Expert advice: how to check for a flat battery

As the weather gets colder we’re going to see an increase in the number of call outs for cars with flat batteries. The reason for this is simple: the older a battery gets, the less able it is to hold its charge in lower temperatures, and the colder the weather, the thicker oil gets so the starter motor needs to draw more current from the battery to turn the engine over. Put these together and tired car batteries are far more likely to fail. Here’s everything you need to know about your battery. 

What to look out for

The weaker a battery gets, the more it struggles to supply the starter motor with sufficient current so listen out for what sounds like the engine turning over more slowly than normal. Also, watch for a warning light in the dashboard (which looks like a red battery) coming on as you’re driving. This can be an early warning that the engine’s alternator isn’t charging your battery properly.

How to check your battery

If your car is showing any of the above symptoms, take it to your nearest garage or fast fit centre and ask them to perform a battery test. Most will do this for free and it will tell you what state your battery is in.

What to do if you’ve got a flat battery

If you can, call out your breakdown provider. They will be able to help in several ways. Many Green Flag technicians carry a piece of kit called the Midtronics Battery Tester. This can show whether a battery has gone flat because you might have left the lights on, whether it needs a boost (more on that later), or if the battery is unserviceable and needs replacing. If this tester is used, Green Flag gives customers a print-out of the test results for peace of mind. If you need a new battery, your technician will source one for you to buy (subject to availability), and they’ll fit it for free.

What not to do if you’ve got a flat battery

Some people think bump starting a car – where you push it along then release the clutch when you’ve got a bit of speed up – is the best way to combat a flat battery. That’s a real no-no now.

The average age of vehicles is around seven years and even cars that old are pretty complicated electrically. When you bump start a car, you get a big surge of power through the electrical system and although it might start, you’ll find it stops pretty quickly and won’t re-start because you’ve fried the ECU (the Engine Control Unit). Replacing that can cost thousands of pounds. If you have to, the best way to cope with a flat battery is to jump start the car. Find out how here.

How to maintain your battery

Batteries always used to be under the bonnet but increasingly we’re seeing them in the boot or even under the back seats. The handbook will say where your car’s is located. First, make sure the terminals aren’t covered with what looks like white fur. This is a by-product of the fumes that come off the battery. A wire brush will get rid of this and a bit of good old copper grease will stop them furring up. Just be careful. Remember you’re playing with enough electricity to hurt!

Tabs and charging

If you test your battery and it’s showing up as deficient you can buy tabs from motor retailers that you drop in to reinvigorate the cells. You used to be able to charge batteries but modern immobilisers are so complex that many manufacturers advise you against removing the battery. And it’s not a good idea to plug a charger in while the battery is still attached to the car. You won’t put 240v through your car but you’re introducing a different source of power which could cause electrical problems.

How long should a battery last?

Manufacturers typically offer three-year warranties on batteries and they usually last a lot longer than that. The average age of the dead batteries Green Flag replaces is six to seven years. Drivers will be pleased to know that the age of batteries is gradually increasing as vehicles become more effective at using the power from them.

Nick Reid is a fellow of the Institute of the Motor Industry and head of transformation at Green Flag