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Old Mar 09, 2005, 07:18 AM
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LiPo "C" Ratings: Useful?

Quote:
Originally Posted by nakelp86
... Im looking at about 2000mah or more Lipo packs for brushless motor.
let just say Lipo is rated at 15C so max discharge would be at 30amps, right?
so ESC could be 30 or 35 amps. for guarding low voltage I will buy protector.
Now to charge this pack and lets say it is 2cell pack, how do I calculate very, I mean VERY safe charge rate? :-)

A very safe charge rate is a matter of definition. Is “safe” that rate which maximizes cell life? Quite safe would be capacity/2, so, for a 1000mah LiPo, 500mah would be quite safe. If it means avoiding possible provacation of a fire, charging contemporary LiPos at 1C is safe. C, the measure of a LiPo’s capacity, is generally quite accurate in supplier’s advertising

But look in the Battery Graph Vault thread - It will be apparent to you that "C" ratings (discharge RATES, defined in terms of C) are generally misleading, at best. So, you need to be even more concerned with calculating a safe discharge rate that with a very safe charge rate. Relying on supplier’s “C” ratings can get you in trouble, unfortunately.

Take the new TP ProLite cells for example: tppacks.com advertises them as “conservatively, 12-14C”. But Charlie Wang specifies discharge cutoff short of capacity at 12 C, and even shorter at 14. These cells will destroy themselves at those discharge rates, if continuously discharged for their full capacity, while remaining well above 3 volts/cell in series, the widely accepted cutoff voltage for ESCs and low-voltage protectors.

See Posts 1, 2 and 30:
http://www.rcgroups.com/forums/showthread.php?t=342795

Post no. 30 appears to implicitly define C rating as the percent of capacity at which the cell reaches overtemperature, not, as has been understood heretofore, the percent at which the cell drops below 3 volts. So, there’s more indefiniteness introduced into the C rating concept; It’s getting more like “Alice in Wonderland” every day.

C Ratings should be abandoned, IMO, and replaced by graphs. In the case of the ProLite cells, the graph would terminate the 12C curve at 90% (1800mah for the ProLite 2000) and, for the 14C curve: 70% (1400mah). The termination would be labeled “Maximum safe temperature reached”. That would give us really useful information, whence we could apply the cell’s safe operating envelope to our application.

- RD
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Old Mar 09, 2005, 07:53 AM
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I agree that some people are only going to be satisfied when they can see accurate discharge graphs of various cells in order to be able to compare every detail of cell performance. It would be good to have this data available for those who are really serious about product comparisons.

However, the average consumer is not going to take the time to do such detailed comparisons any more than they demand performance graphs of computers, TVs or other consumer goods. The average consumer is going to want a simple number to use as a guide.

So, detailed graphs do not eliminate the need for a meaningful C number that will be of more use to more people. We should not give up on our efforts to encourage the industry to clearly define a meaningful C standard. This requires setting a percentage of capacity and minimum voltage number that a cell will deliver at its maximum rated continuous current delivery.

Various suggestions have been made:

The most commonly mentioned capacity percentages under maximum continuous discharge have been 80%, 90% and 100% of rated cell capacity as determined by a slow (.2C or .5C) discharge.

The most commonly mentioned minimum voltage numbers under maximum continuous discharge have been 3.0V, 3.2V and 3.4V.

If a consensus could be reached on these numbers, and if those who really care about this would only use such numbers when assigning a C number to a cell, some of the typical consumers who read these forums might start asking vendors to provide a C rating based on these numbers. If one vendor adopts this standard, they may pick up business and may force other vendors to follow to avoid losing business.

A good discussion on what makes most sense for these capacity and voltage numbers will hopefully lead to the same kind of consensus that is developing around the CBA testing being done by various forum members.
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Old Mar 09, 2005, 08:00 AM
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Very valid points but to date it is has been vary hard to convince Li Poly users that Li PPS are not all the same and that the newer generation cells behave very differently and require different handling to get max. performance and life from them.
I still contend that it is best to use 70-80% as a base for usage. Just because a cell will deliver 10C does not mean it is best to use it at that level. Look at my chart of TP730 Pro Lite (Beta) . That packs while graphed at 6amps.(8.22 C) has been flown mostly in the 2.5-4A(3-4.25 C) range. Two hundred cycles and still out performing any other new pack in its size range which I have seen. Who can say it would last as long being used at 8-10C continuously.
I do agree that graphs are much more revealing than C ratings and I am glad that others are benefiting from the fruits of our labor.
Link to TP 730 ProLite (Beta) Graph.
http://www.rcgroups.com/forums/showp...&postcount=156

