You can buy portable chargers, the AA etc carry them to jump cars, dont think they are particularly cheap and dont think they charge you battery by much I am afraid.
Have heard solar trickle charges are good once battery is charged.
As John say's Xf's are very battery thirsty. I have my CTEK MXS 5 connected as soon as I get back home from a drive out especially in this cold weather. I hope you get it sorted mate, keep us up to date buddy.
James