KERS System-LiPo, LiFePo or supercap

[Tiberius]

I'm looking at a special application, that is probably best described as a mini KERS system (Kinetic Energy Recovery System as allegedly used in F1). It has to deliver a short intense burst of power and then spend a slightly longer time recharging.

Assume the voltage is around 100 (that's not actually so important). The outgoing power burst is current I for time T. The recharge time is 4 T.

In this case the minimum useful is I=60 A, but I=120 A would be better.
At first I thought T would need to be 10 secs or more, but that is based on a KERS system being used once per lap. If you think of it being used once per corner then T=1 or 2 sec is useful.

So one configuration could be 60 A out for 2 secs then recharge at 15 A. What cells would you use for this? If the battery could stand 40 C discharge, then 1.5 Ah cells would do. But how many cycles would that last? It needs to be in the many 100's to make it viable.
If I look at a supercap, then I calculate that about 10 F is needed. How practical is that?

There is of course another approach. That is to say that even though T=1 sec is useful, more is always better. Why not take it out to T=10 mins? That then needs 10 Ah cells and a huge cost and weight penalty. But the rate of cycling the cells goes way down, the discharge is only 6 C, and so we'd expect a longer lifetime. What would be the best cells for that option?

Thoughts, comment, advice is invited.

Nick

 

[liveforphysics]

I think reaching the limit of traction/stoppie/wheelie happens much sooner than you would be over loading something like charging 40c LiPo cells in a 100v string. LiPo cells rated for 5C continuous charging easily handle 15C burst charging.

Ultimately, what you're looking for is simply the lowest Ri solution to make 100v in the lightest possible package. Supercaps have crap specific Ri, and crap energy density.

4 of these packs would give you 100v peak, only cost a bit over $100 USD, and IMO they would work as well as anything could work in a 1,300gram solution that could fit in your pocket. You could run these packs 2P if you wanted to cut Ri in half, and use 8 packs for a 2,600gram solution.

http://www.hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=9940

Or, if you want lower Ri and more energy storage at the cost of additional weight and expense:

4 of these:
http://www.hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=10304&Product_Name=Turnigy_4000mAh_6S_40C_Lipo_Pack


Or, you could just run a LiPo pack as the main energy storage method for the vehicle.

 

[Tiberius]

Thanks Luke, I kind of expected LiPo would come out ahead of a supercap. Are those a recommended make of LiPo? How many cycles would I expect to get out them if I tortured them in this fashion? Number of cycles could be crucial if they take a hit every corner.

As for the wheelie/stoppie/traction thing, that's not an issue. I'm not talking about a wheeled vehicle. But you are right that weight is very important; its a sort of racing flying machine.

Nick