OR rip the guts out of the controllers and make up your own box to hold the batteries and just fit any battery you like (such as lipo or lifepo4 or Nimh stick pack) as long as the voltage ends up the same...you could even glue together the lego to make the box to hold it all in.....think of it more like modding a car where you want a 5.0 V8 in the hole where a 1.2 would have been, it is just a matter of re-fabricating what you need to get the job jobbed.