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Old Feb 19, 2011, 07:59 AM
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Lipo "C" Ratings - A Replacement Overdue?

I believe it is widely accepted that “C” rating of lipos is discredited as a measure of performance and is being used as a sales gimmick by less scrupulous merchants.

The “C” rating is the multiple of the rated capacity of a lipo which the battery is capable of discharging continuously over a full discharge.

A couple of years ago suppliers were claiming “C ratings of 30C when many of them were struggling at 20C and some I tested were only capable of 15C. Gradually development caught up an ratings were starting to look more realistic, but we appear to have now rushed off with another round of wild claiming with 45C, 50C and several claims of 65C.

Apart from being dishonest and misleading the customer, it is unfair on the few makers and merchants who are honest and state real “C” ratings – they miss out on sales whilst the “wild claimers” prosper.

In the IR Meter thread Mark Forsyth said that:-

“My personal internal resistance 'number' for high performance cells is 12000 / cell capacity, when measured after stabilization at room temp (~72F) for 1 hour minimum.. Anything around or below this number I consider to be outstanding performance.”

I very much agree with the principle and although we might debate the 12000 figure, ( he does say “outstanding”) it is certainly close enough as a start. Possibly 15 – 20,000 might be better for a “good” lipo.

On such a basis a number could easily be derived for any lipo cell which could be a “Figure of Merit” or FOM by dividing the 12000 figure by the product of the cell capacity in mAh and the IR of the cell at a standard temperature. This would give an aiming value of 1; lower would be below average and >1 would be good, >>1 outstanding.

Obvious disadvantage is ensuring constant temperature measurements but the advantage is that you don’t need to do a full power discharge to find out whether the rating is genuine or just sales splurge.

I wait to be torn to pieces! – please try to be constructive.

Wayne
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Old Feb 19, 2011, 08:17 AM
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It'll never fly, Wayne. That is far too sensible, logical, practical, and informative. But above all, it would be way too hard for marketing "experts" to just throw out any meaningless number they care to, knowing that most people have no way of checking on its validity or don't care to.

Which is why I'm so looking forward to receiving the IR meter you have on its way to me.

Maybe some day we'll see a bit more "truth in marketing" than there is at the moment. Suggestions and contributions like you have made are a first step in that direction. We can only hope...
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Old Feb 19, 2011, 08:21 AM
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For many manufacturers C rating is calculated from the temperature grow during a full discharge. They think that way: if delta temperature stays below e.g 50 deg. Celsius you can pull that so called continuous current.
It says almost nothing about cycle life and calendar life.
I've got a bad feeling that the formula given above is derived using this oversimplified approach.
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Old Feb 19, 2011, 09:24 AM
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The world of advertising is focused on selling products, not making sure the consumer is well informed. It's just a numbers game they play and if we don't want to lose to them, we must be we informed and not just follow blindly.
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Old Feb 19, 2011, 10:11 AM
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Brilliant Wayne, simply brilliant!

While I would be completely and utterly shocked if manufacturers ever adopted an objective rating system that could be independently verified by end user and used as a valid comparison tool of various battery brands, I'm hoping that an IR-based objective basis will gain traction among informed consumers.

I, for one, never purchase packs unless I have seen published IR measurements or discharge curves (thanks Charles) from a trustworthy source.

Mark
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Old Feb 19, 2011, 10:26 AM
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Quote:
Originally Posted by Wayne Giles View Post
Obvious disadvantage is ensuring constant temperature measurements...
Question for you, Wayne (or for others who might know). In your experience, is there any kind of consistent relationship between IR and temperature? In other words, would there be any reasonably simple way of correcting measurements made at different temperatures to a "standard" temperature?

