Note1: This is a relatively advanced instructable. Don't attempt it if you don't have experience with power electronics. The voltages and currents used in this project can be dangerous and appropriate safety precautions must be used. This instructable outlines what I did to make this project, but it is not a substitute for proper safety training in power electronics. Check with your local community college for availability of classes in your area.
Note2: In addition to the 48V battery voltage, this controller requires a 12V power supply. If your battery pack consists of 12V cells, then you can just tap 12V from the pack. This was not possible for my battery pack, so I used a separate DC-to-DC converter to supply the 12V power. See my other instructable on constructing this DC-to-DC converter.
Note3: This controller is over-designed for this application. The IRFP4468 FETs are rated for a maximum of 195 Amps (each) at 100V. This application will typically use less than 10 Amps at 50V. I have been commuting (10 mile round trip) almost every day for the past 2 months using this controller and it has been trouble free (knock-on-wood :)