mcstar wrote:I think your idea is interesting in premise, but it may be that you'll get a much better experience using something that's lighter and better suited for the task at hand. For instance, 5AH of LiFePo4 at 36V would only weigh about 7lbs and would carry you at full speed both ways, no regen needed at all. You could get such a pack for about $200 from China.
JCG wrote:That is to say, a Li-ion battery cell, no matter how many cells are with it in series, can't charge faster than about 10 A without harming it and its electrolyte, which shows up as a loss of capacity over its lifetime: http://tinyurl.com/5g7kq9. Ultracapacitors can charge as fast as they can discharge - which is extraordinarily fast. The two modules I mentioned earlier can easily provide over 3000 A at short circuit... and will come back for more many hundreds of thousands of times. A data sheet that might prove interesting is here: http://tinyurl.com/6nlrw6
JCG wrote:1) Regen braking is a must. Not just that, but STRONG regen. I need a controller that can send up to 30 A or so back to the ultracaps when I ask it to (gradually at first, of course! Charging a capacitor is like a short circuit).
2) I also hope to be able to use regen on the fly. That is, to be able engage it while cruising on a flat slope, and pedal against the generator resistance to charge the caps slowly if they need it. I envision using a front hub motor, separate from the normal rear wheel and cassette, to do this.
3) Ultracaps, like any caps, will drop in voltage as they discharge. I'm planning on draining them to no more than half of nominal potential at any time (16 V). This would be accomplished by using 75% of the available energy (63 kJ). If my motor is cranking out 500 W, that energy should last up to two minutes in a single burst, but as the voltage drops from 32 to 16 V, the current would jump from ~15 to ~30 A, for example.
JCG wrote:3) Motor: brushless DC direct drive should be a good choice for regen, thanks Ypedal. I assume I should find a 24 V BLDC front hub motor; as it should be able to handle the max 32 V and won't switch off at 16 V (as long as the controller doesn't either), I guess?
4) Controller: I was looking at this one: http://tinyurl.com/6bv4yu, which I guess can fit in the voltage range and passes plenty of current. These Kelly controllers are supposedly programmable too; which sounds nice.
5) Throttle: I would love something that could control motoring and regen in one twist grip, but I have no idea where these are sold anymore, and if they would be compatible with the Kelly controller. Any advice?
Link wrote: Most hub motors are noticeably more difficult to propel than a normal hub just on their own. Though probably worse than most, mine drags me down to less than 10mph if it doesn't have power.
JCG wrote:Re: ruling out the hub motor, it would be better to use a chain-mounted motor instead? I can't say I'm looking forward to the added complexity...
JCG wrote:Miles: The motor drag got me thinking. Is there a rough estimate of the amount of force to be overcome by the hub motor drag at cruising velocity, or a guess at how much power is sapped by overcoming it? It may be possible to pedal the bike normally while drawing only a very small amount of power from the caps.
Miles wrote:JCG wrote:Miles: The motor drag got me thinking. Is there a rough estimate of the amount of force to be overcome by the hub motor drag at cruising velocity, or a guess at how much power is sapped by overcoming it? It may be possible to pedal the bike normally while drawing only a very small amount of power from the caps.
Maybe this will be a guide: viewtopic.php?f=3&t=7223
JCG wrote:Miles wrote:JCG wrote:Miles: The motor drag got me thinking. Is there a rough estimate of the amount of force to be overcome by the hub motor drag at cruising velocity, or a guess at how much power is sapped by overcoming it? It may be possible to pedal the bike normally while drawing only a very small amount of power from the caps.
Maybe this will be a guide: http://endless-sphere.com/forums/viewto ... f=3&t=7223
This is good stuff. Thanks Miles.
Common to all direct drive hub motors is that they are always mechanically engaged. This is both a good thing and a bad thing. It is good because it makes regenerative braking possible. All direct drive hub motors can be made to do regen whether they are equiped to do so or not. Both the BionX and TidalForce both have sophisticated controllers which allow varied braking energy to be stored back in the battery, while most of the Chinese kits like Crystalyte, Golden Island, and Wilderness Energy need 3rd party controllers to do so. Always engaged is a bad because it means that you are always overcoming the rolling resistance of the motor even when you are not using it. Depending on the symmetry and quality of the motor, this additional drag torque can range from imperceptible to feeling like you are always riding with a flat.
JCG wrote:Link - Thanks for all this. I was reading your posts in other places and was hoping you would chime in. I agree, the largest problem with the caps is the voltage drop with discharge. If the controller is ok with it (as the BYU EV-1 drag racer was: http://tinyurl.com/5w7oa5), then great; but if not there is always room for a small DC boost converter (I have some small ones on the way).
In terms of range, no problem. The caps should only be delivering power during acceleration, which should be less than 10 seconds a pop. Regen can occur whenever convenient.
Thanks very much for the motor suggestion, and I'm really glad to hear that the controller sounds good. This is the kind of direction I'm most in need of right now. I'll check on the throttle, my hope is that the Kelly controller might be adaptable to several different kinds of inputs. I'll try to find out from Kelly the usual way that regen is engaged using their controllers (unless someone here already knows). I'm starting to regret my chemical engineering training, only one circuits course...
JCG wrote:Link, if you pedaled your bike, and (assuming you had the ability to run at a very low throttle) supplied just enough juice from your battery to make it feel like you were riding a normal, unmodified bike, can you guage the amount of current you'd be using for a given pack voltage? We can use that to figure out a value for the parasitic power loss.
JCG wrote:Why is there always a good thing with a bad thing? It looks like I need to poll to see which hub motor has the lowest rolling resistance. Is there a favorite out there?
Miles wrote:Maybe one could get an accurate value by driving it with another motor, or weights and string.....
"I find that I can pedal with my 409 and hardly notice any drag." What figure do you put on that?
Anyway, it's the input power needed that we're concerned with, in this case.......
johnb wrote:The 40x motors have a cogging torque of about 0.5 to 1 N-m which translates to about 10-20 watts at 200 rpm. The 409 has the lowest cogging torque.
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