Revive Your Vehicle: Charging a Flat Car Battery Explained

Any vehicle can suffer from a flat battery – even an electric car. Thankfully, charging a car battery is a straightforward process, but there are some important points to consider first.

A car battery will typically last between three and five years before it needs replacing. There are many signs that a battery is getting old, but common symptoms include the engine cranking slower than normal, loss of power to the electronics and a warning light on the dashboard.

Charging a failing battery might get you home, but you should fit a replacement as soon as possible. A garage will usually be able to fit a new battery in a matter of minutes.

It’s also important to drive your car regularly. Without use, even a battery in good health might only last two weeks before it needs recharging. And external factors such as cold weather or extreme heat could reduce this to a few days.

Equally, the cause of a flat battery might be something as simple as an interior light left switched on.

Electric cars have a separate battery for starting the car and powering ancillaries such as the lights and air conditioning, so the same issues also apply.

Why has your car battery gone flat?

2021 New Year flat battery

If you have ruled out the possibility that your car battery is simply old and needs replacing, you need to figure out why it has gone flat. Reasons may include:

• Regular short journeys. As many motorists have discovered with increased working from home, the combination of infrequent use and short trips puts a strain on car batteries.
• Leaving an interior light on. Also check the boot light, as this is harder to spot when you leave the vehicle.
• Using the electrics when the car isn’t running. Did you listen to the radio while you spent the afternoon valeting the car?
• Fitting the wrong battery. If it’s too small, it will lack the power required to crank the engine.
• Extreme outdoor temperatures, either hot or cold.

How to charge a car battery

Buy a car battery in lockdown

Once the car is running, a drive of around 30 minutes – preferably at a steady speed on the motorway – should be enough to return the battery to full health. Don’t stall the engine or you could be back to square one!

If you find that the battery is flat or the car is slow to start at the next attempt, it’s almost certainly time to fit a new one. Prices range from around £50 to £200, depending on the size of battery your car requires.

A modern car with stop/start engine technology requires a different and more expensive AGM or EFB battery. You’ll also require a smart charger, rather than a conventional battery charger.

Here is the process for charging a car battery:

1. Check and clean the battery terminals. If they look dirty, rub them with a wire brush and remove any residue before charging.
2. On older cars, make a note of radio PIN codes or how to reset the clock BEFORE you disconnect the battery.
3. Disconnect the battery. Remove the negative terminal first, then the positive terminal, before opening the clamp holding the battery in place. Using a magnetic ratchet set or spanner will reduce the chances of important nuts and bolts dropping into the bowels of the engine. We’ve all been there.
4. Although it’s possible to charge a car battery in situ, we’d recommend removing it from the car. Position the battery on a flat surface in the garage or shed before connecting the positive cable to the positive terminal, then the negative cable to the negative terminal. Don’t recharge a car battery in the house – there’s a risk of explosion or fire.
5. With the cables secured, switch on the battery charger and wait for the battery to be charged. How long it takes depends on the size of the battery, but it could be up to 24 hours before the battery is fully topped up. Most chargers have a light or indicator to tell you when the battery is ready for use.
6. Unplug the charger, disconnect the cables, then position the battery in the car. Secure the battery by refitting the clamp, before fitting the positive terminal and then the negative terminal. Switch on the car’s ignition, check for any warning lights, then start the engine

If your car spends a lot of time not being used, it’s probably worth investing in a trickle charger. These devices monitor the battery’s health and top it up gradually with electricity from the mains.