The unit handles 26 volts with Li-ion ok, but I think 33 volts will be going too far and could blow the mainboard. The "controller" is like no other, being more like a computer motherboard with a CPU managing a whole variety of functions.
Here's a pic of the
mainboard.
Obviously new NiMh cells will improve the performance, but having a greater range of gears often impairs the range on these bikes. The original 3 speed Lite which gave around 20 miles in a typical set of conditions would drop to about 17 mile range with a 5 speed hub, though this difference varies according to how hilly an area is.
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