I think it would have been better if you put pictures throughout your text because there are so many details we will get lost as we've never seen some of these things before.

Some ideas
#1 Bionx

This has regenerative breaking (page 9) can be mounted to your folding bike and is lightweight and I believe it has a 6v port on it and it can be put into regeneration mode while you are going down hill.

#2 Goal zero has a couple of packs differing in size that would work if you want it separate from the bike.

#3 Cellphone wrist holders