I don't have any access to power, so the only option is to use a solar powered trickle charger. These sit on the dash and send a tiny electrical current back via the cigar lighter to the battery.
I bought it without considering the fact that the cigar lighter would need to be permanant rather than ignition switched live. So in factory guise, there's no way of making the trickle charger perform. I found a workaround.
Wire with 7.5A in-line fuse
I found a strip of wire with an in-line fuse, and cut the ends of. Then crimped a spade connector at each end.
Spade connector on each end
I pulled the cigar lighter fuse from the fuse box, then found a perm live feed from one of the empty DME relay sockets. The one I used is a perm live for a car with a tiptronic gearbox.
Then jsut use the in line fuse to bridge from the perm live DME output into the space left empty from the cigar lighter fuse = a perm live cigar lighter which is still fused just in case.....