Check out Battery-to-Battery chargers from Sterling Power USA, or if you really need 100's of amps of charging power while driving down the road check out the Alternator-to-Battery charger they have.
I haved the Battery-to-Battery charger in my bus. It charges a 24v auxilliary bank at around 20 amps per hour while I'm driving the bus to the Patriots game.
http://www.sterling-power-usa.com/
or
http://sterling-power-usa.com/altern...ychargers.aspx
Multi-stage charging, without plugging into AC power! sweet for tailgating every weekend and not worrying about my batteries being charged.
-Jeremy