Electric Bikes - Possible to recharge battery without plugging in?

Is it possible to completely recharge a battery just from braking and downhill riding on a roundtrip commute? I like the idea of some extra power on windy days . I also like the idea if I can get that power all from my own efforts. It seems bionx has a nice kit and only weights under 15 lbs.

It's an interesting goal but I'm not sure it's one I'd strive for. If you want to leave with a fully charged battery, return with the same, and not plug in along the way, why not just ditch the weight and cost of the electric system? You end up in the same place with money to spare. That's assuming it's a round trip of course.

It's an interesting goal but I'm not sure it's one I'd strive for. If you want to leave with a fully charged battery, return with the same, and not plug in along the way, why not just ditch the weight and cost of the electric system? You end up in the same place with money to spare. That's assuming it's a round trip of course.

Well for me 80% of the time I would be willing to sacrifice some speed to charge my battery. I only need an extra boost probably just 10% of the time ( i.e going uphill or strong headwinds ).

Is it possible to completely recharge a battery just from braking and downhill riding on a roundtrip commute? I like the idea of some extra power on windy days . I also like the idea if I can get that power all from my own efforts. It seems bionx has a nice kit and only weights under 15 lbs.

No. It would require way too much effort.

The crux of the problem lies in the round trip. Your friends going to work, like downhill grades, are your enemy coming home, and vice versa.

The crux of the problem lies in the round trip. Your friends going to work, like downhill grades, are your enemy coming home, and vice versa.

I think you misunderstood my question. Say my roundtrip is 20 miles. I only need a 'boost' for 1 mile. So during the rest of 19 miles , wouldnt I be able to sacrifice some speed to charge the battery for that boost?

duke_of_hazard,

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)

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.


thanks, that does make sense. I guess the human body is much more efficient than any battery/motor.

The motor creates drag and weight, so you have to turn it on to ride. You also need to have it on to utilize the regenerative braking feature. If you can adjust the power, you can use a much lower level of support or none. You would have to use a throttle or higher setting for the hills/winds. The throttle burns a lot of energy, but you could keep energy use low for the rest of the ride. With the braking, it might almost regenerate the same amount of energy used. You'd still neeed a little charge at home, though.

I think you misunderstood my question. Say my roundtrip is 20 miles. I only need a 'boost' for 1 mile. So during the rest of 19 miles , wouldnt I be able to sacrifice some speed to charge the battery for that boost?

I understand your question.

Elevation is elevation not matter how you cut it. Both sides of the ascent are equally cruel in terms of energy.

I suppose it's possible to contrive a round trip that would recharge the battery but my assumption is you don't want to expend more personal energy with an eBike than a bike.

I think your goal is worthwhile if it makes the impossible possible, that 1 mile simply cannot be done without assist.

I guess the human body is much more efficient than any battery/motor.

The issue is really that if you're using a motor to capture energy that comes from human leg-power, you are ADDING the energy-losses of the motor+battery to the energy losses that were already there. If you just don't use a motor, you don't lose any energy in the motor.

Apparently humans can convert food energy into motion energy at about 20% efficiency.

If you consider losses along the system of fuel => powerplant => electric wires => charger => battery => motor controller => motor, the over-all efficiency is much lower than 20%.

If you then consider the fossil fuel energy that goes in to creating the average USA person's diet, the efficiency for the human is also far worse.