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Old Oct 06, 2012, 02:22 PM
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Originally Posted by MikeAnderson View Post
Over on the Lipoly dbase thread mrforsyth tells us:


I've done a seach for "initial energy dissipation" and the quoted post is the only place this is mentioned. I'd like to know more about this "initial energy dissipation". Can I get an explanation please?
Mike,

I assume that you are aware that the IR of a lipo varies inversely with temperature. This being so the IR, and therefore the heat dissipated in the lipo, will reduce as the discharge proceeds and the temperature of the pack rises.

In working out an equation to estimate the real C values of lipos we have to work on an 'initial' IR value at 22degC and hence an initial power dissipation.
Thus we are saying that if a lipo initially dissipates more than 6W/Ah/cell on a fixed current discharge, then the lipo is overstressed and is being operated beyond its capability.
The 6W value is somewhat arbitrary but is based on a lot of full power discharges over many different capacities and 'C' ratings and consistently we found that initial dissipations of >6W resulted in excessive final temperatures and cell damage.
Hope that helps.

Wayne
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Old Oct 06, 2012, 02:44 PM
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Originally Posted by Wayne Giles View Post
Hope that helps.Wayne
Perfect - many thanks.
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Old Oct 06, 2012, 09:07 PM
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Data set observations

I've just had a look at the latest compilation of results in post #1 of the The Lipoly Objective Performance Database thread & have these observations / comments:

1) Nobody has measured cell IR under 3 mohms. Really?

2) Individual cell IR's are not reported - or are they? Is each data row reporting the pack values or the cells within the pack values?
If each row is for a pack, then are the "Cell IR" values reported calculated by dividing the Pack IR by the number of cells, which includes the pack main wires & connectors, or are they the average of the individually measured cell IR's?
Why isn't there a "# of cells" column?
I've been thinking that a measure of pack quality would be the variation of IR's among cells in the pack. (Low variation = high quality) I can't see cell IR variation from the published data.

3) Zero cells (or are they packs of cells?) over 3000 mAh perform better than their labelled current delivery rating. Only one at 3000mAh performed better.

4) Of the 6 cells/packs over 3000mAh capacity all performed at about half of their rated current deleivery rating. (Not enough data points to be statistically significant)

5) Why is the cell resting voltage during testing not reported? Is it insignificant?
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Old Oct 06, 2012, 09:25 PM
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Mike, I'm no expert at this. It's rather new to me.

1) I have measured cells at 1 and 2 mohms, but we list the worst cell so each reading is a single cell. I guess that speaks to point 2 also. Your conclusion is correct , we can only see the worst one and not a variation. The idea when deciding how much current to pull from a given pack is to only pull the max current of the "worst" cell in the pack. Variation is also an important consideration , and many times the cell voltage is too.

After observing some of these variables and being somewhat concerned about them I still think it is best to keep the tool as it is because it is simpler. I keep my own tables of course and I know there are other variables as well. I just have to account for them when doing my packs.

I would kinda like a temp and voltage correction table that is easy to use. One of the other biggest variables I've noticed is the actual capacity , but again entering these variables into the tables can produce quite a mess in short order.
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Old Oct 07, 2012, 12:55 AM
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For cells wired in series, only the worst one matters - it's the limiting factor. Say we've got a 3S battery. It doesn't matter that cells 1 and 3 are doing well at 50A if cell 2 is cooking and puffing at that same current.

Hence, the overall performance of the pack is equivalent to the performance of its single worst cell.
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Old Oct 08, 2012, 05:02 AM
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While I understand the simplified approach, I still lament the missing cell variation data. If the worst cel IR is the same as the best cell IR, that tells me a lot about the pack when compared to a pack where the worst cell IR is a lot worse than the best cell in the pack.

Still, it's better than what we had before which was nothing.
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Old Oct 08, 2012, 03:11 PM
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Originally Posted by gulio View Post
Mike, I'm no expert at this. It's rather new to me.

1) I have measured cells at 1 and 2 mohms, but we list the worst cell so each reading is a single cell. I guess that speaks to point 2 also. Your conclusion is correct , we can only see the worst one and not a variation. The idea when deciding how much current to pull from a given pack is to only pull the max current of the "worst" cell in the pack. Variation is also an important consideration , and many times the cell voltage is too.

After observing some of these variables and being somewhat concerned about them I still think it is best to keep the tool as it is because it is simpler. I keep my own tables of course and I know there are other variables as well. I just have to account for them when doing my packs.

I would kinda like a temp and voltage correction table that is easy to use. One of the other biggest variables I've noticed is the actual capacity , but again entering these variables into the tables can produce quite a mess in short order.
The idea of the system was to keep it simple. In any case the corrections we might try to apply would not be true across a range of different lipos.

The thinking was that if the initial dissipation is >6W/Ah/cell at a particular continuous discharge current, then the pack is being overstressed. We noticed this from many discharges and it really does work amazingly well as a real 'C' forecast tool.

A temperature correction table is not practical as the IRs of lipos all have a negative temperature coefficient but they do not track.

Lipo capacities seem to have degenerated. I measured a lot of single Loong!ax cells some years back and they were all within 1% of rated capacity. I have just checked a 5S 4000 GensAce pack and the cells vary from 2% low to 10% low.

I don't believe that capacity variation is a direct problem, but is likely to cause cell damage if you discharge until the weakest cell is <3V.

In general terms a wide variation in balance, IR or capacity are all signs of poor quality control which is bad news. You don't need to quantify the problem to know that there is a good chance you have a dodgy pack

Wayne.
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Old Oct 12, 2012, 10:32 AM
ago involo ergo sum
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Lipoly Model prediction Chart

I'm a visual data analysis kind of person so I used the Lipoly Performance Calculator to generate 5 families of C-rating predictions and plot them on the chart attached.

