|Jan 20, 2012, 02:22 PM|
A simple LiPo performance tool
This is the “Simple LiPo Performance Tool” Discussion thread.
What it is:
This thread is an attempt to provide a forum to discuss an improved way for the average modeller to get a handle on LiPo (or lipoly) performance using a simplified “IR” derived maximum current figure – normally called the C rating - but one which reflects the performance of the battery in practice better than the one on the label.
It is about a very quick and simple tool which, if you know the Internal Resistance of a battery and its capacity, will give you the maximum current which should be drawn to give good performance and long battery life.
What it is not:
The discussion and tool is aimed at ordinary modellers and is not focussed on expert technical explanations of LiPo behaviour which is a very complex matter. This is at heart an empirical “rule of thumb” method based on considerable practical experience and evidence.
Why it is here:
For most modellers, the important values of a LiPo pack are:
1) The number of cells (the nominal pack voltage)
2) The capacity in mAh (how much energy it can store)
3) The C rating (how much current it can safely deliver)
4) The weight
5) The shape
1, 4 and 5 are simple. For 2 and 3 we normally rely on the label.
Frequently manufacturers overstate the capacity a bit, and it is dependent to some extent on the test conditions, but the opportunity for cheating is limited. Batteries with over optimistic capacity claims can result in overcharging (if based on a C rating) and shorter than expected flight times but the relationship between capacity and performance is fairly straightforward.
The difficult one is the C rating.
Of all LiPo parameters it is the least understood and the most abused by LiPo Marketing departments. When the label says a battery is capable of 150C, it may be a fiction of the label writer or it is possible that figure was obtained for a brief period in a test lab – it is certain it won’t be obtainable throughout a normal flight for the full capacity of the battery maintaining a useful voltage. A continuous C rating (which is the one you expect the label to mean) means the current at which the lipo can be continuously discharged throughout a cycle. A “burst” figure can mean anything unless the time is specified, which it almost never is – the huge C rating the manufacturer is so keen to advertise might only be maintained for a second or two.
On another thread:
Mark Forsyth is making an attempt to create a reference site of realistic C ratings based on the measurement of cell “IR”, or more correctly the cell Effective Series Resistance (ESR). A simple calculator allows you to put in the “IR” number and obtain a recommended maximum current for that particular battery to obtain good performance (minimal voltage drop) and maximum life (acceptable heating).
The spreadshheet he has created to save your own results is attached to this post in that thread:
Please put any discussion and comments here in this thread not the reference site. A reference thread is only useful if it is easy to find things and comments and discussion make that impossible.
Presently the reliability of “C” ratings is a function of the honesty of the supplier, how they are specified, and how they are measured. The general consensus is that for all practical purposes that reliability is now very low (some vendors honourably excepted). This matter has been discussed over the years many times – search on “Figure of Merit” or “Internal Resistance” if you are interested. The forums contain a wealth of information about battery performance from respected regular posters who have devoted enormous energy and time to creating a valuable treasure trove of battery data; and if you are serious about battery performance and want to have the best possible understanding of how your cells will perform you need to spend time with the test data. Threads like the Battery Vault http://www.rcgroups.com/forums/showt...=3#post3401770 contain links to hundreds of tests of batteries at different loads and over many cycles. It is now maintained by Charles (everydayflyer) who with a few distinguished others spends more time in a year flying and testing batteries than most will do in a lifetime.
The problem is that, as Charles himself has noted, many ordinary modellers find the details of voltage/mAh curves and cycle performance too complicated. “A bunch of colored lines that is incomprehensible.”
The intention is not to start a “my battery is better than yours” gab-fest but to provide an unbiased, level-playing-field instrument to enable reasonable predictions and comparisons. If that puts the wild claimers in the battery business to shame at the same time highlighting the (few) honest suppliers who at present lose out by telling the truth, that’s a nice outcome as well.
|Jan 20, 2012, 02:22 PM|
What is "IR" and why is it useful
Internal Resistance (IR) is an empirical (measured) value that describes observed behaviour; it is not a description of what the cell is doing. A LiPo battery used in the way modellers normally use it, behaves to a reasonable approximation as though it were a voltage source in series with a small resistance inside the battery – hence Internal Resistance. A LiPo is not however actually a perfect voltage source containing an internal resistor. It just behaves roughly as though it was and that makes it easy to make some predictions about battery performance without having to analyse the electro-chemistry of the internal workings in awful detail. The equations involved and their solution is not the stuff ordinary folks want to be involved in.
