Purpose:

The purpose of this thread is to serve as a central repository for objective performance data for commercially available lithium polymer (lipoly) battery packs based on real world performance rather than the flawed manufacturer ‘C’ rating system. Besides manufacturer ‘C’ rating (maximum current that a pack can be expected to sustain throughout a full discharge), modelers have had to rely on advice from fellow modelers, scattered various test threads, etc., from which to cull meaningful information to make lipoly purchase decisions. It is hoped that this thread will be an additional source of reliable data that is performance based, not marketing based.

In order to make the data as accessible as possible and provide the modeler with a true indication of how a pack will perform in an actual application, Wayne Giles, John Julian, and myself have developed and validated a simple tool (MS Excel, attached below) that calculates the maximum recommended current, FOM, and True C rating for a lipoly pack based upon the pack’s capacity and measured cell internal resistance. A web-based version of the tool is available here: http://www.jj604.com/LiPoTool/.

How you can contribute:

First off, you’ll need a means to measure lipoly cell internal resistance (iCharger or FMA charger, Wayne Giles’ ESR Meter, and perhaps others).

The aim is to develop a simple but useful database of lipoly objective performance. You can contribute and make it worthwhile by posting results from your own work provided it includes:

1) Battery details: Manufacturer, model, labeled capacity and 'C' rating, age in months and approximate number of cycles.

Your measured:

2) Lipoly cell IR,
3) Temperature at time of measurement,
4) The measuring equipment used

And from the spreadsheet calculator:

5) FOM,
6) Calculated True ‘C’ rating

A reference thread like this is only useful if it remains a data only thread. This thread is not the place for: comments on the validity of IR, claims that one method is better than another, theoretical discussion of the methodology, questions, and other intellectual banter. John Julian has initiated a separate discussion thread expressly for that which is unmoderated and where discussion and comments are encouraged. Discussion thread here: http://www.rcgroups.com/forums/showthread.php?t=1577989

Please note that IR measurement is greatly affected by temperature and readings must be taken 72± 2 degrees F (22± 1 deg. C) after 1 hour minimum dwell at this temperature. If using a charger that takes internal resistance measurements during charge cycle, charge at 1C maximum to limit effects of cell heating during charging. Data that is posted that is measured outside this range will be deleted.

If this method proves useful, posted data will be periodically consolidated to a searchable database for user convenience.

This is a REFERENCE thread. Please post only data to this thread.

This is a useful rule-of-thumb technique that seems to be valid for a range of batteries and conditions and that has been verified over a long period.

We know there are some anomalies, particularly for smaller size, poorer performing batteries but in general it is a lot better than the absurd C rating claims on most lipoly pack labels.

Like all simple rules-of-thumb, like the Watts/Pound rule, we think the usefulness and simplicity for the average modeler far outweigh the exceptions.

The Performance Calculation Tool has been demonstrated to be valid if the cell IR is measured:

1) After 1 hour minimum rest to stabilize charge and pack temperature (or at 1C max charge rate if using FMA charger).

2) At measurement temperature of 72F (22C).

3) Using a reliable IR measurement method. The ESR/IR meter was used for the verification of the method, and it has also been checked with an iCharger 3010B, 306B, and FMA Powerlab PL8.

To use the performance calculation tool, simply input measured cell internal resistance and pack capacity into the appropriate (green) cells and the tool calculates maximum recommended current, FOM, and True C rating. It is recommended that you do not exceed the maximum recommended current in order to prevent damage to your lipoly pack. FOM and True C rating numbers can be used for future comparison. Note that since internal resistance of a lipoly changes over time, the ability of the pack to deliver current without overheating will also change as the pack ages, making the manufacturer ‘C’ rating even more useless. You can use the calculator to easily calculate maximum recommended current and True C rating and write this information on a label and stick to the pack and simply update it as it changes.

As noted in post #1 above, temperature at time of measurement and ‘settling’ time are EXTREMELY important. In order to be valid, internal resistance measurements must be taken at 72± 2 degrees F (22± 1 deg. C) after 1 hour minimum dwell.

Sample screen shot of how the tool works:

The results from the calculator are based on an initial energy dissipation of 6W/Ah which has been found to give acceptable performance without significant degradation for a wide range of lithium polymer chemistry batteries. This figure has been empirically determined and validated by three technically qualified modelers in three countries.

