Car light left on overnight: What now?

If you have left car lights on all night, the battery can quickly run down. It does not make much difference whether it is a halogen, LED or xenon lamp. If you leave the light on for too long, the battery will drain and the car will no longer start.

But in most cases this does not mean that you now have to call a garage or the breakdown service. A kind neighbour or a friend can usually help. In our guide, we explain what you should do if your car battery is dead.

How long can you leave the lights on in the car?

If you accidentally leave the lights on in the car, the car may still start the next morning. But how long can you leave the car light on before the battery is dead?

Most car batteries have a capacity of 45 Ah and a vehicle electrical system voltage of 12 volts. So if you have left the interior car lights on all night and two small bulbs with 10 watts continue to burn, the battery will only be empty after 27 hours.

But this only applies if the battery is new and fully charged - which is rarely the case. Battery lifetime is often considerably shorter and can also be negatively affected by external factors such as cold.

What to do if you left light on in car and it won't start now

For many drivers, this is the absolute nightmare: the car lights have been left on overnight, and now the battery is dead and the vehicle will not start. This does not always have to be a case for the breakdown service. Instead, you can simply ask a neighbour or a friend to jump start your car for you with jumper cables.

To do this, proceed as follows:

  • If your vehicle is parked on a road, first secure the location and yourself with a warning triangle or by wearing a high-visibility safety vest. You will need a jumper cable to bridge the terminals.
  • -Now take the red one of the two jumper cables and connect it to the positive terminal of the other car's battery.
  • Now plug the red cable into the positive terminal on your car battery.
  • Then connect the black cable to the negative terminal of the other car.
  • Connect the other end of the black cable to the earthing point of your car – do not connect the cable to the negative terminal of your car as this may cause sparks!
  • Now ask your helper to start his car.
  • Wait about one minute and then start your vehicle.
  • If the car does not start, wait a few minutes and then repeat the procedure two more times.

If the car does start, do not switch the engine off again immediately – because then the car will once again not start. To be on the safe side, you should drive your car for between 10 and 20 minutes, preferably without running power guzzlers like the seat heating. By this time the car should have “recovered” and you can switch off the engine without worrying that it won't start again afterwards.

If the car does not start after the third attempt, you should not make any further attempts to jump start your battery – otherwise costly damage may result. In such a case, it is best to call the breakdown service, who will in the vast majority of cases be able to start your battery with a booster. Then you should also drive the car for at least 10 minutes before switching off the engine again.

If you have the lights have been left on in car for a very long time, it can also happen that the battery is completely discharged. This is extremely rare, but nevertheless possible. In such a case, the battery has to be recharged, and it is best to go to a car workshop for this. Under no circumstances should you charge the battery from the mains at home, i.e. from the socket – this is extremely dangerous!