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How to charge a flat car battery

How to charge a flat car battery

Dan is an experienced motoring journalist who has more than 20 years of experience. He has been the editor of titles such as Fast Ford and Redline, and his latest project was converting an old Renault Trafic into a family campervan.

Engine hardly turning over? Starter motor sounds like it's on its last legs? Your battery could require some attention.

Is the battery to blame? 

If the car turns over strongly but the car won't start, the battery isn't to blame. Immobilisers are often guilty of this, and sometimes a fix is as simple as locking and unlocking the car and trying again.

If you turn the key and the car's completely dead, check to see if any lights have been left on. If so that's your culprit. If nothing happens at all – no click, no dashboard lights, no signs of any lights being left on – check the battery terminals aren't loose. 

If you're sure the battery is to blame, and jump-starting isn't possible, you'll need to try charging the battery. 

How to charge a car’s battery

  1. You can connect a mains battery charger (they cost upwards of £20), which you connect directly to the car's battery. You can either do this with the battery in situ, or you can take the battery out first
  2. Battery chargers are fairly self-explanatory: connect the '+' and the '–' to the battery terminals and turn the charger on. Most have lights to tell you the state of charge and when the battery is full.
  3. If you remove the battery from the car, don't charge it in the house – a faulty battery (or charger) can potentially cause an explosion and/or a fire.
  4. Alternatively, if the vehicle spends time sitting around, not being used, a trickle charger (or conditioner) may be a better option. It actively monitors the battery's health, and can remain connected for extended periods.

Learn more about a battery's amp-hour rating and what it means for its operating life.

How to test the car battery

How to test the car battery

  1. You can do a simple battery test with a basic multimeter (buy them for around £20). Turn the dial onto the '20V' setting and connect the red probe to the '+' side of the battery terminal and the black probe to the '–' terminal. 
  2. It will then give you a voltage reading. '12.6v' means there should be sufficient power to start the car, so if it's not starting and you're reading 12.6v then the battery isn't at fault. 
  3. If the reading lower, then you may experience issues, and if it's any lower than 12v it's technically flat, so starting will be virtually impossible. 
  4. With the engine running the voltage should read between 13.8v and 14.4v.