If you have real problems with the battery going flat, long term Bjorn, it may be worth looking at this -
Optimate DC to DC 12v chargerA bit of a pricey option but this would allow you to charge your battery from another 12V battery which you could then take home to recharge when necessary.
I've not used it myself but others who have no access to a mains power supply (e.g.using a 'lock - up' garage) have used it with great success.....

James