Critically it is important to understand there is no “internal resistor” so you can use some resistor rules (two cells in parallel have half the IR of one) but not others (IR varies dramatically with temperature in a way a resistor does not). That is why the Effective Series Resistance (ESR) term is really preferable.
Pedantic FOOTNOTE: “ESR is normally used for Equivalent Series Resistance which is the series component of a reactive AC circuit. As used here it just means the effective resistance to current flow within the battery measured in Ohms.”The first plot of the IR of three batteries measured at a range of temperatures was made some years ago when IR numbers were a lot higher but nicely shows how different “IR” is from a true resistor and how important it is to specify the temperature at which testing took place. Not only does the resistance vary significantly with temperature, for this particular test at least it appears that the curves are not the same shape for different batteries.
Importantly, IR is a measured number not an intrinsic property, so that different methods of measuring IR give slightly different numbers.
Of the four tools I have used to measure IR: Wayne Giles’ ESR/IR meter (the only instrument I know designed solely for our purpose), the FMA PL8 charger, and the iCharger 3010B all use the Kelvin method by measuring cell voltage through the balance leads to avoid errors caused by resistance of the battery leads and connectors. The Hyperion SuperDuo does not.
I found that the numbers are all slightly different although they are consistent across different batteries if using the same method. Based on a range of test results, my conclusion is that the ESR meter, PL8 and iCharger produce close, but not identical, numbers for IR
The important caution is that, when comparing IR numbers you can only reliably do so if you know at least the test temperature AND the method used to measure them.
Why use this number?
If it is so prone to measurement variables, why use IR at all? Well, C rating is largely now meaningless since that also needs to be specified under particular conditions - and never is, and never will be, since it is not in the manufacturer’s interest. However a great deal of practical experience by a number of knowledgeable folks over several years and in several countries suggests that, properly specified, IR is a “good” measure. It is fairly easy to do with the right equipment, easy to understand and, with relatively few controlled conditions, values can be compared. Practical experience to date has shown it is a good guide to real performance. Hopefully this discussion thread can enlarge the field of practice. For example there is some evidence that different manufacturer’s batteries respond differently to temperature – do IR predictions for Brand A vs. Brand B stay valid in Norway in winter and Arizona in summer? We don’t yet know for certain.
What is the importance in practice?
Typically a LiPo discharged under increasing currents displays a set of voltage/time curves something like the second picture.
These are for a small single cell of nominal 160mAh capacity. Not what most folks fly but they are nice because they are easy to load up to very high C ratings on normal test equipment, the results are more obvious, and they are cheap - so killing them in the process is not as painful.
The same sorts of results occur for bigger batteries at their equivalent discharge rates.
Note how as the discharge rate increases - measured as “C” which equals the actual current divided by the manufacturer’s mAh rating - the available voltage to the power system drops (the Voltage shown in the solid lines and measured on the left hand axis), and the Temperature rises (dotted lines; right hand axis). Eventually at high enough C the cell simply cannot maintain any useful voltage.
Temperature rise is a function of the internal heating (high IR and/or higher current means that more heat is generated inside the battery), the physical configuration of the battery (big batteries have lower IR but poorer surface/volume ratios, multiple cells have poorer heat dissipation than a single cell since they have less surface area exposed), and the external cooling conditions (ambient temperature, air flow etc.)
This next graph is commonly seen however, particularly if the cells have high IR or are discharged at a rate well beyond their practical limits. This plot is for three supposedly identically specified 160mAh cells discharged at 3.2A or 20C. The Hyperion can just cope with the current and maintains a cell voltage of about 3.4V.
Normally, up to a certain current, the voltage curve falls with increasing current and continues to decrease with time. However at some fairly high current, the curve dips, and then rises again as the internal heating of the battery accelerates the chemical reactions inside and produces more volts. The battery is certainly being abused at this point and will have a short life.
