Usual thing with all batteries,only a small space you can only fit a small battery hence they won't last long,that is until some other new technology comes on stream to do away with the wires to your auxiliary socket.
Of course if you don't have it plugged in you lose all the automatic options such as it starting as soon as you turn on the ignition and stopping when you turn it off. And you know that the very day you forget to switch it on manually will be the day when you need its evidence.
Thanks for all replies, I did not know the post had been moved. It looks like I will have to put up with a cable which is no big deal. I just wondered if there was an alternative and BT made a good point regarding sods law.