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Old Mar 25, 2012, 11:52 AM
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Essentially, yes. Internal resistance causes a voltage drop within the cell when current is drawn, in accordance with Ohm's law (V=IR). Lower voltage under load means your power system will draw less current.

Note that the internal resistance will cause increased heating within the cell. The amount of power being dissipated internal to the pack is a function of the internal resistance X current squared (P=I2R). As a pack ages, this power dissipation and associated heating can become significant and accelerate the early retirement of your lipolys.

As you can see, as packs age and internal resistance goes up, the pack's ability to sustain voltage under load diminishes and the pack can be damaged if run too hard. Effectively, 'C' rating changes over time. For the most part, the 'C' rating written on the side of of most packs is useless marketing chatter and will have you damaging your packs if you follow it. It's precisely this reason why Wayne, John, and myself are attempting to make sense of this and give the modeler something useful to get the most out of his lipoly investment, and why I included the 'True C' rating in the formula.

Follow the 'True C' and maximum current recommendations and you will have happy lipolys.

Mark
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Old Mar 25, 2012, 01:46 PM
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Quote:
Follow the 'True C' and maximum current recommendations and you will have happy lipolys.
I intend to; hence the questions, the spreadsheet and investing in a meter. Thank you for the hard work. Cheers.
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Old Mar 31, 2012, 09:34 PM
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I am also interested in a predictive measure of battery performance. The method and tool presented here seems to be a nice and easy way of doing this.

I will make up a pair of leads for my Fluke meter to plug into the balance connectors of my packs which will give me a manual method of measuring and calculating IR for each cell. Is there a suggestion for standardising conditions under which we measure IR so that we eliminate as many extraneous variables as possible? For example, a fixed temperature, standard load current (presumably in C to load bigger packs at a more representative draw), etc.?

Jeff
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Old Apr 01, 2012, 12:15 AM
ancora imparo
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Jeff, you can't measure battery IR with a normal multimeter on the resistance range if that is what you have in mind (although I don't think it was - just covering the bases!).

You need to measure the cell voltage drop at known current load and then calculate the IR. So at minimum you need an accurate cell voltage reading, an accurate pack current reading and an accurate time of measurement. That's what the ESR/IR meter and the chargers do. They use different methods to do it and so come up with slightly different results. Personally I think the ESR/IR meter method is the closest to how we expect LiPos to behave in real life, but it isn't 100% clear exactly what each of the chargers do and they may have some internal magic that assists.

You could achieve the same result with a Fluke on cell voltages and an accurate low loss current measurement such as an Emeter or similar wattmeter. Be mindful that the time of loading will affect the results at high current.

Mark may want to advise on the conditions he recommends for consistent results as he did the work on this not me.

John

Quote:
Originally Posted by ParkJeff View Post
I am also interested in a predictive measure of battery performance. The method and tool presented here seems to be a nice and easy way of doing this.

I will make up a pair of leads for my Fluke meter to plug into the balance connectors of my packs which will give me a manual method of measuring and calculating IR for each cell. Is there a suggestion for standardising conditions under which we measure IR so that we eliminate as many extraneous variables as possible? For example, a fixed temperature, standard load current (presumably in C to load bigger packs at a more representative draw), etc.?

Jeff
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Old Apr 01, 2012, 12:57 AM
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>> ... intent of the thread ... practical rule-of thumb ... assist modelers ... anarchy reigns ...Speculation

alright here's my USD 0.02. Bluntly undiplomatic but nonetheless in a constructive spirit:

You guys are missing the point. To evaluate the performance of a pack, I take it to the field and fly. Any scientific measurements, figure-of-merit etc is useless, if - as observed - the numbers are all over the place.

Stating the obvious (100 % "my opinion" / experience)
- for consistent top-notch performance, I buy quality LiPos. Simple as that. I need only two sets and charge the other one at high C while flying.
- I store at ~3.8 V, not fully charged
- I operate at room temperature (they don't like cold)
- I discharge within 3..4 minutes down to 3.2 to 3.5 V (it will drop very rapidly within the last few mAh). Avoid excessive current draw at the end and let them shoot of their mouths that it will kill the LiPos. It doesn't kill mine.
- Helis only: If power with today's technology is a problem, you're either into something stupid (drag racing), competing on international level or haven't learned to fly.
- the latter is the easiest to fix, so why not spend the time flying/simulator? A light-handed flying style will make you look like you've got 200 % the power. What you can get from voodoo-scientific optimizations is marginal at best.
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Old Apr 01, 2012, 02:01 AM
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Hi John,

I was planning to measure open circuit and loaded voltages across the cells and pack and use the voltage differential together with load current to calculate IR values.

The pack will heat up during measurement and this may affect the IR values measured. I was hoping that measurement duration of 10s to 20s would not have too much of an impact.

Does load current during measurement have a great effect on measured IR? If not, then a fixed current draw across a range of packs would be easiest to implement.

Jeff
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Old Apr 01, 2012, 02:26 AM
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In my experience, pulse duration for voltage drop measurements should be kept as short as practical to avoid I2R effects. Wayne Giles expended a great deal of time and effort researching and perfecting the ESR Meter. My experience with the iCharger 106B+ and 306B has demonstrated that they too are a tremendously accurate predictor of actual performance, and consistent with Wayne's amazing meter.

