Thread: Discussion A simple LiPo performance tool
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Old Jan 23, 2012, 02:23 PM
Wayne Giles is online now
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Quote:
Originally Posted by Alan Hahn View Post
I applaud your effort, but one question I have is how are you going to define "C"?

The only actual definition I have ever seen was off a Lipo "bible" on the old FMA site. I believe the definition was due to Kokam.

Anyway the definition of "C" was the max current draw that would bring the total discharge down to some fraction of the total cell capacity(it was close to 100%, but I don't recall the exact %), and keep the pack (maybe cell) temperature less than ~140 degrees F.

The assumption was anything above this temperature will damage the lipo. Other specs gave the ambient starting temperature, and the allowed (if any) air cooling

Sorry I don't recall the numbers better, but I did go and look for it, but the info seems to be gone now.

Presumably that is what "C" originally meant. Clearly it is related to the cell internal resistance, the temperature coefficient, and size of the cell. Also "damage" has a wide degree of interpretation---immediate damage, decreasing the # charge/discharge cycles.........

So to be meaningful, I think you need to get a handle around what we actually mean by "C". Not too sure how you are going to do this, simply because some people expect hundreds of cycles, some seem to be happy with 60. To these people, "C" would have a completely different significance.

Anyway, good luck!
Alan,
Hope John doesn't mind me jumping in here; I haven't asked him, but I think I understand what you are saying. We should really separate how the makers specify "C" ratings and how the modeller understands them.
The modeller is told and understands that the C number is the multiplier he can use to determine the max current he can take from his Lipo.
Various ways that makers specify it, and I have read several, all seem designed to avoid anything that can be measured by the user. Equally, they are irrelevant to any practical situation in the field.
The idea of the Mark's calculator is to produce realistic "C" number, as understood by the customer, that he can use to calculate the maximum sensible current he can take from his Lipo, at a reasonable efficiency and without unduly stressing the pack.
Experience has shown that just putting the IR and cell capacity into the calculator will give an answer which is surprisingly accurate over a wide range of lipos and capacities.
The few honest suppliers whose packs really meet the C ratings stated are losing out to the many suppliers who make such wild claims that are patently laughable.

Wayne
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