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Old Sep 06, 2012, 11:05 PM
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Do you trust the IR readings from the PL-8 ? I'm not sure I do. How about this? It only takes one bad (high IR) cell to fall on it's face thus ending the discharge early. Is this possible or were you monitoring each cell's voltage during?

In any case, I think you paid too much.
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Old Sep 06, 2012, 11:38 PM
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Rick,

You did not indicate the listed capacity of the packs but from the usable capacity deltas between 2A and 20A draw, I'm with gulio here and would say the reported IR numbers are way outta whack, at least at first analysis.

Were the IR measurements and discharge tests performed when the packs were at the same temperature? Do you have access to an iCharger for comparison?

Also, are you sure that the main discharge connectors have no issues and your charge lead on your PL8 is not super long or of small gauge? A lossy connector or a bad charge lead will show very poor performance in a discharge test but this will be masked by the IR test as the charge lead resistance is eliminated. This may mean that your packs are just fine and you have loss elsewhere in your setup.

In my experience, IR is an extremely accurate predictor of pack performance and your problem may be beyond the cells themselves.

Mark
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Old Sep 07, 2012, 12:15 AM
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Sorry, I edited my post above. These are all 5000mah packs.

Quote:
Originally Posted by gulio View Post
Do you trust the IR readings from the PL-8 ? I'm not sure I do. How about this? It only takes one bad (high IR) cell to fall on it's face thus ending the discharge early. Is this possible or were you monitoring each cell's voltage during?

In any case, I think you paid too much.
I was reading each cell separately on the discharge. Granted, some packs had one or two cells dipping a tenth lower than the others but the others were falling on their faces too.

Quote:
Originally Posted by mrforsyth View Post
Rick,

You did not indicate the listed capacity of the packs but from the usable capacity deltas between 2A and 20A draw, I'm with gulio here and would say the reported IR numbers are way outta whack, at least at first analysis.
Mark
A bad connection would have given me a hot spot on the leads/connectors. I am using 45A APP's crimped with the OEM APP tool. The wires and APP's on the OEM FMA PL8 lead were not even warm.

Quote:
Originally Posted by mrforsyth View Post
Were the IR measurements and discharge tests performed when the packs were at the same temperature? Do you have access to an iCharger for comparison?

Also, are you sure that the main discharge connectors have no issues and your charge lead on your PL8 is not super long or of small gauge? A lossy connector or a bad charge lead will show very poor performance in a discharge test but this will be masked by the IR test as the charge lead resistance is eliminated. This may mean that your packs are just fine and you have loss elsewhere in your setup.
At the field two were very warm, the other two were ambient which was only around 103F.
Today I was at 75F during tests on all packs.

Quote:
Originally Posted by mrforsyth View Post
In my experience, IR is an extremely accurate predictor of pack performance and your problem may be beyond the cells themselves.

Mark
The fact that my FMA Corsair with an upgraded power system only went 1.25 laps around the flying field was my first hint that there was something wrong with the fully charged pack. The .25 of a lap was into a bush so, lucky for me, the plane suffered very little damage. [whew]

I guess my questions here is how I can have an IR that, at worse on the worse pack (I entered 7 for IR, 5000mah), is a true 13C rating but can't even hold a 20A draw for longer than a minute at most. I expected this to be, and it has proven such, more accurate.
Could it be a new chemistry in these newer 40C rated packs? [shrugs]

Rick
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Old Sep 07, 2012, 12:19 AM
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Quote:
Originally Posted by rampman View Post
At the field two were very warm, the other two were ambient which was only around 103F.
Today I was at 75F during tests on all packs.
IMO, there's your answer my friend.

The negative temperature coefficient of lipolys will show dramatic difference in internal resistance at 103 vs. 75F.

Can you measure IR again at ~72F and report your findings?

Cheers,
Mark
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Old Sep 07, 2012, 01:22 AM
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Quote:
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Today I was at 75F during tests on all packs.

Rick
Nope. Today i did all tests at 75F.
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Old Sep 07, 2012, 01:26 AM
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I just put ky 12 light bulb discharger on the packs and the voltages fell off really fast again.
Someday these may end up as someones power pack to charge their 2200 and smaller packs.
Rick
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Old Sep 07, 2012, 01:48 AM
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Ok, thanks for the update Rick. So, the reported IR's were taken at 75F and not 103F as I misinterpreted.

