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Use a solar panel to keep your car battery charged

battery solar charger

Martynn Randall is technical editor at Haynes and has been with us for 27 years. He's written more than 60 Haynes publications and has owned more than 85 cars and 60 motorbikes... so far!


Leave your car unused for more than a few days and there's a risk that the battery could be depleted. 

Modern cars have several electrical systems – such as the immobiliser, convenience and engine management systems – that draw current from the battery, even when the vehicle is locked and not being used. Over a few days, those little bits can leave you with a horrible ‘click’ when the key is turned. This is a common occurrence when you leave a vehicle in an airport car park and head off on holiday for a week or two.

If the car was parked at home, you might be able to leave it connected to a battery maintainer, but that's impossible when you're away from a mains socket. Luckily, there is an alternative: a solar panel that can be plugged into the vehicle's diagnostic socket (OBD).

They’re readily available, inexpensive and many come with a plug compatible with the design of diagnostic sockets fitted to all cars since about 2001. You may also be able to buy an OBD power cable separately. The solar panel pictured below is made by Ring but others are available.

Ring solar charger

The OBD sockets in cars all have the same terminal pin assigned, which is connected straight to the battery positive feed. So even with the ignition switched off, that pin allows direct access to the battery.

All you have to do is place the solar panel on the dashboard or attach it to one of the windows (with the sucker pads provided), then plug the cable into the diagnostic socket. 

The socket is normally under the facia on the driver's side, although this can vary, so have a look in your Haynes manual. 

That’s it! Nothing more to do. The panels don’t produce a lot of power, but it should be enough to keep the battery topped up, especially if the sun's beating down like it has been in the UK for the past few days. The panels also work well even when direct sunlight isn’t available and, because they are fully automatic, they will only pass charge to the battery when the output from the panel is greater than the current voltage of the battery.

It could just avoid that horrible feeling you get when you discover the battery's flat and all you want to do is get home as quickly as possible.