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How Hard To Change Car Battery? Mistakes to avoid

Car batteries last anywhere from four to eight years, if your battery is older than that, congrats you’re doing well. Every car will need at least one new battery in its lifetime, knowing how to change a battery correctly is a skill worth having.

Changing a car battery is not difficult. However, in modern cars, it’s important to use a computer memory saver before removing the old battery. Many modern vehicles may also require battery coding. 

Seven steps to successful car battery replacement, include:

  1. Fit memory saving tool and remove car keys
  2. Remove ground terminal then positive terminal
  3. Remove battery clamp and battery
  4. Fit new battery and clamp
  5. Fit positive terminal, then ground terminal
  6. Remove memory saver tool
  7. Code battery

In this post, you’ll learn why we need to use a memory saver tool and what happens if you don’t. You’ll also learn how to change a car battery correctly and why we should code or register the new and what happens if we don’t.

What Is A Memory Saver Tool?


A memory saver tool also known as a KAM tool is a simple device used to supply power to a car’s computers while the battery is disconnected.

Why do we need to use it?

All modern car systems like engines, transmissions, HVAC, MMI, ABS brakes, restraint systems, etc. are all managed by computers, more commonly known as control units or controllers. Every system will employ its own dedicated control unit.

They use clever learning software that adapts to criteria like weather conditions, your driving style, vehicle component wear, and age.

Information collected by controllers is stored in the computer’s memory, disconnecting the car battery without first powering the controllers wipes their memory and causes drivability issues.

What happens if I don’t use a memory saver tool?

Vehicle specific scanner

The car may display issues like erratic engine idle, awkward transmission changes, ABS, traction control, warning lights illuminated, steering angle warning, erratic HVAC, window control, etc. 

Most of these minor issues self-repair by simply driving the vehicle but some have been known to persist until they have been calibrated by a scan tool. Usually means a visit to the main dealer.

Or you can buy your own scan tool that includes calibrating function and battery coding which you’ll learn about later. You can check out the full-function scan tool I use here on the Auto electrical repair tools page.

Tools You’ll Need To Replace Car Battery

Battery changing tools

Removing a car battery usually requires loosening or removing just three fasteners. The terminal fasteners (which are usually the same size) and the battery hold-down fastener.

Terminal fasteners – Just basic tools are needed: a selection of wrenches, 10 mm, 12 mm, and 13 mm are common sizes for the terminal fasteners. 

Battery hold down – Ratchet, long extension, and sockets are best. 10 mm, 12 mm, 13 mm are common sizes for hold-down fasteners.

Safety Kit You’ll Need when changing a car battery

Changing a battery isn’t without risk but if you follow the procedure it’s quite safe. You’ll need safety eyewear and gloves.

If you need tools, check out check my favorites on the Mechanics tools page, they’re all conveniently delivered by the nice people at Amazon.

Battery replacement personal injury risks include:

1 Explosion

The risk is low but battery acid is flammable and shorting the battery could ignite battery acid vapors. Removing the ground side terminal first and fitting last as per the guide will eliminate the risk of shorting.

2 Battery acid

The old battery may leak acid, use gloves when handling, and keep upright at all times.

3 Battery weight

Batteries are heavy, many times they’re fitting in awkward places. Accidentally dropping a battery on your toe or the car fender will cause damage.

1 Fit Memory Saving Tool

Fitting a memory saver tool also known as a KAM (Keep Alive Memory) tool is easy. A good tool will give the operator the option of connecting to another battery using crocodile clips or a 12v plug. 

As you are changing the battery out, it makes sense to have a donor car parked close enough to plug the tool into the donor car 12v socket. 

Now the tool is powered up, next we need to get that power into your car’s control modules. To do that the tool is equipped with an OBD plug that plugs directly into your car’s OBD socket located under the dashboard on the driver’s side. 


The tool illuminates to signify all is good. You’ll find my favorite memory saver tool here on the Auto electrical repair tools page.

Before removing the battery terminals, go ahead and remove the key from the ignition and place it in your pocket. Many vehicles lock the car automatically when the battery is reconnected.

2 Remove Negative Terminal First, Then Positive Terminal

Before removing the battery you’ll need to locate it. Most are under the hood, but others may be located in the trunk or elsewhere. If you can’t easily locate it, try the driver’s manual or google your model. Your battery may not be visible, but you’ll no doubt need to remove plastic covers to gain access to the battery. 

With the memory saver tool in place and working, the ignition key in your pocket, and the battery located, go ahead, locate and remove the black negative terminal first.

