TBark, I've got an older version of the Montana with a spare battery that I'll never use the spare. Looks similar to this one:
Spare battery
If you're interested in the spare, PM me when you get the Montana 750i and I'll send you the spare. I'll confirm that the battery fits before sending it to you.
I'll never use the spare because my older unit takes AA's as an alternative, but the unit resides on my quad for navigation. And literally sips battery so the pack itself will easily give me 2 solid days of use before I'd have to look at AA's....
J.