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Old Feb 02, 2012, 04:47 PM
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Quote:
Originally Posted by vlad_vy View Post
There is any relation between cell IR and possible maximum charge rate?
My 2 cents...

Given that the ability of any battery to accept charge current is directly dependent on its internal resistance (among other factors), yes, there is a relationship between maximum charge rate and measured internal resistance. However, quantifying this with any level of precision is a nebulous task as Wayne and Charles have indicated.

From a practical perspective, I have found that there are limitations on charge rate in that a lipoly cell with higher internal resistance will have a lower termination voltage. Lower termination voltage means lower usable capacity. For all packs that I have tested (FOM .5 and higher), the 'sweet spot' charge rate is about 3C-5C and results in short charge times and more than acceptable flight times. Higher rate than this results in only slightly shorter charge time but at the expense of lower available capacity.

Given the time, I will typically charge at 2C as I nearly always will charge multiple packs in parallel while preparing to go to the flying field (park, ball diamond...) and use my chargers at their maximum available current output.

Mark
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Old Feb 02, 2012, 11:26 PM
ancora imparo
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How true that is and a nice post on the other thread. But I hope I didn't give the impression I was doing some kind of Fascist thread policing earlier.

This is an unmoderated thread and people are free to post whatever they want. What I did ask was that postings respect the intent of the thread which was to discuss a practical rule-of thumb which could assist modelers where currently anarchy reigns. Speculation about details without any supporting evidence might be good as an opinion-fest but it just confuses those who might be looking for help.

Charles, given that you pioneered higher charge rates and know more about it than anyone else (and almost certainly have more supporting hard evidence about LiPo behavior in real life than all the armchair theorists in the world put together), I was personally looking forward to anything you might post on the relationship of IR to charge rate. It's not something I have ever looked at and I sure hope I didn't turn you off.

It's not a topic I had originally thought of, but there is no a-priori reason why carefully measured IR under controlled conditions might not predict useful charge performance. It's fairly obvious that we are talking about two different electrochemical scenarios between charge and discharge, but this is an empirical approach. That is - is there a reliable correlation between an observed behavior and a measured parameter based on adequate observation and measurement. That's all.

John

Quote:
Originally Posted by everydayflyer View Post
A post of mine this morning along these lines.

http://www.rcgroups.com/forums/showp...43&postcount=8

<snip>

Most LiPoly cells are not designed for our use and some just happen to work for us but no manufactur is going to spend tens of thousands of dollars doing suitable of use test for us.

Charles
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Old Feb 17, 2012, 02:34 PM
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IMO, the FOM is redundant. As I understand it, FOM is as follows:

less than 1: below average
1: average
greater than 1: good
much greater than 1: outstanding

The relationship as you have it between FOM and C isn't linear, but it's pretty close. Thus, you could get rid of FOM and just say:

less than 22C: below average
22C: average
greater than 22C: good
much greater than 22C: outstanding
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Old Feb 17, 2012, 05:06 PM
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I see what you mean about having two figures both specifying the current capability of the lipo.
FOM was thought up in another thread and it is, I think, included as a bonus. The fact that a "Good" Lipo has an FOM of 1 is easier to fix in the memory than 22C although they correspond.
The "C" rating has become so discredited that the FOM was suggested as an alternative.
Although they are both a function of the capacity and IR, there is a square law involved in the real "C" rating so that it rises a lot more slowly than the FOM.

Wayne
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Old Mar 07, 2012, 04:17 AM
BrainFart RC-Pilot
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The calculator is interesting.
http://www.jj604.com/LiPoTool/

But when to measure ? After being fully charged? Or at storage level?
The IR drops if the pack is fully charged. At least my iCharger does show this very clear.

I measure in my hangar (around 20degrees) and half charged. Also a day after...or at least a few hours later. That is the reference for me.
This way I can check the quality of the pack. Checking the IR at the same level each time.

This helped allot:
http://www.rcgroups.com/forums/showp...ostcount=10608

I like this calculator, but IMO would be handy to have also a reference when to measure. (charged or storage level)

Also it does display the single cell IR value.
But at the balance board this IR is different from the main lead.
I guess, you need to multiply with cell amount of the pack, to get the IR of the pack main lead with this calculator. So maybe adding a cell count 'entry' to set the amount of cells, so that you can see the main lead IR value.

my2cent
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Old Mar 07, 2012, 12:22 PM
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Quote:
Originally Posted by TreeDiver View Post
But when to measure ? After being fully charged? Or at storage level?
From testing performed by myself and others and as recommended here, charge state has little effect on IR. However, temperature has a major effect. As such, packs must rest for at least 1 hour to stabilize at a consistent temperature in order for the measurements to have validity. I take my measurements at storage charge as this happens to be easiest for me.

