Most car batteries that are in good condition will last at least two weeks without needing you to start the car and drive to recharge it. But if you don’t intend to drive your car for some time for whatever reason, you should still start it up once a week to recharge the 12-volt battery.

Want to know how to avoid getting a flat battery if you’re not driving your car often? And what you need to do if you go to drive your car and the battery’s dead? Read on to find out what to do, and what not to do, when maintaining your car’s battery.

How often should I start my car if I’m not driving?

If you’ve not driven your car for the past two weeks or more, regardless of how old the model is, you should start it up at least once a week and keep it running for 15 minutes. By doing this, you allow the alternator (or ‘dynamo’ in older models) to recharge the battery and ensure the engine is kept in good condition too.

When you do this, remember to turn off the lights (if you need to use them because it’s dark and your car doesn’t do this automatically), otherwise, you’ll find that it’s completely flat when you next go to drive. This is what’s known as a ‘deep discharge’, which the lead-acid batteries in cars aren’t designed to handle. In fact, if this happens multiple times it can reduce the lifespan of the battery by a third.

What should I do if my battery is flat?

The chances are that if you’ve been driving for a long time, then you’ve had to jump-start your car at some point.

Also known as a ‘boost’, this quick-fix method starts a vehicle with a discharged/dead battery by connecting with another car’s battery or another power source through jump leads.

Jump starts can sometimes be necessary if you’re out and about on the road and need to get moving quickly. However, a better alternative for long-term battery maintenance is using a designated battery charger/maintainer while regularly testing your car’s wattage between charges.

It’s important that you know how to jump-start your car if you ever come to your car and it’s got a flat battery. It’s also just as important to know what to do when you want to recharge your battery.

Here’s a simple step-by-step guide to jump-starting and charging your car battery.

How to jump-start your car

When you jump-start a car, you’re using external electricity to recharge a flat battery enough for you to switch the engine on.

The most common method of jump-starting a vehicle requires you to have jump leads (these cost around £10 from Halfords).

Here’s a simple process to jump-start your car safely:

  1. Position the car with the working battery close to the car which is flat so that the jump leads can reach both vehicles’ batteries, taking care to move metal objects out of the way and get rid of any loose clothing and jewellery which could cause it to short-circuit and generate a nasty zap.
  2. Connect the first lead – first, make sure the engine of the working car is switched off and attach the red jump lead’s crocodile clip to its positive (+) terminal, before attaching the other end to the positive (+) terminal of the dead battery in the other car.
  3. Connect the second lead – attach the black jump lead’s crocodile clip to the negative (-) terminal of the working car’s battery, then attach the other end to an earthing point (metal point on the engine block/chassis) away from the flat battery and fuel system.
  4. Wait for a few minutes and then start the engine of the working car, letting it run for a minute.
  5. Once this time has passed, start the engine of the car with the flat battery and let both cars run for 10 minutes.
  6. Turn off the engines of both cars and disconnect the jump leads in reverse order to how they were attached (removing the black lead from the car with the flat battery first and finishing by removing the red lead from the working car.) Make sure the leads don’t touch one another in the process.
  7. Try restarting your car – if it doesn’t start, it’s likely that there’s a more serious problem which you need to contact the AA/your breakdown recovery provider about.

Remember: After you’ve done a jump start, you’ll need to keep the vehicle’s engine running for around 30 minutes to allow the alternator time to charge the battery sufficiently.

Charging your car

It can be detrimental to a car battery’s health if you let it go completely flat by not driving it, or by leaving the lights on, for example.

The good news is that it’s relatively easy to check the health of your vehicle’s battery and charge it. All you need is the right equipment and the know-how to do it.

First things first, you’ll need to make sure you have a multimeter/voltmeter (around £8 from Halfords) and a charger that is suitable for your car’s battery type and engine size.

To help you make the right choice when it comes to buying a charger, you should consider:

  • Type of battery - lead acid batteries work with most kinds of chargers, but calcium/gel batteries need a higher voltage than conventional ones. Also, if your car has stop/start technology, you’ll need a ‘smart’ charger that is compatible with these vehicles.
  • Engine size - most chargers will have an upper limit for the size of the engine in the car which it chargers. This is because the bigger the engine is, the bigger the battery will be, so you should check this when looking online/in an autocentre.

Six steps to charging your car

Once you’ve chosen the right charger for your car, you should follow these six steps to recharge it safely and get your motor back on the road.

  1. Test the voltage of the battery – using a multimeter/voltmeter set the dial to read DC voltage (indicated by a solid line and a broken line above/to the side of the letter V). Set the dial to 20, which will then let you measure between 0-20 volts.
    • Connect the red cable to the positive (+) terminal and black cable to the negative (-) terminal of the battery, making sure the engine is switched off and preferably the car should have been parked overnight to get an accurate reading.
    • Take the reading, which should be no lower than 12.6V in order to be fully charged. 12.2V is generally considered around 50% charged and anything below 12V means it’s discharged.
  2. Before charging, check that the contact points (or 'terminals') on top of the battery aren’t dirty or corroded. If they are, use a piece of sandpaper to clean them, ensuring you wear gloves so none of it touches your skin.
  3. Disconnect your battery – take out the negative (black) lead first and follow this up by taking out the positive (red) lead, before moving them both to one side. Disconnecting your battery will probably reset your dashboard, any seat memory settings and your radio, so you may want to have the security codes to hand in order to reset these afterwards.
  4. Connect the charger – keep the charger as far away from the battery as possible, using the length of its cables, then attach the clamps of the charger to the right terminals (remember: positive to positive and negative to negative), before plugging in the charger to the mains.
  5. Turn on the charger – you’ll need to read the manual of your chosen charger to find out whether it stops automatically once it’s fully charged or whether you need to disconnect it manually. A smart charger, for example, will monitor battery condition and adjust its charge cycle to fit with the requirements of the car and avoid overcharging and potentially damaging the cells.
  6. Disconnect your charger and reconnect your battery – make sure the engine of your car is still turned off and remove the charger clamps. To reconnect the battery, plug the positive lead (red) in first and then the negative (black) one.

Remember: If your battery is still losing charge after you’ve done this, it may be that you need to replace it. You can use one of the many autocentre free battery checks in order to determine this.

Maintaining an electric car battery after not driving

Electric cars and hybrids have 12V batteries (also known as ‘auxiliary batteries’), like standard vehicles, but they charge differently. The most common way to do this is by pressing the start button to turn it on, which will automatically operate a charging system which in turn gives the battery more power.

If you’re concerned about how much charge the auxiliary battery in your EV has, you should start the vehicle up at least once a week for 10 minutes each time for a sufficient recharge.

At the same time, you should check the status of the main onboard battery (this can be done through most electric cars’ dashboards) to see whether it needs charging. Even though you may not be driving, lithium-ion batteries in EVs can gradually lose the capacity to hold a charge over time, and you may see the effects of this in the form of loss of range/percentage.

Should your car need charging, make sure this is done through a designated home charge point or public station. While each EV comes with EVSE (electric vehicle supply equipment) that includes a Type-2-to-3-pin cable for charging from a mains socket, this can overload the outlet and potentially be dangerous.

Remember: Check your vehicle’s handbook before choosing to maintain the 12V battery by plugging into your domestic mains socket.

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