Tried this?
 

Has anyone tried one of these, could be practic no?
http://www.4-motorcycles.co.uk/acatalog/BATTMAN.gif
MHBATT - Battmann Solar Battery Maintainer

An innovative method of managing battery drain. Solar trickle charger to manage battery drain in parked vehicles. Can be connected to the cigarette lighter or direct to the battery. Green LED shows when Batt-Man is providing charge. Will not overcharge or damage batteries.
Price: £24.99

Mattias

[This message has been edited by Matt595 (edited 14 March 2005).]

No.

Just adding something else that;

is easily broken
is easly stolen
adds to the luggage

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Better off disconnecting the battery. And spending the money on gas or beer.