The aim of the Max Current calculator is to give a very simple tool to estimate the maximum current any particular battery is capable of while maintaining a decent cell voltage and limiting the internal thermal heating.
It is not a theoretically derived tool but one based on a number of years of observation of the performance and heating of batteries under test and in flight. The conclusion is that placing a limit on the power dissipated in the battery as a function of its capacity is a remarkably good guide to the maximum current rating. This has proved true over a range of commonly used battery sizes down as far as the tiny single cells used in the Micro fliers from ParkZone and the like and hopefully can be extended by others experience.
|Jan 20, 2012, 02:23 PM|
Variation of IR
What you need to know:
1) Comparing IR values is only valid if they are measured the same way
2) Comparing IR values is only valid if they are measured at the same temperature
Details we might discuss later:
There is much debate about the actual method to use when measuring cell IR and hence even if two vendors both used the same Kelvin technique, they would come up with different answers. Both can be equally precise but each will claim theirs is “more accurate”. All of the four ways I have measured IR use a different combination of technique and timing, and so it is not surprising they produce slightly different answers.
Note that the “industry standard” method of measuring IR of small batteries which are used under normal low drain conditions actually measures cell impedance by applying a 1kHz signal. The values obtained do not reflect how we use LiPos and give results significantly lower than any of the above methods on our flight batteries. If you want to see how complex the whole business of test method is, here’s a quick summary: http://batteryuniversity.com/learn/a...milar_readings
Another difficulty is that different batteries respond differently to temperature change. In general IR will drop as temperature rises and this is easily demonstrated just by warming a cell with your hand while measuring IR with the ESR meter. However there is not a common curve that describes all batteries. Additionally, the temp dependence curve is also a function of the test current. This is actually a fairly tedious and labour intensive task to get decent experimental results unless you happen to own a temperature controlled environment in which you can soak batteries for at least a few hours before testing.
This graph shows how IR varied at four different temperatures for four sample batteries. It was a fairly crude test but does show how large the increase in IR is when going from 25°C to 5°C.
It would be great if anyone has significant good empirical data on temperature coefficient variation and also some hard data on the fairly well recognised ‘winter failure syndrome’.
Since this post was first written I have done some careful testing of the IR variation of a number of LiPO packs over the range 0-50 degrees C.
The second picture shows some typical results for identically sized packs. All LiPo packs show increasing IR as the temperature falls. It is not a linear relationship but the curve is not some simple mathematical one that is the same for every pack. It is roughly exponential but the curves for two different packs may actually cross over. That is a LiPo with higher IR at room temperature than another may have lower IR at a higher temperature. The is significant in terms of the well know practice of pre-warming LiPos for competition use or the "Winter failure" effect.
So what’s the Conclusion?
Overall, precise prediction of cell performance from a simple measurement of IR is probably too much to ask. However it is very much better than what we currently have and is easy to do and understand for the average punter. It provides good guidance based on a fair history of practical controlled test results. Provided you use the same measuring tool each time and note the test temperature it is a reliable comparative method.
There is no doubt that the measured IR correlates directly with performance; At least some of us now have a very good idea how a pack will perform on full power discharge tests before they are used by measuring the IR first after a couple of forming low C cycles.
|Jan 20, 2012, 02:24 PM|
Other links of interest
Some other links of interest that are about the same sort of issue:
The public battery spreadsheet
Battery Efficiency thread
|Jan 20, 2012, 02:25 PM|
Some practical results
So how well does it work in practice?
I have run a number of tests on batteries with identical specifications and found that the “Max current” number from the calculator is a pretty good indication of the reasonable maximum current in practice for the sorts of batteries I use.
For example I tested a pair of new identical Turnigy 3S 1300 mAh 20-30C and got an IR of between 17 and 18 mOhm. Suggested max C is then approximately 21 Amps and when I tested them at 20A they were clearly at or just beyond the sensible limit. The cell voltage is under 3.5V at mid-run, discharge curve is flat or slightly “dipped” and the temp rise is into the “hot” region.
