Install a second battery in the boot, a deep discharge lesuire battery for a caravan is best. Then install a split charger which will always insure that the main battery is charged before diverting charge to the second battery.
http://split-charge-relays.co.uk/ gives lots of info.
When I moved my battery to the boot I bought an after market battery tray from ebay which I bolted in the boot, I then used a battery cable from an M3 to connect this to the under bonnet fuse box and ran another cable to the starter. I then used some 18mm MDF to make a box for the battery and covered it in boot carpet from a scrapped R45 so it looks factory fit. I wasn't installing a second battery but the principle is the same.
On my T16 maestro I did similar to above except I adapted the battery tray.