As I have a the W124 cabriolet in the garage...
My much cherished E30 325i cabriolet spends most of its time (when my sisters garage is not available) outside, where there is no mains power....
So I have recently opted for a solar trickle charger which Iv connected direct to the battery via the supplied crocodile clips....
Dont know how good these are, much debate on this forum and other sites into their effectivness, so time will tell....