing a car battery is usually a quick job, and one that can save you a surprising amount of money.
In this guide, I’ll explain how to change a car battery safely and correctly, what tools you’ll need, and how to make sure you buy the right replacement. It’s suitable for most petrol and diesel cars with a standard 12V battery. In addition, if you’re not sure whether the battery is the root cause, we have a detailed guide on how to know if you need a new car battery that’s worth checking before diving in.
How to know when it’s time to change a car battery
Before reaching for the tools, it’s worth confirming that your battery really is to blame. Here are the usual warning signs:
- The engine cranks slowly or doesn’t start at all
- Your dashboard shows a red battery warning light
- Electrical systems such as windows, wipers or the radio stop behaving as they should
- The battery case looks swollen or is leaking
- The car has not been used for a while, particularly in cold weather
What you’ll need to do the job
To change a car battery, you don’t need a huge tool kit or expensive equipment. Here’s what I typically use:
- A compatible new car battery (check your manual or current battery label)
- A socket set or spanners (10mm or 13mm are the most common sizes)
- Protective gloves and safety glasses
- A small wire brush for cleaning the terminals (optional really)
You may also want to take a photograph of the old battery in place before you start, to remind yourself how the terminals were connected. Sounds basic, but I’ve been there and some batteries aren’t nearly well enough marked.
You’ll also find our car battery buyer’s guide helpful when choosing a replacement, and we’ve listed some of the best car batteries you can buy to suit different budgets and specs. If you’re after recommendations for your toolkit, we’ve also tested the best car battery testers and car jump starter packs, both handy things to have around if your battery is misbehaving.
Step-by-step: how to change a car battery
- Turn everything off
Ensure the car’s ignition is off, keys are removed and all lights, fans and accessories are switched off. Unplug any devices from the 12V socket. For peace of mind, leave the driver’s window cranked open in case the car locks itself when you hook up a new battery (ask me how I know!). Also, keep the key and alarm fob in your pocket. - Think about the alarm
Also, in some cars you need check for an alarm battery back-up. To do this, leave the ignition switched to on (without the engine running) to drain down the back-up once you’ve disconnected the main battery. - Locate the battery
In most cars, the battery is under the bonnet. However, in some models – including a number of BMWs, Jaguars and Citroens – it may be in the boot or beneath a rear seat. In a Porsche Cayenne, it’s under the passenger seat, and in the old Mercedes-Benz A-Class, it’s in the driver’s footwell! Your owner’s manual will confirm the location – failing that, YouTube or Google will.
- Disconnect the negative terminal
Always start by removing the negative terminal, usually marked with a minus sign (-) and a (often, but not always) black cable. Use a spanner to loosen the clamp, then carefully lift it off with a twisting motion.
- Disconnect the positive terminal
Repeat the process on the positive terminal, marked with a plus sign (+) and a red cable. Be careful not to allow your tools to touch any metal bodywork. Also, be mindful of any additional wiring or fuseboxes that may be fitted to the positive terminal.
- Remove the battery clamp
The battery is likely held down by a bracket or strap, usually secured with one or two bolts. Loosen and remove these, then carefully lift out the old battery. These can sometimes be quite awkward to reach, so a socket and extension bar are very useful to have. Batteries are heavier than they look, so use both hands and lift with your knees – and don’t be shy about getting someone to give you a hand. - Clean the terminals and tray
Use a brush to clean any corrosion from the terminal connectors and check the tray beneath for rust or battery acid damage. This is also a good time to inspect the battery cable ends. - Install the new battery
Place the new battery in the same position, ensuring it sits firmly and the terminals are in the correct orientation. Reattach the clamp or bracket to hold it securely in place. - Reconnect the terminals – positive first
Connect the positive terminal first, then the negative. Tighten both clamps so they are snug, but do not over-tighten. I often find nipping them up so there’s resistance is more than enough, because battery terminals are more fragile than you might think. Be prepared for the alarm to go off, so have the fob close to hand. - Start the car
With the new battery installed, switch the ignition on, look for warning lights, and make sure everything presents as it should. Then start the car – it should fire up immediately. If not, double-check the connections or ensure the battery is properly charged, although in 40 years of tinkering, I’ve yet to find a new battery that isn’t.
Once you’ve fitted the new battery, it’s worth learning how to look after it. Our guides on how to charge a car battery and how to maintain your car battery will help make sure you don’t need to do this job again any time soon.
