Good morning Frank....AH is the power the battery has, it is the volts that you need to be careful about. A 36V battery will not work in a 48V bike, not enough voltage just as a 52V+ will not work as there is potentially too much voltage going to the electronics. It may work for a time, but you run the risk of frying the electronics.
If you have a low AH your time that you will be able to ride will be diminished as the battery has a low capacity. Same as with a car battery, a smaller battery will work, but it will not turn over the motor as long as a higher capacity battery.