the hybrid motor system seems to be on a lot of peoples minds. It would probably have to be pretty pricey for a few reasons.
1. You wouldn't get away with a lead acid battery, you need to run a generator for four to six hours to charge those babies. It might extend the range, but it won't continuously charge. Therefore think of a Lifepo4 of NimH setup (priuses use NimH)
2. You will need a decent generator for the voltage to charge the batteries. This will add weight and slow you down.
3. Regenerative braking should be strongly considered. A kit like Bionix would be helpful for this but also expensive. Hopefully all of the charging you would gain by trying to slow the trailer with a big generator would be beneficial.

Thats all
let us know how it goes though, I think their are quite a few of these projects underway. I'd like to see one turn out well.

ebikehub.

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The grand central station for electric bike information.