It is true that this may seem overly simplistic, and it is entirely possible that the formula could be refined with additional variables and various adjustments - however the effect would be to make it completely useless for all except the expert and knowledgeable. With acknowledgments to those who are truly expert, we think the average flyer will find the calculated maximum current and 'True C' value figures from the spreadsheet to be extremely helpful. If all possible variables are accounted for, we think the average flyer is left scratching his head. Consequently, the calculation is deliberately simple but our data shows it to be surprisingly effective in predicting lipoly performance.

Rather than get involved in armchair debate, we would much prefer that modelers actually use the performance calculator and poke holes in it from a practical perspective if they find it to be less than useful or providing inaccurate performance predictions.

We believe that there is a majority of modelers that are less concerned with the theoretical derivation of the formula and would be grateful that someone is trying to make sense of the horribly flawed 'C' rating system. It's these that we want to encourage to try the calculator. If it proves wrong in practice we will gladly say so. However, crude rules-of-thumb like the “Watts/pound” rule have proved enormously useful in the past to help make eFlight practical and successful.

Once again, to comment, praise, lambaste, suggest improvements, or debate the merits of the tool, please refer to the discussion thread here: http://www.rcgroups.com/forums/showthread.php?t=1577989.

Reserved

Example of measured data:




The excel file for the above attached data is attached below in .zip format for you to download and use to track your lipolys. The formulas are embedded within the file and will automatically calculate max recommended current, FOM, and True C for each pack. You simply need to input relevant pack data and highest measured individual cell IR.

The data from the spreadsheet (screenshot, actual file, etc.) can then be posted to this thread.

Thunder Power PP65 1300 pack gives 10 milliohms per cell at 21c using a wayne meter. That results in a "FOM" of 0.92 and a "true C" of 21 per the spreadsheet.


Has anybody compared IR figures obtained from different chargers/meters? How do they compare? Seems to me the wayne meter numbers are consistently higher than those from an FMA charger.

Originally Posted by biskit

Has anybody compared IR figures obtained from different chargers/meters? How do they compare? Seems to me the wayne meter numbers are consistently higher than those from an FMA charger.

Great questions for the discussion thread here: http://www.rcgroups.com/forums/showthread.php?t=1577989

Mark

While we're in the testing mood, a cheap turnigy flightmax 30c 1300 pack gives 9-11 millohms per cell at 21c with the wayne meter. Using the 11mohms of the worst cell gives a "FOM" of 0.84 and a "true C" of 20 from the spreadsheet.

Oops, sorry for the question in the data thread!

Hi everyone,
I have posted the following data in support of the efforts being made by Wayne, Mark and John. My tests are based on a collection of packs I have from various origins (not all model flying) and the results should be fine apart from the Cycle count/age info which has been estimated rather than recorded.
I hope it adds to the total of the thread, more "grist for the mill".
regards
Bob

How do you figure the max Amps and true c values

My small collection of packs. FP = Flight Power. The FP packs are 2 x 1300 3S joined permanently to make a 6S pack.

this is awesome! Im gonna plug in all my batteries!

So I am pretty interested on the IR state of my batteries and testing it with my tricopter performance. My icharge gave me about 10mohm. Since the spread sheet script is protected I looked around different posts for a linear approx between True C and max AMPs. I realize that this is a non-linear relationship. I then got the Specs of my eflite motors, park 450s, and the weight of my tricopter to figure its performance ie WATTs/LBS and what each arm was seeing. I them tested it on my battery I pulled a total of 375 watts with a max amp of 36.75. I calculated a c rating of 16.75. By the was this is a 2200 nano tech 3s 25-50c. I know temp plays big in the calculations. I back calculated my IR to be about 8.0 mohm. I placed that into my other equation and figured that I have about 25watts due to total IR^2 losses which sound about right for eff of 92%.

Although this has a FOM of 0.774 it still delivers enough performance to fly my tricopter very nicely. 147.33 Watts/LBs is near the range which could be used for unlimited 3d performace IAW eflite. Maybe C-rating is really quite lower then on the packs or my cals are justed totally hosed, but the program on this thread sees to be working better then a thumb rule. if you account for losses it may very well be spot on. In playing with the numbers an FOM of 0.4196 would still get me about 100 watts/lbs which will still fly a tricopter ok or can be graduated to flying parkflyers with less power demands.

From my homemade IR setup.
All measurements at 25.2C.

1 common sense lipo for sale - "like new" - $40

My 3S lipo collection. Buyer beware on the new Rhinos as seen in data (40C #5-8). Billowys way overrated too and went downhill faster than older Rhinos and Hyperions.