The main thing is to be extremely consistent with respect to temperature and allow lipolys to rest for at least 1 hour prior to taking any measurement.

Mark
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Old Mar 07, 2012, 01:03 PM
BrainFart RC-Pilot
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Hi Mark,

Okay, all clear. I overlooked. The 1 hour is important and the temperature.
Directly after charged I noticed that IR was lower, but as mention.... I should wait at least an hour. Thanx
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Old Mar 07, 2012, 01:07 PM
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Good to know, I will jump in here and post results on the packs I have. I am using the iCharger 306B and have larger Gens Ace 25C 6S 3800mah packs for my helicopter. I started playing the IR game yesterday and logging data. Was really interesting seeing the packs go from 6-8mOhms per cell to 2mOhms just by taking them out of the 0 celcius fridge where I store them and let them heat up to room temperature (62F in my house).
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Old Mar 07, 2012, 01:26 PM
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Yes, as you fine gentlemen have learned, IR (and thus performance) is extremely dependent on cell temperature. It is for this reason that I will never fly any of my lipolys in my high performance planes if they are less than 70 degrees F (~21C) and prefer that they be over 80F prior to flying. Reason is that the increased internal resistance not only hurts performance but also causes increased power dissipation (heating) within the lipoly.

Taken to an extreme, this self-heating can damage a lipoly. Wayne Giles has termed this phenomenon 'winter failure syndrome' and I think this term fits nicely. It basically means that a lipoly becomes damaged by attempting to draw significant current when the pack is very cold and has dramatically increased internal resistance.

Lesson here - warm lipolys are happy lipolys, but only when being discharged. Storage is a whole other story...

Mark
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Old Mar 07, 2012, 01:41 PM
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Yeah Mark,

It's funny I learned the "winter failure syndrome" years ago when I flew electric planes outside daily. When the winter months came my power and flight times dropped dramatically. I now do the same and keep the batteries warm just before flight.

I will take some new IR readings today right out of storage, and again an hour later after the batteries have warmed up.
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Old Mar 07, 2012, 03:35 PM
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Ok testing has begun...

I took these packs out directly from storage in my fridge where the ambient temperature is between 1-5 C. I went in order from Pack 1A to my last Pack 3B when measuring. All packs are stored at a storage voltage of 3.85V per cell using an iCharger 306B for IR measurement/charging.

These packs have 5 cycles through them and have been discharged to about 50% each time while be charged at no more than 1.3C and no less than 1C.

At (1-5 C)

Gens Ace 25C 6S 3800mAh packs... 1 month old (Feb 2012)

Listed from cells (1,2,3,4,5,6)


Pack 1A: (9,7,8,13,7,7) total 51 mOhms

Pack 1B: (7,8,8,8,8,7) total 46 mOhms

Pack 2A: (8,7,8,8,7,7) total 45 mOhms

Pack 2B: (7,7,9,8,7,7) total 45 mOhms

Pack 3A: (6,6,8,7,6,7) total 40 mOhms

Pack 3B: (8,8,9,9,8,8) total 50 mOhms


I am now allowing the packs to warm up to room temperature for the next hour and I will take another IR reading and post back.
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Old Mar 07, 2012, 04:28 PM
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Shortman,

Here is an old post which demonstrates how temperature dependant the IR of lipos is. It also shows how the dissipation within the pack varies with IR and temperature.
The "Winter failure" syndrome that Mark refers to is the early failure of the pack rather than the failure of the pack to perform. If you look at the internal power dissipation at 5degC, it is not surprising that they are damaged. The discharge plots demonstrate the classic "Sag and recovery" which I believe does the damage.
At your room temperature I would expect the IR values of your packs to a fall to well under half the values you have read so far.

http://www.rcgroups.com/forums/showt...7#post16669929


Wayne
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Old Mar 07, 2012, 04:50 PM
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Thanks for the link Wayne, very informative. Going to retest the IRs now :P.
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Old Mar 07, 2012, 05:00 PM
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Part Two of IR Testing

Packs were allowed to warm up to room temperature (17C or roughly 62/63 F) for an hour
Still at a storage charge of 3.85V per cell

New IR readings...

Pack 1A: (3,2,3,5,2,2) total 17 mOhms

Pack 1B: (3,3,3,3,3,2) total 17 mOhms

Pack 2A: (3,3,3,3,3,3) total 18 mOhms

Pack 2B: (3,2,4,3,3,2) total 17 mOhms

Pack 3A: (3,3,3,3,3,3) total 18 mOhms

Pack 3B: (3,3,4,4,3,3) total 20 mOhms
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Old Mar 16, 2012, 11:24 PM
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May be a Dumb question here...What is the expected IR of a New 4S 4400 mah Pack?? A 4s 6500 pack? Should not be a difference?? Correct?
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