----
Dave here is a paste from one of my post in the Tanic versus PolyQuest
Thread:
I agree with the comments on the past generations of 10C cells but from RD B's Graph and my calculation:

Added: RD B's graphs show Tanic 2150 dropping to 3.2 at .3 Ah (14% of cells capacity)
iRate 2200 dropping to 3.2 at .6 Ah (27% of cells capacity)
Thunder Power Pro Lite 2000 dropping to 3.2 at 1.9A (95% of cells capacity)
All of these at 10C.
-----------
I could be off a little on these calculations due to the fact that a mostly flat line crowded graph is hard to read with any great degree of precission but they should be close enough,at least Charlie should be happy with them.
Charles
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Old Mar 09, 2005, 08:47 AM
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Dave, problem is, the C rating definitions you mention will not preclude failure of the ProLite cells, because they can perform as specified regarding discharge rates above a defined voltage for nearly full capacity. They are failing from OVERHEATING, while discharging according to specification.

- RD
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Old Mar 09, 2005, 08:53 AM
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Not incidentally, All my temperature readings on all my graphs are suspect in the extreme; they are probably way too low.

Both Charlie Wang's and Brad S's professional equipment show much higher temps. at corresponding points in their discharge graphs. Charlie says the graphs per se agree quite closely.

It remains possible, as I originally suspected, that their is a failure mechanism other than, or in addition to, overall high temperature, in these lightly constructed, very high discharge-rate cells.

- RD
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Old Mar 09, 2005, 08:56 AM
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There will always be those who fail to read and or follow instructions however:
TP
IMPORTANNT SAFETY INSTRUCTIONS AND WARNINGS v 1.01

Operating Temperature

2} During discharge and handling of batteries, do not exceed 160 degrees F.

------------------------
I don't know about the rest of you but I have no intension of knowingly being in use of a 160 F LI Poly battery.

I think it is also important to point out that just because ,you shorted,over discharged ,damaged in a bad landing or by droping a LI Poly battery
and it did not slef destruct right then and there does not mean that it is as good /safe as new.

Charles
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Old Mar 09, 2005, 09:00 AM
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Good point, Charles! But along the lines of Dave's comments: How does the Tyro measure temperature in use, even if he's aware of the need?

- RD
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Old Mar 09, 2005, 09:18 AM
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What you need is

Maxium safe power desnity (watts per lb) for continuous operation without overheating
Ditto, but 30 second burst.
Derating factor for 'pack' operation.
Energy density at 50% maxiumum power rating as defined above. (watt hours per pound or some such)

By taking POWER rather tha CURRENT you get a figure that incorporates voltage sag. It also defaltes those who would tout '15C' cells that sag far more and weigh far more (amd cost far more) than equivalent slighty largeer capacity '10C' cells.


If you assess what sorts of uses these batteries are put to, those are really the tWo performance specs that are actually relevant.

Competition flyers who are not looking for very long motor rUns - maybe 3 minutes or less - only really care about watts per pund in short burstS, and are preapred to sacrifice pack lifetime to get it.

MOST other fliERs want energy desnity. If you are looking for 10 minute plus flights, at say 50% throttle, you are not going to be running at more than 10C maximum 4C average anyway. So > 15C type cells are a bit of a waste. What is more important to most people is a rugged cell that does 6-8C, maybe 10C without sagging too much, and lasts forever at 4-6C. And weighs in as light as possible and costs as little as possible.

That's energy density - as long as the cell will do 8C, energy density, reliability and cost are really all that matters.


I've got a spreadsheet with loads of cless on it, but it needs real wold data from everyday fliers graphs.


What you need for max power is voltage and current that didn't overheat the cell. and cell weight.

Wht you need for energ density is the integated power over the discharge cycle at say 6C to a 3v cut off point.

I am not clear what these CBA gadgets put out, but a little software could do both and come up with a figure of quality.


Key in weights, and prices, and the figures in terms of power and energy versus weight and price just drop right out.
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Old Mar 09, 2005, 09:19 AM
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Quote:
Originally Posted by RD Blakeslee
Good point, Charles! But along the lines of Dave's comments: How does the Tyro measure temperature in use, even if he's aware of the need?

- RD
Everyone who flys Glow has to have in addation to plane,radio and fuel , Glow drive,electric starter,fuel pump,Loaded Volt meter to check flight pack,tach,paper towels,cleaner, and 1/2 truck/SUV of misc. support equipment.

