http://www.xantrex.com/productcart/pc/viewPrd.asp?idproduct=141
This is the only 13.8V DC 750 aH charger I could find on the web, but I have no idea about how big the metal end of the cord is that would go into the POB's charging jack, and I don't even know if it is the right kind of charger.
Hopefully someone will know if this would be the right kind of charger.