With such water volume variation, it seems that a Banki maybe the logical solution.
I DO NOT SUGGEST A PELTON, a Vertical Turgo is a possibility.
You need a carefull analysis -- 460 to 90 watts (real around 430 to 60 Watts).
My suggestion is a high voltage generator, 240 Volts AC like a 1.5 HP induction motor as a generator, or a PM 3 phase brushless Smart Drive motor from Fisher & Paykel with two set ups, DELTA for high power and STAR for low power and transformers at the load to bring the voltage to the charger ( I suggest a C40)
Let me know your plans and maybe I can help you -- where are you located ?.
If you use an AC induction motor, we could test a prototype ELC( Electronic Level Controller) to regulate the AC output voltage level.
Regards
Nando