A few year ago, I made some electric bikes for my parents. The black bike was made from only parts, where as the red bike was really a conversion from a regular bicycle. I did add new brakes and a luggage rack.
The bikes are powered by 10Ah 44.4V Lipo packs that weigh just 3kg. That’s amazing power density. Not the biggest battery I have assembled but it’s still very light for that much power.
A note on making batteries. Please be careful. A momentary slip of your
soldering iron or hand can cause a major short circuit melting things. You can get a very bad burn as the battery terminals melt or be injured in a different way. Just be very careful. I have gotten a very bad burn from large cells shorting before and it sucks…a lot.
I’ve made 2 x 15Ah 72V battery packs before and they are pretty cheap to put together. The main difference when you get larger is that the standard RC LiPo charges can’t handle the number of cells in the pack, so you have to add a battery protection board to monitor and balance cells while charging. It also prevents over discharge of the cells.
UPDATE: This article was written a few years ago. I only recommend using 18650 cell battery packs. I no longer make LiPo based packs for bikes