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Old Oct 21, 2013, 05:38 PM
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Originally Posted by KRProton View Post
Just asking for clarifications on a few things with my ESR meter;

To use the FOM (Figure of Merit) calculator, you read the resistance of each cell in your pack, then enter the single highest value (most resistance) in the calculator, right?

For consistency to compare all your packs (and to compare to packs those cataloged in the forum) readings should be taken at 72F (22C). This is a fine tool for finding my best and worst packs and comparing to others in the forum. However, the packs I am most concerned about the ones I use for racing I preheat. So then, to find real-world maximum discharge Amps (C-rating) for these packs, would it be prudent then to also take resistance readings at the temp at which I will be using these packs?

In other words, I assume the FOM calculator takes into account normal heating of packs, but what about pre heating?

Thank you.

Tim
Tim,

Yes both your assumptions are correct.

The FOM and the Lipotool are only broad guides and both work very well overall in predicting performance .

Obviously you cannot compare readings of a preheated lipo with those taken at 22*C so for a good general comparison the 22*C figures should be used.

Preheating lipos which is often done for competition is a separate field but reading the IR there will still give you very useful information which is even more useful in your specific case. The temperature coefficient of the IR of lipos varies so that reading IR at higher temperatures is more relevant to your application as that is the temperature range yours will be working in.

Some lipos with very high claimed C ratings have a higher temperature coefficient so that they are effectively sacrificing performance at lower current ratings to enhance their performance at higher currents. Ie they need to be hot to deliver the max current at minimum stress. It seems it is self-heating which does the damage.

Wayne
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Old Oct 21, 2013, 09:50 PM
Tim Lampe; Hobbico R&D
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Champaign, IL
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Thanks for your reply and expertise Wayne.

I hope what you say about the higher temperature coefficient applies to the batteries I am using in my scenario ( high C-rating packs in F5D racing) because my prized packs are showing higher IR than I expected. I have (and tested with the IR meter this evening) six packs, all 5S 1800mAh packs connected with 4mm bullets in series from a 2S pack and a 3S pack to make 5S. Four of the packs are "older" Genesis Power 65C packs with the 3S cells leftover from another plane I flew last year and the 2S packs purchased new this spring (not optimum to mix older cells with newer, but was my only option at the time).

My other two packs are also 2S + 3S Thunderpower 1800mAh 70C which were introduced into the rotation about two months ago and are just now broken in but have been used in a race (so they should be performing optimum).

All the Genesis Power packs are right at 57 Ohms resistance with the individual cell resistance hovering around 11 - 12 Ohms for an FOM of around .59 and a max amps of .31.

The Thunderpower packs are even higher resistance with the pack being about 61 Ohms and the cells ranging from a low of around 10 to highs of around 13 Ohms for an FOM of .51 and max Amps of 28 - 29A.

I fly these packs for a race full-throttle for around 70 seconds, then throttle as necessary for landing. Average current draw during a race is about 50A and the packs typically take a nice 1100mAh to recharge.

Did I just find out there is no Santa Clause (shouldn't subject those packs to 50A)? Or, is everything okay and there is something I am missing (such as since I'm not running the packs all the way down to 20% I can use higher Amps, or batteries used for racing simply aren't going to last as long)?

So, is my world upside-down and I'm destroying my packs quickly because I'm drawing around 50A but according to the figures and the calculator they're good for only around 30A? This was in ambient summer temps (average 70 - 80F, but I did begin heating them in my car toward the end of the year when the temps started to cool).

It's odd to me that the IR of my recently better-performing TP packs have higher IR across the board than my apparently tiring Genesis Power packs.

And, maybe, without pre heating I'm damaging my packs, so heating should be part of the regimen before every flight?

Toward the end of the year it seemed as though my older Genesis Power packs were not able to hold voltage toward the end of a flight (plane slowing), but the Thunder Powers were going strong. Wonder how much of that is due to battery quality (?) and how much is simply due to cycles on the older batteries. I didn't think pulling a conservative 1100mAh at a soft 50A out of a 1800mAh 65 - 70C battery would be very much of a load.

Tim
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Old Oct 22, 2013, 08:12 AM
Tim Lampe; Hobbico R&D
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Let me rephrase my long-winded questions from the above post;

First though, Ill preface by stating that Im anal when it comes to the care and handling of my LiPos I always balance charge, I never store fully charged, I discharge to no lower than 20% (usually only 30% - 40% actually), I charge only the evening before or usually the morning that I fly, I never expose my LiPos to excess heat (such as leaving in my car during the day), etc., etc.

In-flight, Im using 5S 1800mAh 65C batteries (comprised of a 2S + 3S pack connected in series with 4mm bullet connectors) and subjecting them to only about 50A discharge (only like 30C!) for about 70 seconds taking out only around 1100mAh.

