I have thought about this too as I think I have a battery about to go. Lithium batteries are nothing new to me as i have dozens of them for my model airplanes. It wouldnt be that hard to make one able to do what it needed to do for a bike, the hard part comes in with charging them. They do not like to be over charged at all, over charging a lithium polymer battery will most likely end up in flames. I have been looking at the A123 systems M1 lithium cell as we use thoes too. Much more forgiving and can sustain some pretty high amp draw but still would need a deticated charger on the bike. I bet the comercial ones mentioned have a charging circuit built right into the battery, probably why they cost more... I assume they are using lithium polymer batteries in them and even though they are pretty tough batteries they will start to get weak after about a year of constant use...(think laptop battery) I like to have a go at building one but to many things on my plate already as is..

Darren
IN-S40