I know how hard I looked for half-decent instructions for the dual battery mod, and I have found them! Go here for the mod:
http://foy.foy.com/minircmod/moddocs...attinstruc.htm. That is the "Piggy Back" style, which by some people, have said that it does wheelies off the start. Which it doesn't. It is a great mod, and DEFINITELY worthwhile if you are a speed freak, and easy to do.
The only reason I posted this thread is because I remember how hard I looked for instructions, I actually went through all of the pages of threads in this folder, and it took me about an hour before I found one. I hope this will be as helpful and successful to you as it was to me!
Also, if you want the charger for it, here is the link:
http://foy.foy.com/minircmod/moddocs...attcharger.htm. Temporarily, I used a 9 volt battery, and the car charger off of the actual controller itself. It works, and I charge it for about 2 minutes.
My car now goes as fast as about running speed, but I will still need to test that. Buuut, I hope you have as much success out of this modification like I did!