According to 300mpg.org when doing a conversion, volts ~= speed. The advantage of converting a scooter and not a big bike, is that no one is expecting very much speed, so we can get away with a lower-voltage system. Even once you have decided your voltage requirements there are a whole load of other things to think about: 1. Lead Acid or Li-ion? Lead acid has the advantage of being cheaper, but the major disadvantages of being bigger, heavier and holds less power - and also if you drop the bike you’ll end up with acid all over your shoes! Li-ion is more expensive, has more difficulties with charging and discharging, but is lighter/smaller per KWh of capacity. 2. BMS Part of the problem with Li-ion batteries that they are made up of lots of little batteries, each of which must stay within the prescribed charge and temperature ranges or else and also be as close as possible to the other batteries in the pack. Most big or EV batteries are just clumps of much batteries wired in series and parallel to provide the current voltage and amp hour output. A BMS is a little circuit board that ensures that each of the cells is equally charged, and that the power is not being drained from one faster than any other. 3. Form factor Building a battery is, therefore, possible. It is probably the best way of getting the right size (as well as discharge rate and capacity) of battery we need. However, building a battery is also dangerous (high voltage) and requires specialised tools (like a spot welder). It would be best, therefore, to find a method of building a battery that doesn't require spot welding.