I have to say that I'm finding it difficult to believe that it's theoretically not possible to have even a 40C cell over 2000 mAh capacity @ 2mOhm IR.

In any case it does illustrate that low IR = High C-rating
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Old Oct 12, 2012, 10:45 AM
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I have to say that I'm finding it difficult to believe that it's theoretically not possible to have even a 40C cell over 2000 mAh capacity @ 2mOhm IR.
You are correct Mike, and I have never seen a graph of a lipoly pack that was discharged multiple times at 40C or above and survived. It is not only theoretically impossible, it is impossible from a practical perspective (at least with lipolys that I have ever seen / tested) as simple physics tells us that the heat produced internally within the pack will destroy the pack in short order.

Note that manufacturer's C-ratings even when true will tend to be more generous than those produced by the tool as the manufacturer ratings will allow for a good amount of overheating that causes damage. Conversely, the C-rating produced by the tool is intended to slightly conservative and not cause a damaging heat rise during discharge. In other words - the tool's C-rating is what is expected by most modelers that I have talked with.

Mark
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Old Oct 13, 2012, 09:06 AM
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Originally Posted by Wayne Giles View Post
Thus we are saying that if a lipo initially dissipates more than 6W/Ah/cell on a fixed current discharge, then the lipo is overstressed and is being operated beyond its capability.
The 6W value is somewhat arbitrary but is based on a lot of full power discharges over many different capacities and 'C' ratings and consistently we found that initial dissipations of >6W resulted in excessive final temperatures and cell damage
I've been thinking about burst discharge applications like electric gliders, especially in the high performance realm. I can do ten 200 amp discharges for 3 seconds at a time spread out over a couple of minutes (one burst every 24 seconds) and the pack doesn't get hot. (It DOES warm up though) So the 'allowable' initial heat dissipation must be much higher than 6W/Ah to allow that.

Most manufacturers label the burst discharge C-rating at twice the continuous C-rating.

The Lipoly performance model will predict that C-ratings are twice the continous C-rating when the initial heat dissipation is 24w/Ah/cell.

MaxAmps = SQRT(InitialHeatDissipation*CellCapacity/MeasuredCellIR)

Any thoughts on this?
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Old Oct 13, 2012, 10:35 AM
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Mike,

Yes, we discussed a multitude of ways to make the tool more complex but ultimately decided that keeping it as simple as possible would make it most useful for the largest number of modelers. After all, it is titled 'A simple LiPo performance tool'.

Adding complexity was discussed in post #3 of the data thread: http://www.rcgroups.com/forums/showp...85&postcount=3.

Further, none of us who validated the tool fly hotliners or other high performance gliders with a defined flight pattern and thus would not have been able to validate a burst calculation number through empirical observation. This does not mean that the data derived from the tool is not useful for such applications. It will simply require some data collection, observation, and fine tuning on the part of the end-user to determine what minimum FOM works best in his specific application.

As you can see, the intention was to serve the largest number of modelers as possible with a performance indicator that is based on objective measurement rather than marketing whim and the tool is meant to be a simple 'rule-of-thumb' rather than a scientific derivation for us anorak types.

Mark
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Old Oct 13, 2012, 03:02 PM
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Quote:
Originally Posted by MikeAnderson View Post
I've been thinking about burst discharge applications like electric gliders, especially in the high performance realm. I can do ten 200 amp discharges for 3 seconds at a time spread out over a couple of minutes (one burst every 24 seconds) and the pack doesn't get hot. (It DOES warm up though) So the 'allowable' initial heat dissipation must be much higher than 6W/Ah to allow that.

Most manufacturers label the burst discharge C-rating at twice the continuous C-rating.

The Lipoly performance model will predict that C-ratings are twice the continous C-rating when the initial heat dissipation is 24w/Ah/cell.

MaxAmps = SQRT(InitialHeatDissipation*CellCapacity/MeasuredCellIR)

Any thoughts on this?
Mike,

When doing a lot of constant current power discharges and noting IR before the testing, I became conscious of the correlation between IR and battery performance. In measuring the IR and calculating the dissipation within the pack, we found that an IR and current level which equated to a cell dissipation of more than 6W/Ah invariably resulted, after a full constant current discharge, in excessive terminal temperature and/or a sag and recovery voltage plot.

Almost certainly it is the excess temperature which does the damage which is a result of the internal cell dissipation.
Whether this dissipation is generated by a constant current or a series of pulse currents is irrelevant. It is the total number of Joules of energy dissipated as heat over a known time which will decide whether the cell is being abused.
(BTW I think your example should be one pulse every 12 secs or less pulses or a longer time)

As Mark says, surge ratings are variable in time and amplitude so that it is impossible to specify a standard whereas a full constant current fixed discharge is simple and tests exactly what the maker claims as a "Continuous 'C' rating"

Wayne
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Old Oct 13, 2012, 11:34 PM
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I just want to know why packs are not delivered with cell IR rating from the manufacturer. When I used to race electric touring cars the Nicads and NiMh race packs had individual IR rating labels on each cell.
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Old Oct 13, 2012, 11:54 PM
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I just want to know why packs are not delivered with cell IR rating from the manufacturer.
Because then it makes the marketer's job harder. Much easier to fudge C ratings than IR values.
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Old Oct 14, 2012, 12:02 AM
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Because then it makes the marketer's job harder. Much easier to fudge C ratings than IR values.
Buying Nicad or NiMh RC touring car packs was easy. You paid a premium for low IR, longest run time and highest average voltage. All tests were done using a standard testing methodology and equipment. No marketing voodoo allowed.
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