[nota]

Originally Posted by dad911

Using 18v battery on 24v mower shouldn't hurt either, will just spin slower.

18v battery on 12v bike could hurt bike (50% overvoltage)

Do you know how much current bike draws under full load? If it is under 5 amp, something like this would work: https://smile.amazon.com/DROK-Waterproof-Synchronous-Converter-Adjustable/dp/B00CGSHR2S/ref=sr_1_1?ie=UTF8&qid=1506392433&sr=8-1&keywords=18v%2Bto%2B24%2Bv&th=1

on line reports say the bike will live at 24v and go faster
so 18v Li [really 20v full charged to about 15v cutoff] should be fine

should be 320 watts at 12v =26+amps , but at 18v = under 20 amps