I had a thought while riding. I have an old set of 24V 3.3A Nickel-metal hydride (NiMH) batteries which I used to use with my old Amplifier A7 stand up scooter. They are very small and compact. Two of them in series would produce 48V and be very easy to easy to place and carry on the ebike.
I discovered during my first week of commuting with the ebike that I was only using 4AH on the way in to work and 4.5AH on the way back. This 48V NiMH pack would provide enough juice if used judiciously.