just found this which may be of interest to those that lay their car up for the winter,
link.
People normally use an optimate which costs £40-50, this seems like good value @ £8, easily wired into your existing battery charger. I've had one for 3 weeks, maintains a good 13+ volts in the battery

my old pa has a 12v system on his boat, standard car battery etc, he uses a solar trickle charger which has the same regulator and stuff. Not much use in a dark garage but even on an overcast day it provides a good charge. Saves having leads out from the house if there is no power in the garage...