put our Heads Together
Join Date: Jun 2003
Location: southeast pennsylvania
Bikes: a mountain bike with a cargo box on the back and aero bars on the front. an old well-worn dahon folding bike
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yes, in 20 miles of riding, it's possible to charge a battery enough for one mile of electric assist. But it will make your over-all trip more difficult. Here's why:
The amount of energy you get back after using your e-bike wheel as a generator is about 30% of what you put in. Some energy is wasted as heat in the process of converting motion to electricity in the motor. Some energy is lost as heat when the electronic controller converts the alternating current electricity coming from the motor into an appropriate direct current voltage for your battery. Some energy is lost in the battery itself. Also, if you use gearing to run a smaller motor at higher RPMs (which makes higher motor efficiency possible) you will lose some energy in the gearing.
When you reverse that process to use your motor to help you, each of those steps involves some energy loss again.
Another factor going against you is that a battery loses some energy if it is left sitting for just one day. In my experience NiMH batteries will lose most of their energy in just a couple weeks, although lithium batteries are much better.
So on the whole, it's a much more efficient option to just ride slowly, in a low gear, on the up-hills and ride a little quicker on flat ground, rather than to use electric assist on the uphills and pedal against electric-regeneration when you are not on an uphill.
If the whole process of generating electricity and then using it to power your motor were something more like 90% efficient, then you would probably be better off using the motor in that way due to the way that wind resistance works. That kind of efficiency might be possible if you were willing to pay millions of dollars for parts and engineering, since small affordable motors achieve efficiency of up to 96% or so (not including energy losses in controller and battery)
Last edited by cerewa; 04-24-09 at 09:23 PM.