Another pair of fairly new identical Zippy 3S 2200mAh 40C batteries came out at a recommended max. current of 48 Amps (which is 22C despite the 40C label).
40Amps is the maximum I can accurately test a 3S without a lot of hassle but at that current they were quite comfortable. Looking at the mid-run cell voltage which is under 3.6V and temperature which was “warm” I would not want to push them a lot further however and 48A seems an entirely reasonable maximum number.
The results are in the first two graphs.
And finally for something that surprised me, given that the max. rating tool was developed largely by observational and measurement on “normal” 2200 mAh flight packs. I have had an interest in the small 1S cells used in micro fliers for some time and thought I would try out the calculator on a sample of three cells which I knew well.
I picked a 160 mAh Hyperion and Thunderpower which have similar characteristics and a Turnigy Nano which I know to be inferior at higher rates. From previous testing I know these “25C labelled” cells struggle to reach 20C in the case of the first two and 15C for the Turnigy. At these rates they get very hot and the voltage is just adequate.
Measured on a modified ESR meter at 1.6A rather than 16A I got the results in the table.
I then tested all 3 at 10C and got the results in the third graph which is consistent with practice. The Hyperion and Thunderpower are doing fine by 1S standards, the Turnigy is just making 3.4V mid discharge.
Then I compared them using 10C for the Turnigy and 15C for the other two which as close as I can get to the calculator predictions. See last graph. (There is some variation in the temperature curves – the Hyperion curve in the previous graph is incorrect and should be ignored due to poor sensor contact.)
At the recommended maximum currents predicted by the calculator all 3 cells are performing almost identically and at what I would regard as the limit of voltage drop and temperature rise. The Hyperion is showing reduced capacity – it has had a pretty tough life and I’m not surprised. However the CURRENT capability predicted by the calculator is quite remarkably accurate.
In short, this calculator developed from normal full size packs seems to predict surprisingly well at the tiny end of the battery world as well.
|Jan 20, 2012, 02:58 PM|
Joined May 2006
I applaud your effort, but one question I have is how are you going to define "C"?
The only actual definition I have ever seen was off a Lipo "bible" on the old FMA site. I believe the definition was due to Kokam.
Anyway the definition of "C" was the max current draw that would bring the total discharge down to some fraction of the total cell capacity(it was close to 100%, but I don't recall the exact %), and keep the pack (maybe cell) temperature less than ~140 degrees F.
The assumption was anything above this temperature will damage the lipo. Other specs gave the ambient starting temperature, and the allowed (if any) air cooling
Sorry I don't recall the numbers better, but I did go and look for it, but the info seems to be gone now.
Presumably that is what "C" originally meant. Clearly it is related to the cell internal resistance, the temperature coefficient, and size of the cell. Also "damage" has a wide degree of interpretation---immediate damage, decreasing the # charge/discharge cycles.........
So to be meaningful, I think you need to get a handle around what we actually mean by "C". Not too sure how you are going to do this, simply because some people expect hundreds of cycles, some seem to be happy with 60. To these people, "C" would have a completely different significance.
Anyway, good luck!
|Jan 20, 2012, 03:04 PM|
Alan, thanks for that. The whole point is that this value of C is from a measured value of IR and a factor found from practice to give good results - not a definition. If you can be patient enough till Mark gets his calculator thread going it will be clearer I think.
|Jan 20, 2012, 06:53 PM|
|Jan 21, 2012, 09:57 AM|
Whould those who are very knowledgeable about such please explain the following to me?
A Billowy 3S 2250 which was new 5-14-2010 has been discharged at some rather high rates including a single cell at 110 Amps.(then reconfigured as a 3C) ,has been flown to less than 3V per cell by mistake during which it got extremly hot , has 76 cycles on it ,(12 since the gross overdischarged / high temperature flight) has been stored at room temp.(72-80F) and when tested just now ...
Power Lab 8 ,charge rate 1.10 (1/2 C) battery at 72F (has been setting for well over a month).
Perhaps I sould mention the Preset I am using is a custom one where no balancing takes place until 4.1V per cell.
