BMS battery sell a 10ah 48v LiFePo4 battery for a similar price to a 12ah 36v battery. It seems to me that the 48v battery is a better bet, but it is 10% heavier. Also, I hear what you say about replacement. Are both these batteries likely to last the same amount of time?
It depends on what you want and what your controller is-
Going off the energy that the battery stores. 36V x 12Ah = 432 W 48V x 10Ah = 480 W ( +11%)
so if you travel at the same speed using both batteries you'll get 11% further using the 48V battery (which matches the weight difference).
If your controller isn't restricted you'll be able to go faster, but range drops with speed. Is your controller restricted to 15mph to comply with local laws?
Battery life will depend on the number of Amps that you pull, but because these two ratings are fairly similar then life should be similar. What is your controller rating and the battery rating? A battery rated at 2C* can give out up to 2 x its Ah rating for short periods without compromising life too much but should normally be under 1C* Higher performance batteries are available which are rated at up to 20C at higher cost. Anything over this and the battery life will be reduced more quickly.
* 36V 12Ah C is 12A, 2C is 24A. 48V 10Ah C is 10A, 2C is 20A
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