Given my handling practices and what I thought relatively mild loads, I thought my batteries had a pretty easy life, so Im surprised when Im coming up with IR numbers around 11 12 Ohms/cell and resulting FOM figures around .5 and max discharge current of only around 30A.

Are my batteries not as good as I thought and Im abusing them (based on the IR numbers and FOM figures)? Are they simply degrading (building internal resistance) due to normal use and cycles, or are my batteries fine and is there something Im missing?

Tim
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Old Oct 22, 2013, 09:12 AM
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In my experience, 30C is actually a very high load for any lipoly battery and continued discharges at such rates will most assuredly cause undue heating and affect usable life expectancy. Your experience appears to be consistent with this.

Do you have access to an infrared thermometer? If so, I suggest that you monitor surface temperature of your packs after discharge. Any temperature above ~120F after discharge would give me concern as this means that internal temperature is much higner and will cause accelerated degradation.

The maximum recommended continuous current from the Lipoly Tool will produce less heating and will allow you to extract maximum longevity from your lipolys. Unfortunately, high performance race planes, EDFs, etc., will frequently exceed this. The net result is that lipolys will suffer and will need to be replaced sooner than desired.

Mark
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Old Oct 22, 2013, 09:20 AM
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Quote:
Originally Posted by KRProton View Post
In other words, I assume the FOM calculator takes into account normal heating of packs, but what about pre heating?
I read somewhere (can't find the thread) that some manufactures use chemistry in the packs so that they can take more current but the mixture they use is not as good at lower currents as other manufactures. That makes no sense to me because at lower currents you are not using very much current so it doesn't need much from the battery. Does that makes sense to anyone? Only reason I bring it up because if true that might affect the readings? Only thing I can think of is maybe some batteries have bad IR readings at low current tests but at high currents the IR is very good. I am aware that lipo batteries in general do have better IR when they warm up but maybe some do this a lot more than others because of the mixture?
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Old Oct 22, 2013, 09:38 AM
Tim Lampe; Hobbico R&D
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Champaign, IL
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Quote:
Originally Posted by mrforsyth View Post
In my experience, 30C is actually a very high load for any lipoly battery and continued discharges at such rates will most assuredly cause undue heating and affect usable life expectancy. Your experience appears to be consistent with this.

Do you have access to an infrared thermometer? If so, I suggest that you monitor surface temperature of your packs after discharge. Any temperature above ~120F after discharge would give me concern as this means that internal temperature is much higner and will cause accelerated degradation.

The maximum recommended continuous current from the Lipoly Tool will produce less heating and will allow you to extract maximum longevity from your lipolys. Unfortunately, high performance race planes, EDFs, etc., will frequently exceed this. The net result is that lipolys will suffer and will need to be replaced sooner than desired.

Mark
That all sounds plausible to me and pretty much consistent with everything in these forums (but from what I've been reading, I thought 140F was the max. LiPo temp before damage). After a flight my LiPos come out nicely warm to the touch (not scientific I know, but based on my experience my LiPos aren't getting exceedingly hot) and never puffed. But yes, I do have an infrared thermometer.

I'm not going to change my drive to lower the load, so yes, maybe just have to cycle new batteries into the bunch more frequently. Next step would be interesting to get IR numbers off a brand new pack, or recently broken-in pack. On the other hand, my latest TP packs - only a dozen or so flights - have the highest resistance around 12 - 13 Ohms/cell.

Thanks Mark!

Tim
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Old Oct 22, 2013, 09:40 AM
Tim Lampe; Hobbico R&D
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Originally Posted by xStatiCa View Post
I read somewhere (can't find the thread) that some manufactures use chemistry in the packs so that they can take more current but the mixture they use is not as good at lower currents as other manufactures. That makes no sense to me because at lower currents you are not using very much current so it doesn't need much from the battery. Does that makes sense to anyone? Only reason I bring it up because if true that might affect the readings? Only thing I can think of is maybe some batteries have bad IR readings at low current tests but at high currents the IR is very good. I am aware that lipo batteries in general do have better IR when they warm up but maybe some do this a lot more than others because of the mixture?
From what Wayne Giles has told me a few times, some LiPos seem to be constructed to have a better "heat coefficient" where they can tolerate, or are purposely designed to withsdand higher discharge when warm than another battery warmed to the same temp - or something to that affect.

Tim
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Old Oct 22, 2013, 10:00 AM
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Both Wayne and John Julian have performed some significant testing on both internal vs. external temperature as well as temperature coefficient of various lipolys. Their testing shows (quite logically) that internal temperature of a lipoly can be significantly greater than exterior surface temperature when discharged at high rates. I personally try to ensure that my lipolys never exceed 120F surface temperature after discharge as I am fully aware that the internal temperature can be much greater. This is particularly exacerbated with higher cell count packs as they have less surface area per cell than lower cell count packs to shed heat during discharge.