E flyers need to realize that a watt/amp. meter,and IR Temp. gun are necessary support equipment.

Yes you can crank glow by hand and you set neddle by ear but props on fingers hurt and with a tuned pipe you will miss the neddle and toast an good high performance motor before you hear it sag in flight.

Added:
While I do not wish to see advances limited ,I would also point out that even though I have been flying RC for 42 years am an AMA CD and Leader Member(for many years ,approx.28) I can not fly a Turbin Powered Aircraft legally in the USA. I have no problem with this just pointing out that here are reasons for limitimg useage of some products by the unqualified/certified.

Remember that part in Sticky which says that beginners should not use Li Poly Batteries?


Charles
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Old Mar 09, 2005, 12:17 PM
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Generally speaking, the people participating in this thread are mostly hardcore e-flight junkies who spend way more time and money on this than the typical casual flyer. You guys can get as complex and complicated as you want with your graphing, and it will really be useful to many of us. But it is simply never going to filter down to the masses.

Regardless of what is accomplished by coming up with a complex set of measurements to rate LiPos, eventually someone is going to have to translate it into something simple that the average casual flyer can understand.

If it turns out to be true that certain LiPos can hold high voltage under maximum rated discharge and fail due to overheating, then that creates a new problem that complicates things. Has there been more than one instance of this failure, or is this just speculation about the one that did fail? If this is more than just an isolated case, then it may be that those cells should not be recommended to LiPo novices.

In any case, if a cell can hold high voltage at maximum rated discharge yet fail from overheating, then the maximum rated discharge must be too high. If discharged at a lower rate, heat should cease to be a problem. So, depending on the outcome of a thorough investigation, it may be that these cells would need to be downrated.

But to the best of my knowledge, this is still purely speculative based on one known failure in one test with absolute cause yet to be conclusively determined. In other words, it could have been a defective cell.
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Old Mar 09, 2005, 12:30 PM
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Actually, there are two now, Dave: The ProLite 730H1 which I graphed also failed. Your cautious reception is appropriate, however.

Two cells is not a large sample and my results are at variance with Charlie Wang's and Brad S's testing.

But I think it's a bit extreme to dismiss my test results as "speculation".

- RD
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Old Mar 09, 2005, 12:52 PM
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Guys, this is slightly off topic but do any of you remember seeing pictures of an opened cell posted here or RCU a year or so ago, and the thread it was in?
I’ve been searching everywhere for it, but cant find it.
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Old Mar 09, 2005, 01:17 PM
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1A, I'm gonna quit testing and do searches for a nickel per.

Word "unrolled" turned it up:
http://www.rcgroups.com/forums/showt...light=unrolled

teehee - RD
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Old Mar 09, 2005, 01:21 PM
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There have been several photos of LiPos cut open. I recall that Crazy Ted was one of the posters.

RD, I just finished reading about the second puffed cell. Something odd going on somewhere, for sure. I did not mean to imply that your test results were speculative, and certainly did not mean to dismiss your efforts. I meant that it would be speculative to assume that there was an inherent problem with the cell when there was only one known failure with cause yet to be determined.

Now there have been two such failures with two different sizes of the same type of cell, which tends to diminish the odds of individual cell defects. This points more toward a problem inherent in all such cells, or a problem with the test equipment or test procedure. Since others who've performed similar tests have reported no such failures, odds would seem to favor an equipment or procedure problem.

Perhaps it's best to keep that conversation with the threads discussing those cells, and keep this thread more focused on general cell rating.

I remain convinced that a maximum continuous discharge C rating while delivering a specific percentage of cell capacity and remaining above a specified minimum voltage without cell puffing or failure is still a valid cell measurement tool for the average consumer. But I'm open to discussion about why this would not work.
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Old Mar 09, 2005, 01:39 PM
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Quote:
Originally Posted by RD Blakeslee
1A, I'm gonna quit testing and do searches for a nickel per.

Word "unrolled" turned it up:
http://www.rcgroups.com/forums/showt...light=unrolled

teehee - RD
Thank you very much!
Do you take paypal? I hope it didn’t end up costing me over a buck!
After a couple of days of searches, I thought I'd ask.


Dave, thanks I'll check into Crazy Ted posts also. I seem to remember one with major discoloration but it's been a while ago.

Yes, I think it’s very important to figure out why both tests resulted in puffed cells (in the other thread). It should give better understanding for lipo safety and handling.

Jim
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