or if you want a "easy" way to keep your 12v topped up invest in a caravan charging relay and have it installed and keep your 12v in the boot ( in a battery box of course
) and it charges it up till its full then automatically cuts off so when youre driving about it charges ( if you have a towbar and two "light" sockets then chances are that you already have this system in place i think its the grey one that is normally the charging/auxirally soket but please check before hand....)