The most obvious choice is a normal 12v accessory socket (fag lighter), these fit into the inside of the side panels, but you have to be very carefull as the diameter is the same as the space.

The other alternative is to use a 16a DIN socket, these are a smaller diameter - it's what I have. I use a 2 way 12v accessory extension with a DIN plug on the end. I run the extension into the tank bag and I can charge the phone, laptop (A5 netbook), camera, camcorder - anything that uses a 12v adapter.

You need to run up an auxillary circuit with a relay so that you it cuts power when you take the key out.

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Never underestimate the power of stupid people in large groups.