This WU is based on the batteries fitted in standard cars for the British market. Always check with your service manual or authorised service centre before performing any DIY maintenance on your own car.

What is a Car Battery

Not surprisingly, a car battery is a battery that powers your car. Wait, I hear you call, my car runs on Petrol (or Diesel... or LPG... whatever). Even though that's the main fuel, a battery is still needed for three main things.

• Starting the engine
• Providing the spark in the ignition cycle (not on diesel engines)
• Powering the auxilliaries (eg the radio)

NB: I'm not dealing here with electric or hybrid cars such as the Toyota Prius which have batteries to provide main traction.

What are the specs of a car battery

Car batteries are big (6 x 9 inch footprint, about 8 inches high would be typical) heavy things that provide an output of 12 volts. Of course, you can get 12 volts from 8 little 1.5 volt AA cells. But you'll be hard pushed to provide the 100-200 amps that are needed to turn over a car engine to get it started.

In fact, car batteries are made up of 6 lead-acid secondary cells in series with each other. Each one of these has a nominal output of about 2 volts, hence the total output of 12 volts. Additionally, these "wet cells" have a very low internal resistance, meaning they can supply a huge current. And their large size means they can store a lot of charge.

For comparison, a typical D-type cell will provide about 1.5 volts, and will have a capacity of about 2-4 amp-hours. An amp-hour is a simple way of expressing how much power the cell can provide. 4 amp-hours means the cell can provide 1 amp for 4 hours, or 4 amps for 1 hour, or any other combination of amps and hours that multiplies to 4. A typical car battery on the other hand provides 12 volts, and has a capacity of about 50 amp-hours (of course this depends on the size of the battery). Furthermore, a normal D-cell can only provide a current of about 0.3 - 0.4 amps. A rechargable D-cell may be able to provide slightly more than that - perhaps up to 5 amps. Car batteries typically provide 300-600 amps for starting the car, and could theoretically provide over 2000 amps (although this may permanently damage them).

Different cars use differently rated car batteries, and may offer two or more alternatives (eg a high power battery designed for drivers who make lots of short journeys.) Usually, when purchasing a car battery from a third party seller (such as Halfords in the UK), they will have a list of all the standard battery options for all cars.

Where do I find my car battery

In most cars, the battery is located under the bonnet (hood in the USA) near one of the front corners. In some cars (eg the Alfa Romeo 166) it's behind a panel in the boot. And in a BMW Z1 I couldn't find it at all!

Charging a car battery

In normal use, a car battery is being constantly charged by the car's alternator. This means that after the initial start (which does drain the battery), a few minutes driving will recharge it back to full power and keep it topped up, even if you're using lots of additional electrics (eg headlights). This is why high power batteries are made for people who do lots of short journeys - to ensure the battery can keep starting the car even if it hasn't been fully recharged.

Batteries are usually purchased charged. However, it doesn't do any harm to drive around for 20 minutes after fitting a new battery, as it may have been on the shelf for a couple of months after being charged. All rechargable batteries leak charge slowly.

You can also buy battery chargers. These can be useful if you know you have problems with the battery and haven't had time to replace it. However, charging up a car battery takes 10-12 hours, and requires you to disconnect the battery from the car's electrics. So it's usually not worth it.

If you're caught out with a flat battery, there's two common ways to get a car going with a totally flat battery.

The other time you may need a charge is if you're leaving your vehicle for an extended period of time - say a few months - it's possible for the continuous drain (radio to keep its memory, alarm system etc) to run the battery down. You can actually get solar chargers that sit on the dashboard and plug into the cigarette lighter to keep the battery topped up in this situation. Of course, they don't work if the car is garaged!

Topping up a car battery

Older car batteries had vents to allow any gas produced inside to escape safely. However, this meant that the water inside (it's dilute sulphuric acid) slowly evaporated - and therefore you needed to top them up from time to time with distilled water.

Newer (since perhaps 10-15 years) batteries are sealed with only a pressure release valve. This don't need topping up - but as they do contain liquid acid inside, they should still be kept upright as much as possible.

Changing a car battery

If you're told at a service that the battery's low, you can let them charge a fortune for a new battery, and then charge for the labour to replace it. Or you can do it yourself. I hold no responsibility for any damage you cause your car or yourself doing this!!

Go to an appropriate shop (eg Halfords) to buy a new battery. The assistant should be able to help you pick the right one. Be careful - car batteries contain a lot of lead, and are therefore very heavy! Also, make sure you never touch the two battery terminals together, or both to the same piece of metal. The 2000+ amps that could flow can melt most metal and possibly cause the battery to overheat and explode.

When you get home (or in the shop's car park if you're desperate)....

• Stop the engine and pop open the bonnet.
• Find the battery.
• Lift up the plastic covers that should be over the terminals.
• Check that the positive and negative terminals on the new battery are the same way round as the old one - they certainly should be. If not, return it and speak to the assistant.
• Start with the positive terminal - this is often coloured red and/or marked with a + sign. Use pliers to gently loosen the nut (only very slightly) and ease the wire off the positive terminal.
• Repeat with the negative terminal - this is often coloured black and/or marked with a - sign.
• Loosen sufficiently any physical restraints on the battery. Most cars simply rely on the weight of the battery to provide the main stability for it, and only have a couple of small bars holding it in place.
• Carefully lift the old battery out and place it somewhere safe.
• Put the new battery in, the same way round as the old one.
• Re-attach the physical restraints.
• Attach the negative cable, being careful not to overtighten the locking nut.
• Repeat with the positive terminal.
• Smear some medium weight grease (eg vaseline) over the terminals. This helps to prevent corrosion.
• Replace the protective plastic covers.
• Close the bonnet and try to start your car!

Once this is done, as suggested above, you may want to drive around for 15 minutes to ensure the battery is fully charged. As part of this drive around, go to a recycling centre or domestic waste dump and give them your old battery. Don't just stick it straight in the skip or bin! In most places it's illegal to dispose of old car batteries in any other way.

You may find that the radio needs reprogramming, or if you have a coded radio that you need to re-enter the code as it's been disconnected. Most engine management systems can cope with the battery being disconnected for short periods of time. Again, check with your service centre if you are concerned.

Glindsey told me about "Battery Savers" that you can plug into the cigarette lighter while you change the main battery to keep power for the radio, management system etc. Probably worth looking into if you're concerned, or your service centre tells you there can be a problem.