Their testing also shows that some formulations of lipolys do indeed have steeper temperature coefficient than others. This can allow them to hold higher voltage during high rate discharges. The downside is that they perform poorly at typical temperatures and they also heat up a lot more during discharge, which can result in lower usable cycles. While they may be better in some applications, they're certainly no panacea.

Mark
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Old Oct 22, 2013, 10:32 AM
Tim Lampe; Hobbico R&D
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Understood!

Thanks again Mark!

Tim

P.S. My 5S packs are comprised of one 2S pack plus one 3S pack (2200mAh shown in the attached photo, but using 1800mAh presently).
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Old Oct 22, 2013, 01:53 PM
Tim Lampe; Hobbico R&D
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'Nother question....

When in a LiPo's life time will it have its lowest resistance? When brand new, never used? Or right after break-in? Other?

Tim
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Old Oct 22, 2013, 02:04 PM
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Tim,

Yes, I agree with all that Mark has said. I have never yet tested any Lipo which really can cope with a continuous discharge of >35C without showing excessive stress such as sag and recovery due to self heating or excessive terminal temperature.

John Julian has done alot of investigation into how the temperature coefficient varies in different packs and if you look at the plot he has done in the link below you can see how different lipos vary over the temperature range. The Revo 60 is a case in point; high IR at low temperatures but very low at high tempearures. This means that it will be outperformed by much lower C rated packs at low temperatures.

http://static.rcgroups.net/forums/at...110-Graph3.jpg

I think the best you can do is to try to compare IR values at or near the temperatures you will operate the packs at. That may be difficult, but if you do you must ensure you are at the correct and, very important, the same temperature for each pack.

Wayne
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Old Oct 22, 2013, 02:08 PM
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Quote:
Originally Posted by KRProton View Post
'Nother question....

When in a LiPo's life time will it have its lowest resistance? When brand new, never used? Or right after break-in? Other?

Tim
I have found that most lipos fall in IR after the first 1 - 5 cycles, some only fall after the first cycle. Generally the fall is 10 - 25%,
From then on the way is only upwards for the IR, but only slowly unless you are stressing them hard.

Wayne
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Old Oct 22, 2013, 02:25 PM
Tim Lampe; Hobbico R&D
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Champaign, IL
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Originally Posted by Wayne Giles View Post
Tim,

Yes, I agree with all that Mark has said. I have never yet tested any Lipo which really can cope with a continuous discharge of >35C without showing excessive stress such as sag and recovery due to self heating or excessive terminal temperature.

John Julian has done alot of investigation into how the temperature coefficient varies in different packs and if you look at the plot he has done in the link below you can see how different lipos vary over the temperature range. The Revo 60 is a case in point; high IR at low temperatures but very low at high tempearures. This means that it will be outperformed by much lower C rated packs at low temperatures.

http://static.rcgroups.net/forums/at...110-Graph3.jpg

I think the best you can do is to try to compare IR values at or near the temperatures you will operate the packs at. That may be difficult, but if you do you must ensure you are at the correct and, very important, the same temperature for each pack.

Wayne
Thanks again as always Wayne!

Yea, determining IR at specific in-flight pack temps would be just about impossible. I suppose I could take a few temp readings of packs after a flight to get a base line, then on the workbench (in a controlled environment) heat all my packs to that new base temp. For me, guess the best thing is to analyze data logged in my ESC (voltage) so see how well packs are maintaining voltage in flight to weed out those that go bad as they log cycles.

Looks like my packs might last half a season (50 - 60 cycles depending on the number of packs I have in my rotation) which I suppose is not crazily unreasonable given what I have learned (what you have taught me).

I'll look at the graphs you referred me to as well.

Funny thing is, my 3S 30C packs that I use in other, regular models show lower resistance. Probably because they may actually be 30C and I'm loading them accordingly.

Tim
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Old Oct 22, 2013, 02:27 PM
Tim Lampe; Hobbico R&D
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Originally Posted by Wayne Giles View Post
I have found that most lipos fall in IR after the first 1 - 5 cycles, some only fall after the first cycle. Generally the fall is 10 - 25%,
From then on the way is only upwards for the IR, but only slowly unless you are stressing them hard.

Wayne
This partially confirms the notion of breaking in packs (I mean if it contributes to longer life/more cycles).

Tim
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Old Oct 22, 2013, 03:08 PM
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Lots of real, accurate information here - thanks to all for the effort. I've never tested any packs that hold up to over 35C discharge either, although I've ran many across the bench claiming 40-100C.
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