Choosing the right battery
When shopping for a new battery, it’s important to get one that matches your car’s specification. The main things to look out for are:
- Size and fitment: Batteries come in various sizes and terminal layouts, so make sure the new one matches the old unit or what’s listed in your handbook
- Ampere-hours (Ah): This measures the capacity. A higher Ah rating gives more power reserve, which is useful in cold weather or for short trips
- Cold cranking amps (CCA): This shows how well the battery performs when starting in low temperatures
- Battery type: Cars with start-stop systems usually require AGM or EFB batteries, which are more expensive but better suited to modern electrical demands
Most motor factors, battery retailers or online tools, such as eBay assured fit or Amazon Garage can help you choose the correct model.
Battery safety tips
Even though learning how to change a car battery is simple, safety is still important. Follow these tips:
- Always remove the negative terminal first, and reconnect it last
- Wear gloves and safety glasses if you’re not confident to protect against acid spills or corrosion
- Never smoke or use open flames near a battery
- Don’t use metal tools that might inadvertently bridge battery terminals, as this can cause sparks or damage
- Dispose of your old battery responsibly. Most local tips, and garages or parts suppliers (that you bought from), will accept them.
Is it worth changing a car battery yourself?
In most cases, I’d say so. If your car is a fairly straightforward petrol or diesel model, learning how to change a car battery can save you both time and money. While garages may charge £150 or more to supply and fit a battery, the part itself can often be bought for £50 to £120, depending on the spec.
If you’re dealing with a complex hybrid system, a luxury car with multiple ECUs, or you simply don’t feel confident, there’s no shame in leaving it to a professional. I would, to be honest. But for the majority of cars, DIYing a battery change remains one of the simplest – and rewarding – maintenance jobs you can do yourself.
Still have questions? Our car care advice section is full of useful tips for keeping your car running smoothly, including the best car trickle chargers to keep your battery topped up without overdoing it.
FAQs: What you need to know about changing a car battery
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Can I damage my car by changing the battery myself?
If you follow the correct procedure, changing a car battery is unlikely to cause any damage. The key is to disconnect and reconnect the terminals in the right order – always remove the negative terminal first and refit it last. Most modern cars are relatively robust, and a simple battery swap won’t upset things unless you reverse the polarity or short a circuit accidentally.
However, in newer vehicles with advanced electronics, you might experience warning lights or temporarily lose convenience settings like radio presets or electric window calibration. Also take a note of your radio code (if you have one). These often reset automatically after a short drive. If in doubt, consult your handbook or use a memory saver tool to preserve your car’s settings. -
How long does a car battery last?
Most car batteries last between three and five years, depending on how the car is used and the quality of the battery itself. Batteries deteriorate gradually over time and are affected by factors such as cold weather, short journeys, and infrequent use – all of which reduce their ability to hold charge and start the engine effectively.
If you often drive short distances, your alternator may not get a chance to fully recharge the battery, which can accelerate wear. A regular maintenance charger, or occasionally taking the car on a longer run, can help extend battery life. But once the battery struggles to start the car consistently, it’s usually time to change it. -
What happens if I connect the battery terminals the wrong way round?
Connecting a battery the wrong way round is a serious mistake and can cause instant electrical damage. Reversing the polarity can blow fuses, damage control units and even fry sensitive electronics like ECUs or infotainment systems. On older cars, you might get away with a spark and a blown fuse, but on modern vehicles, the repair bill could be considerable.
To avoid this, always double-check the battery’s terminal markings before connecting it. The positive terminal will usually have a red cover or marking and the negative a black one. If in doubt, refer to your vehicle handbook or take a photo of the old battery before removal. -
Do I need to reprogram anything after changing a car battery?
In most cases, no specialist reprogramming is required after changing a car battery. The car will often relearn key settings such as idle speed, climate preferences and radio stations after a short period of driving. Some cars may initially idle erratically or trigger warning lights, but these usually resolve without intervention.
However, certain models may lose features like electric window one-touch operation or steering angle sensors, which might need manual recalibration. This is especially common in cars with stop-start systems, advanced driver assistance, or infotainment setups. If in doubt, check your handbook or speak to a technician before starting the job. -
What’s the best battery charger to maintain my car battery?
A smart trickle charger is a good investment if you don’t drive regularly or are storing a car over winter. These chargers monitor the state of the battery and top it up only when needed, helping extend its life without risk of overcharging. Brands like CTEK and Ring Automotive offer well-reviewed models that are easy to use and compatible with most 12V lead-acid batteries.
Look for a charger with built-in protection against overcharging and reverse polarity. If your car has a stop-start system or uses an AGM battery, make sure your charger supports that battery type. Connecting the charger is usually straightforward, and it’s a good way to avoid having to change the car battery prematurely.
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