I have a Rocky Mountain Fusion 10 mountain bike that I'd like to convert. Here's a link.
As you can see, the frame is pretty cramped. I'm not sure I can mount a battery in there. Is there a mini battery available? Is installing a rear rack for a battery a good choice?
I recommend using the Bafang USA Direct website to help with this. For each battery size, they have a handy pdf file that is a printable template of the battery size. You can print out this template and use it to test fit before actually buying a battery. Battery sizes are pretty standard so once you find a size that works, you should be able to shop with Johnny or elsewhere for the actual battery. Very useful of feature of that site.