How a car’s battery works

Battery removal and replacement image

A car’s battery, like all batteries, stores electrically charged energy in the form of chemicals. Car batteries use lead-acid technology and are designed to last for many years but they’re not like phone batteries and don’t like to be repeatedly drained and charged. So if you run it down by leaving the lights on or have a ‘mystery drain’ somewhere else in the car, it’s likely that your battery won’t be in the best of health.

This is an easy task and requires only basic tools. In most cases, this procedure will take only half an hour or so.

What is the lifespan of a car battery?

The battery on your car has to work hard, and that means dealing with all kinds of loads in all sorts of conditions. Over time, the effectiveness of a battery will be compromised. But how long will it be before your battery is of no use and you need to replace it, and what will the signs be?

The general rule of thumb is that a battery should last around three years. This is in the case of a battery in a car you drive every day, through all seasons. In a car you use less often and keep indoors, say a classic car or something similar, you can stretch this out to around five years.

There is nothing you can really do to stop a battery from running out of life. While you may look at it as nothing more than a big block, the reality is that there are all kinds of chemical reactions going on inside your battery, and over time the chemicals at play lose their effectiveness.

How can I tell when my car battery needs replacing?

The first sign that the battery is dying is when the car becomes harder to start. When a car is running, it’s the alternator’s job to pump electricity through the car. However, it needs to be brought to life with the battery. If the battery is starting to fail, your car will be come slow and sluggish to start, and that’s if it even starts at all.

Batteries also perform differently depending on the temperature. Your old battery might be fine through the summer, because the ambient temperature is higher, so the battery has better conditions to work in. Come that first cold snap, the battery can fail, because it won’t have the power to overcome the cold.

The other giveaway is that things stop working when the engine isn’t running. The windows might become slow, the stereo might lose it stored settings, the lights might not work… all signs the battery has had it.

How do I perform a car battery health check?

The first visual check comes in the form of the 'bubble'. On top of some batteries, there will be a small (less than a centimetre in diameter) ‘window’. Look directly down this. In a healthy battery you should see green. In a flat battery you’ll see black or red.

The other way is to put a multi-meter on it. Set the meter to 20v and put the red probe on the positive terminal and the black probe on the negative terminal. The reading should be 12.6v or higher. Anything less, and the battery isn’t in good health. You can try charging it with a trickle charger, then leave it a few days and test it again. If the reading is still low, it’s definitely time for a new battery.

Your Haynes manual will give you the specification you need for your car. Don’t buy any old battery that fits, you need the right one, with the right power and the right specs.

When to change your car battery

It’ll be obvious when your car’s battery is flat: the central locking may not work and the engine won’t turn over. However, it’s not always easy to know when the battery needs to be replaced. Once the car has been started (via jump leads/jumper cables, bump started or battery trickle-charged) take the car for at least a 20-minute run to charge the battery. Turn off the engine and restart it the next morning. If it’s flat again you need to investigate further.

First, check that the battery leads are secure. Then 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 you need to change your battery, Haynes will walk you through all of the necessary steps.

All cars are slightly different, so if it is time to change your battery, find your car for specific instructions.


Make sure you follow the correct battery disconnection procedure to avoid causing problems with the car's electrical components

How to change your car battery

This is a clip from a sample video

A very brief summary of the task:

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

Why you should change your car 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 the car’s systems, such as the electric windows, audio system and more. This is time-consuming and we strongly recommend fitting a new battery as soon as possible.

Tools you will need

Only basic tools are required for this job

  • Socket set
  • Spanner set

Parts you may need

  • Battery
  • Memory saver (to avoid resetting systems)

How much does a new battery cost?


Battery $100-$400
Garage fee savings $200-$400


Every car is different, so before you view the full instructions, find yours…