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Old Mar 29, 2010, 04:46 AM
Merlin45 is offline
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flyit, crashit, fixit.
crsahing around in mid-europe
Joined Jan 2007
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Quote:
Originally Posted by Alan Hahn View Post
I am not sure why you think smaller lipos, with C ratings (1500-15C, 1500-20C, equivalent 2000-15C) will provide the same current at the same output voltage as as a 20C 2200mAHr pack--which as you say is too heavy and big for the application.

It is obvious that the C rating (even though it is an ill-defined quantity) doesn't directly correlate into providing your definition of acceptable power--even the 20C 2200 pack would fail to provide your definition of acceptable power for the C rating I think.

I will agree with you that cost is not always a good indicator of the internal resistance of a pack. However it isn't completely anti-correlated either ("high"cost does not equate with high resistance).
thanks for taking the trouble to respond. I do agree with what you say (direct correlation/ill-defined etc).

Iím not saying that smaller will provide the same current as larger.
What I am saying is, that it seems that higher capacity with lower-C, is more likely to satisfy than a smaller capacity, even when compensated for by a higher-C rating. So better to buy a cheapo with more mAh and lower <C>.

My (no-name) 2200 20C pack supplies 20A (=9C) to the system easily, and I have taken that to be my target. A much larger battery would not give a higher Ampere reading, because the system is happy with 20A, but lower capacity, with higher-C should crack it, so I thought!

BUT, I had thought that a high-quality (?) Kokam 1500 20C pack SHOULD be able to supply 30A;- well letís say 25C to be on the safe side! However, I only require it to supply 20A, which to my thinking would be be absolutely no problem, IF the C-rating is realistic. It only managed 18A (=90%) though.

So, would a Kokam 1500 25C or 30C actually achieve the target? I doubt it, but donít know for sure. Not prepared to spend big money to find out, as the 20C doesnít live up to itís claim.

The fact that it can only supply 18A is what gets me. WHY, why, why??
Where's my thinking gone wrong?

From what Charles says, Kokam are just not all theyíre cracked up to be, which seems to answer the question. However, it also raises another:

Is this only a question of the claimed-C/or Cell-quality? and; is it unrealistic to assume that a 1500 20C (real) can supply 20A, or are there other, better Lipos? (or are they all unrealistic?).

Thanks for your thoughts on this.
Bob
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