I realize doing something like that would add a bit of complexity to your formula, but temperature would of course have to be taken into account in some way. Applying a "correction factor", if such a thing exists, would be easier than trying to make measurements at a certain temperature.
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Old Feb 19, 2011, 10:32 AM
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It will never be standardized, but 25C Gens Ace LiPos outperform "40C" LiPos.
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Old Feb 19, 2011, 10:52 AM
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Quote:
Originally Posted by SYMAWD View Post
It will never be standardized, but 25C Gens Ace LiPos outperform "40C" LiPos.
Can you please direct me where you read that as im in the process of buying some Gens lipos thanks
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Old Feb 19, 2011, 10:55 AM
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Quote:
Originally Posted by Harry D View Post
is there any kind of consistent relationship between IR and temperature?
There is a strong correlation.
Temperature increases (no more than 60 deg. Celsius) - IR decreases.
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Old Feb 19, 2011, 11:03 AM
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Well, yes, I know that. I was asking about some numerical correlation.
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Old Feb 19, 2011, 11:06 AM
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Arrhenius equation?

ln(1/R) ~ 1/T
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Old Feb 19, 2011, 11:17 AM
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Quote:
is there any kind of consistent relationship between IR and temperature? In other words, would there be any reasonably simple way of correcting measurements made at different temperatures to a "standard" temperature?
I wonder how many data points that would require?

While I agree that IRs are a valide indicator ,up to a point and that Ohm's law will let one calculate what a given cells or battery is capable of at X amps. there is more to it that that IMO.

A lower IR will result in less heating and in some cases at low to moderate discharge rates or lower ambient temperatures may in fact result in less voltage being delivered. In other words heating does lower IR and thus cells perform better.

I saw this often years ago while doing CC discharges with lower C (higher IR) cells. They would hold more volts under load near the end of discharge and often deliver more capacity also.

An extreme example is when a LiPoly will drop below X volts and then recover and deliver more volts through out the remainer of the discharge to near the end where the knee (sharp turn down point is reached.

Example of major drop and then heated rebound

http://www.rcgroups.com/forums/showp...9&postcount=46

A more moderate example

http://www.rcgroups.com/forums/showp...7&postcount=23

I have also seen LiPolys with cells at extremely close IRs for all cells at 72F have a cell increase by 25% or so over the others at 90F however the cell with the higher IR in fact often out performs the others during a CC discharge and that is hard to account for using just cool IR readings.

Attachment 3 shows inbalance ,attachment 4 shows matched IRs. CC discharge shows very good load sharing of cells down to point that discharge should have been stoped.

http://www.rcgroups.com/forums/showp...95&postcount=1


Charles
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Old Feb 19, 2011, 11:17 AM
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LeszekJ, Is that an answer or another question?

What I had in mind was a simple correction factor, confirmed by actual, practical IR measurements on the lipo cells we are talking about here, using measurement devices such as Wayne's IR meter, that could help in resolving the temperature issue that he mentioned.
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Old Feb 19, 2011, 11:25 AM
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Quote:
Originally Posted by Harry D View Post
Question for you, Wayne (or for others who might know). In your experience, is there any kind of consistent relationship between IR and temperature? In other words, would there be any reasonably simple way of correcting measurements made at different temperatures to a "standard" temperature?
Harry,

I did some investgation on this a couple of years ago and expected (I never learn!) that the plots of IR v Temp. for different Lipos would be parallel but at different levels. If you look at the plots attached you can see that the two lower ones (A and B) are approx. parallel, but the third one (C) is way off and actually crosses plot B.
I think that makes it impossible to have a simple correction factor for all packs.
'A' was a Loong Max pack, 'B' was an Electrolite and 'C' was a pack of unknown origin (Chinese) I was testing for a third party.
You can see from these plots how important temperature is in such measurements and is a weakness of my suggestion for a FOM. To obtain valid results the IR must be measured on a cell which has been soaked at the specified temperature for several hours. This plot took me days to produce.
BTW I know suppliers will never fly it, but at least it draws attention to the situation.

Wayne
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Old Feb 19, 2011, 11:58 AM
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Graph showing changing IR at different temps.

http://www.rcgroups.com/forums/attac...0&d=1281885883

That is the same battery that link in above post showed discharging evenly during most of the discharge.

http://www.rcgroups.com/forums/showp...95&postcount=1

Third attachment.
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