Cells at 60% SOC 3.894 / 3.902 / 3.892 volts
At 68 % SOC cell 3.92 / 3.949 / 3.939 volts
IRs 4.1 / 3.8 / 3.8
At 71% SOC cells 3.963 / 3.969 / 3.961 volts
IRs 4.1 / 3.7 / 3.8
At 77% SOC 3.986 / 3.994 / 3.982 volts
IRs 4.5 / 4.5 / 4.1
At 90% SOC
Cells 4.105 / 4.105 / 4.105 volts
IRs 3.9 / 3.8 /3.8
At 98% SOC
Cells 4.1589 /4.159 / 4.159 volts
IRs 4.0 / 4.0 / 4.0
Cells 4.206 / 4.206 / 4.206 (chargre rate at 506 mA and dropping)
IRs 4.2 / 4.2 / 4.2
added when year date was corrected )11:11 AM 1-21-2012
100% SOC charge complete
Cells 4.204 / 4.204 4.204
IRS 4.2 / 4.2 4.2
What I find most confusing is that my sample (yes could be a ringer but I highly doubt it) has been what I consider rather abused and still has IRs less than 1/2 of other's posted IRs.
I have read post that SOC (state of charge) does not have much effect in cell IRs also have read that 10% difference between cells is normal .
Is there a normal difference for same cells at difference states of charge?
Update: took LiPoly off of PL8 with each cell still showing 4.2 mOhm..
Connected to iCharger 306B ,went to Special Function ,check IR
7 / 6 / 6
,went to Monitor cells 4.20 / 4.20 / 4.19 volts
|Jan 21, 2012, 10:21 AM|
Joined Oct 2004
Good question Charles and one that will hopefully be answered as more objective data is gathered. As you know, I have 6 such packs and all are performing nearly identically and worse than my other packs of similar size. All packs are used in the same application and stored at room temp (below 3.85 V/cell) so only variables in my case are pack brand, # of cycles, and age.
Speculating here - Your pack could be from a different lot than my 6, application may be very different, etc...
I was hoping to uncover this mystery when I posted my results of my 3S 2200mAh packs in this thread: http://www.rcgroups.com/forums/showthread.php?t=1450517 but have received few comments.
This highlights the potential benefit of having a good database of objective performance data as well as a place to discuss the data. IMO, with a good set of data in one place, these sorts of questions would be much easier to address.
|Jan 21, 2012, 10:51 AM|
|Jan 21, 2012, 11:01 AM|
No I have not however FWIW while I do not have one of Wayne's IR checkers some who have both his and PL8 have reported IRs close to same using both and Wayne and myself have obtained very similiare results on A123 Systems 2200 mAh cells which IMO would make a very good base line test to compare different devices as they seem to be extremely constant compared to LiPolys.
I have not had a scope in years and thus do not really have any accurate means to measure a load of such short duration.
|Jan 21, 2012, 11:52 AM|
Joined Nov 2006
(yes could be a ringer but I highly doubt it)
Charles, your test batteries were randomly slected from our first shipment of 100 units. No Ringer - we would not do that.
|Jan 21, 2012, 12:24 PM|
I know that but some like to suspect any battery being tested which was not purchased at random. For many years I tested a bunch of Thunder Powers and many pointed out that their TPs did not perform as well or last as long as mine and that I must be getting very slected cells / packs.
Hard to convince many that proper care and feeding has an awful lot to do with the end results.
|Jan 22, 2012, 04:52 PM|
Joined Feb 2007
I have wondered about anomalies like these and wonder if the most likely cause is just simple quality control, or rather lack of it.
I ran tests on two new 3S2200 25C Nanotechs and found their IR's to be nearly double what we would expect and the power runs proved that they were really only capable of about 17 - 18C. There being two, it was not a single rogue pack yet a 1300 25C pack bought at the same time was fine, actually showing a lower IR figure.
Other people have measured 2200 25C Nanotechs at around half the values I saw, still not brilliant, but acceptable.
Is this just QC or do the pack assemblers buy cells from more than one manufacturer ?
Whatever the reasons there is a fair amount of lottery in it
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