Puzzling indeed, especially the pack that had a worse case cell IR of 3.4mOhm yet only delivered 241mAh at 20A for a 5000mAh pack that was fully charged.

Given this, I would almost have to say that your PL8 is thoroughly confused and is not properly reporting internal resistance of extremely poor performing packs. I do not own a PL8 but have found that my iCharger 306B has been absolutely spot-on in all circumstances and has never given good numbers for a poor performing pack nor has it given bad numbers for a good performing pack.

Do you know any flying buddies that own iChargers who would let you borrow it for a few minutes? Wish I lived closer and I would loan you mine for a few days...

Mark
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Old Sep 07, 2012, 12:19 PM
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Yes. I think someone at my flying field has 1 that I can borrow at the field.
Up to this point my PL8 internal resistance measurements have seem to be pretty accurate.
Thanks to all for the inputs. Oh. I have some 3S and 4S 5000mah 40C Sky lipos for sale if anyone is interested.
Rick
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Old Sep 07, 2012, 04:06 PM
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Rick,

I have seen a similar reaction in testing a GensAce 2200 55c pack which I had measured IR at 3 mohms/ cell but decided that as it was claiming to be a 55C pack that I would run full discharge runs at 20C, 25C, 30C, 35C. .... etc up to 55C.

It survived the 30C run (just) but part way through the 35C discharge, the voltage suddenly collapsed below 9V. (3S pack) which automatically terminated the discharge at about 60% of the capacity. When the pack had cooled down it appeared OK but I measured the IRs and they had now increased to 11, 13 and 23 milliohms and remain there so that the high discharge had obviously permanently damaged the pack. Presumably the 23m.ohm cell was the one which suddenly 'let go' and then partially recovered.

When people say that there is another factor other than IR which affects lipo performance they are overlooking the fact that, by definition, it is the IR that dictates the performance of the lipo, but the 'effective IR' at THAT particular moment.

Wayne
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Old Sep 10, 2012, 12:02 PM
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Old Sep 10, 2012, 12:03 PM
That's a funny word
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HK charger that measures IR of each cell according to pg 17 of manual. http://www.hobbyking.com/hobbyking/s...ph_Screen.html

(just in case Mark needs another charger)
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Old Sep 10, 2012, 01:27 PM
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Haha. Yes, I am down to 10 lipoly chargers now, 7 of which are balancing chargers.

Would be curious to know how well the IR measurement works. Perhaps I'll order one after I whittle down my current collection...

Mark
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Old Sep 11, 2012, 01:39 PM
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In that would I measure the cells go? 3. 85v or 4.2? for I serve the data for the calculation of the web (Lipoly Objective Performance Calculation Tool )
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Old Sep 11, 2012, 11:24 PM
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Hi Mark, JJ,

Any chance that the data in the Database thread could be put into one spreadsheet located in the first post? At the moment, we have to go through the entire thread (which I hope is going to get much longer as more data is posted) to find info which is relevant to our application.

I would volunteer to do the job if I wasn't so busy. Hopefully someone with more time will step up. I could help with some data entry if required.

Sorry to be so whiney especially when the data in there is already pretty helpful as is. I think it would be even more useful to have it nicely structured in a form we could sort by Name, True C, Capacity or any other field we choose.

Jeff
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Old Sep 12, 2012, 12:03 AM
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Hi Jeff,

I have already started to compile all of the data and hope to have it all on one spreadsheet in the next few weeks once I get a break from all of my work travel. As you see, I added a poll to the database thread to get a feel for whether fellow RCGroup members felt that the Performance Tool and Objective Database were useful.

I'm happy to see that many have responded to the poll and I'm even a little surprised that lipoly manufacturers and vendors have not voiced more displeasure that the flawed 'C' rating system that has no basis in reality is being exposed for what it is. Really glad that many find the tool and the information that it provides to be useful.

Thanks for the feedback and I hope to have it all in one place very soon.

Cheers,
Mark
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