Which terminal is the negative terminal?

Remove ground battery terminal

The negative side terminal also known as the ground cable is identified by the negative minus symbol (-) on the battery casing. The ground cable’s outer insulation is colored black. Both are marked in the picture above.

Loosen the terminal fastener but no need to remove the fastener. Now twist the terminal left and right to help loosen it.

Tuck the terminal away from the battery and ensure it doesn’t make contact with the battery post.

Why remove the black negative terminal first?

Removing the negative terminal first reduces the risk of shorting the battery positive terminal by striking it accidentally with metal wrenches etc.

Now remove the red positive terminal

It’s identified by the plus symbol (+) and red cable insulation. The fastener size is usually identical to the negative terminal. Wiggle the terminal left to right to help loosen it and lift it clear.

3 Remove Battery Clamp and Battery

Go ahead and remove the battery vent hose (if fitted), it’s a small tube connected to the sidewall of the battery (pull to remove).

Battery vent hose

The battery hold-down clamp is as its name suggests, it holds the battery firmly in place. They tend to come in two main flavors, a discrete clamp at the foot of the battery and a bracket that stretches across the top of the battery. My car has both types, I’m so lucky!

Foot battery clamp

The clamp at the foot of the battery is the more challenging type to remove. You likely can’t get your hand in there. You’ll need a ratchet, long extension, and socket. A 12mm, 13mm, and 14mm are common fastener sizes for battery foot clamps.

Battery hold down clamp

The bracket across the top type is easiest to remove, one or two fasteners and it’s free.

Go ahead and lift the battery out, try and keep it upright. Dispose of the battery at a recycling facility or a parts store will accept it. Storing an old battery in a garage isn’t advised, battery acid is flammable.

4 Fit New Battery and Clamp

Check the battery tray area for signs of corrosion, when batteries leak acid it often attacks the metal tray area. Now’s a great time to clean it and apply a rust converter, you can check out the rust converter I use here on the Rust repair tools page.

Position the new battery making sure the poles are on the correct side to match the polarity of the terminal. Now refit the battery hold-down clamp and vent hose, if fitted.

5 Fit Positive Terminal First, Then Negative Terminal

Positive battery terminal
Fit red positive terminal first

Before fitting the terminals, clean them using sandpaper. 

Fit the positive terminal first, push it down onto the battery positive post as far as you can get it before tightening the fastener.

Now go ahead and fit the negative terminal in the same fashion.

Battery negative terminal
Fit black negative (ground) terminal last

Applying some dielectric grease to the terminals will help protect them from corrosion. Fit the plastic covers and you’re done with the wrench swinging.

Battery grease

6 Remove Memory Saver Tool

It’s now safe to remove the memory saver, simply unplug from the OBD first, then from the donor car. 

7 Code New Battery

Battery monitoring sensor
Battery monitoring sensor

If your battery has a sensor as per the above picture, then your vehicle is fitted with a battery management control module and your new battery needs to be coded to your vehicle. Most European manufacturers use a battery management system and many US models are adapting this system too.

Modern cars as you know are run by computers and manufacturers are adding an ever-increasing number of power-hungry features. Battery power is crucial to all systems and in an effort to improve battery performance; it too has a control module to look after its needs.

The battery management system (battery control module) monitors the battery state of health and load. In order for the battery control module to do its job correctly it must be notified when a new battery is fitted and the output of the new battery. 

The battery control module uses this info to optimize the charge rate.

It optimizes charging based on; the size of the battery (ah – amp hour), state of charge, demand, battery health, temperature, and how old the battery is.

What happens if I don’t register the new battery? 

Battery size and code
Battery code and amp-hour (Ah) details

If your car is equipped with a battery management control module and you fail to adapt the battery to the vehicle (also known as coding the battery or battery registering) your new battery may be overcharging. 

As a car battery ages, it degrades, and to help keep that battery at its peak, the control module increases the rate of charge. If your car doesn’t know it has a new battery fitted, the control module will continue to charge at a higher rate which will lead to premature battery failure.

How to code the battery?

A battery coding tool is required. All new batteries have a battery code right on a sticker, on the casing. Using a battery coding tool or more multi-function bidirectional tool, connect the tool through the OBD port and select the battery management control module.

When prompted, input the battery code and Ah rating and hit enter. The system will update and recalibrate the charge rate.

I’ve listed my favorite bidirectional scan tool which is, of course, capable of coding batteries and much much more, you check it out here on the Auto electrical repair tools page.

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