neptronix said:
C rate does matter. It means that the cells perform better even at lower discharge rates. If i run my cells at 10C constant, they perform horribly even though they are way under spec. They just get warm and turn battery charge into heat.
Heat is not always your enemy. Did you know that many EV's have pack heaters that draw additional current to heat the cells so they will perform better? Lithium and Lead Acid actually deliver a higher voltage under load, and more WH, when heated up. If you want to extend the cycle life of your cells, making the pack bigger to reduce your depth of discharge has a big impact - buying higher c-rate cells to reduce heat at low-c discharge, not so much.
Anyhow, first I have to ask why you would be running at 10c continous in the real world, which would discharge your ebike pack in
6 minutes? That is a very short commute. It is my opinion that to be useful as a commuter, an ebike needs to have at least 1/2 hour of runtime, YMMV. That 1/2 hour run time translates to a 2c continious discharge.
I think what you are saying is that your 20c lipo, when discharged at 10c, has a high voltage drop and generates a corresponding heat load. What you are asserting, based on that, is that higher C batteries have a lower internal resistance, which generates less heat during discharge. However, this is NOT always the case, some cells may be better able to dissipate heat and hence have a higher C rating - assuming the manufacturer is not just making up the specs anyhow. Given the range of IR available in current chemistries commonly available to this hobby, yes the effects of IR will be significant at a 10c continious discharge, but that is (5) times typical demand.
At lower C ratings on quality cells, the impact is so small as to be insignficant. It would be interesting to see the differences in voltage drop/heat generated, say between 20c and 30c lipo discharged at 2c. I would expect differences to be very very small, despite a 50% increase in C rating - and this would debunk your assumption. A higher C-rate gives great bragging rights, but it does not make a significant difference at real-world discharge rates.
Of course, there are always crappy cells out there that will heat up like a toaster oven. I figure the cheap cells of both chemistries don't fit into this discussion. I only run midrange LiFe (>=5c) and above, I don't buy the 1c/2c/3c lowrange LiFe because it is heavy and the cost savings over Headways or a123s is not enough to make lowrange worthwhile to me. Most people run "safer" HobbyKing LiPo, and don't run cheap no-name 1c/2c/3c LiPo because it is more likely to burn.
neptronix said:
like for example if you wanted to drag race etc.
We might be arguing different points. Note that my post said. "My personal belief is that c-rating is largely irrelevant in EV's - unless you are building an exhibition bike that only runs for 2 minutes." Drag racing would fit into the exhibition category.
However, if you build an exhibition bike that needs to discharge at 10c cont, then you anticipate how the cells perform at that rate when designing the pack:
1) Determine the target voltage you want on that system, and build to that voltage at the discharge rate you require. If you want a pack to run at exactly 48v, and the cell runs at 2c=4v/10c=3v, then build 10c pack with 16 serial, and a 2c pack with 13 serial. Both packs will deliver 48v for most of the discharge cycle, because you have sized the voltage drop to the load.
2) Ventilate the pack accordingly. Heat is expected in drag racing, one of the billion things one has to accomodate when running hardware close to its limits.
neptronix said:
...i worry about calendar life more than cycle life since i am a fair weather ebiker and not a daily rider.
Then, even if c-rate had the level of impact you assert, it does not matter to you either, right? Since you expect your cells to die of age not cycles, does it even matter if your cells get warm at 2c?
-JD
