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
True, caps are more power dense than batteries, but, FWIW, even the highest-powered bikes around here don't use more than 100A max. Plus with batteries you don't have to deal with the whole voltage/remaining capacity relationship.
84kJ might be decent for a cap, but it won't get you very far. 84kJ works out to about 23Wh, and that's only if you drain it all the way flat. The average ebike uses 25Wh to go one mile. I have a 72V 4.5Ah SLA pack that I used to use on one of my bikes. Accounting for Peukert and voltage sag, I can get about 500kJ out of it. This was just BARELY enough for me to make the 4 mile or so trip to work, and I have no hills, usually no wind, few stops, and was somewhat gentle on acceleration.
Might need a little more supercap, but I like the idea. I contemplated a small bank on my bike to store regen energy and use it either for acceleration or in conjunction with my battery pack for a short boost in top speed once, but tossed the idea since it wouldn't be worth the ~$200 it would have take me to do it.
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.
1. 30A isn't much to ask from a motor being used as a brake.
I think there's a Kelly or two that can handle it without issue.
2. You're probably not going to want to pedal against the motor while it's acting as a brake unless you're really itching for a workout. 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.
3. A motor will generally draw whatever the controller allows it to. My "500W" Golden will pull 2kW peak. The Crystalyte 5-series motors (rated for 750W) will handle 5kW+ no problem. Doc's reaching the limit on his, but he's running 100V and 100A.
You'll also see less power as the voltage in the caps drops. Since the amperage is what the controller limits, you could potentially draw 3.2kW from a 100A controller at full charge, but that will drop to 1.6kW from the same controller further down the line.
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?
3. Yah. But it doesn't have to be rated for 24V. Motors don't care what voltage they get as long as it's not enough to destroy something, so any one will work. The C'lyte 4XX motors are about right for what you're looking for. Maybe a BL-36, too.
4. That controller will work. Kelly makes good stuff.
5. Me neither. Only thing I know of that works like that is the one on the Vectrix. You might be able to get one from them, but I'm not sure of its technical details or compatibility with the Kelly.