I'm no stranger to building large battery packs - considering the largest packs we build total around 8.3 kWh. I'm taking a new approach to our next prototype ebike build, though - building an ebike as efficient as possible. We built a 36v 15ah pack special for one such ebike. It will probably actually have three wheels, and be designed to lay down on to facilitate getting the rider out of the airstream as much as possible.
The battery pack is a nominal 577 watt hours (it's actually 37v nominal, 15.6 amp hours but we typically underrate our packs) and weighs in around 7 lbs. This battery, on a trike like Odyssey's Mk. 5, would be good for a range of approximately 25 miles with an energy economy of 20-25 watt hours per mile, but I am aiming for an economy of 10 watt hours per mile initially (a 57 mile range) and upwards of 5 wh/mile (114 miles) after all the cowlings are in place and the vehicle has been tuned to be as efficient as possible. It's going to look like a streamlined street luge, with many of the components onboard 3d printed.
One of the biggest considerations right now is the drivetrain. I've already settled on using a three phase motor (called "brushless DC" because the motor operates as if it's a brushed motor but it's commutated in the controller externally) instead of a brushed motor, but I haven't decided on a method of power transmission yet. I'm considering friction drive - while the transmission of the actual power may be less efficient than a belt drive, the motor can be retracted the second the torque transfer is gone. If a "pulse and glide" concept is utilized, the duty cycle might only be in the 25% range - leaving absolutely no transmission drag 75% of the time, and a very light weight required for the "transmission" on top of that.
I'll write more blog posts as this build continues.