A snippet of my testing has been laid bare here: http://www.rcgroups.com/forums/showp...9&postcount=40.

As one can readily see from the 1:1 correlation with discharge curves, IR measurement was deadly accurate at predicting in-flight performance.

To your query - performing open and loaded voltage measurements to calculate IR is technically possible but I'm doubtful that it will be consistently repeatable unless stringently controlled. This is where a carefully thought-out measurement technique is tremendously beneficial. Pulse duration, load, and temperature at time of testing are extremely important parameters that must be controlled in order for the testing to provide useful results. I've found that the iChargers and Wayne's ESR Meter make IR measurements to be very easy and amazingly repeatable.

You might do a search on some of Wayne's earlier posts on this subject. He has vast experience with lipoly ESR measurement technique that has stood the test of scrutiny from many peers.

Mark
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Old Apr 01, 2012, 02:46 AM
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Originally Posted by gravityKills View Post
Any scientific measurements, figure-of-merit etc is useless, if - as observed - the numbers are all over the place.
Your comments are much appreciated. The fact is that IR measurement, when controlled, is amazingly consistent and anything but "all over the place".

Agree that flying is believing but objective IR measurement can reveal subtle differences that the subjective experience of flying cannot resolve. IR measurement is also far more practical than flying. Put a pile of 50 lipolys in front of me and I can sort them by level of performance within a matter of minutes with my ESR Meter. The same exercise would take me days with my planes and would be nowhere near as accurate.

Mark
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Old Apr 01, 2012, 03:06 AM
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Originally Posted by mrforsyth View Post
Your comments are much appreciated. The fact is that IR measurement, when controlled, is amazingly consistent and anything but "all over the place".

Agree that flying is believing but objective IR measurement can reveal subtle differences that the subjective experience of flying cannot resolve. IR measurement is also far more practical than flying. Put a pile of 50 lipolys in front of me and I can sort them by level of performance within a matter of minutes with my ESR Meter. The same exercise would take me days with my planes and would be nowhere near as accurate.

Mark
Agreed... using my iCharger 306b has been extremely easy and I'm able to check my packs conditions and document them over time. I made a little notepad document to keep track of their performance and so far it has been a great help in keeping tabs on battery health and performance.
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Old Apr 01, 2012, 04:37 AM
Uh Oh Jungo
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Hi Mark,

Thanks for your reply and explanations. Seems that my assumptions were a little simplistic. Will look into getting an iCharger 106B+ since I was planning to get another charger. The ESR meter looks good too.

Would it be correct that with either an 106B+ or Wayne's ESR meter and assuming we make measurements at an agreed upon temperature., that we can then make pack comparisons based solely on IR measurements alone? It would be a nice step forward to be able to make reasonably reliable quantitave comparisons of expected pack performance by user measurements from all over the world.

Jeff
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Old Apr 01, 2012, 05:54 AM
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Hi Mark,


Would it be correct that with either an 106B+ or Wayne's ESR meter and assuming we make measurements at an agreed upon temperature., that we can then make pack comparisons based solely on IR measurements alone? It would be a nice step forward to be able to make reasonably reliable quantitave comparisons of expected pack performance by user measurements from all over the world.

Jeff
Jeff
I think that the answer to your question is an emphatic "Yes".

In doing a lot of full discharge testing of lipos I always measured the IR of the pack before testing at 25degC using a pulse load and an oscilloscope and it quickly became apparent that there was a very close correlation between the overall performance and the IR.
This to be expected as it is equivalent to adding a resistor in series with the motor which will drop volts and get hot as it wastes energy which would otherwise be delivered to the prop.
Measuring IR with an oscilloscope was cumbersome and slow so I built the meter and I find that I can forecast the results of full discharge performance runs so well that many are redundant.

Mark and John Julian have done a lot of investigation using both their I chargers and PL8 chargers and the meter and apart from the very rare anomaly, measuring IR at a known temperature and putting it into Mark's calculator, will give you a real "C" rating for the Lipo, which is a real measure of its performance. You can find the calculator at http://www.rcgroups.com/forums/showt...1#post20484169

Wayne
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Old Apr 01, 2012, 06:30 AM
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Wayne,

If this works out as expected, you, Mark and John have done the rest of us a great service.

I look forward to the day where we can tabulate the IR's of popular packs as a sticky in this forum and have an easy cost vs performance comparison which cuts through all the "max C rate" hype that we have put up with for so long.

Good job with the meter too. Hopefully someone will carry it in Aus.

Jeff
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Old Apr 01, 2012, 07:32 AM
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Or the web version at

www.jj604.com/LiPoTool/

If that is more convenient.

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. You can find the calculator at http://www.rcgroups.com/forums/showt...1#post20484169

Wayne
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Old Apr 01, 2012, 08:00 AM
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>> Agree that flying is believing
that's not what I said Quite the opposite:

A flight test will reveal the actual performance over the full discharge of the pack with the real-world load, not some small-signal / linearized guesstimate.
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Old Apr 01, 2012, 09:18 AM
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Originally Posted by ParkJeff View Post
Will look into getting an iCharger 106B+ since I was planning to get another charger. The ESR meter looks good too.
Jeff,

In addition to Wayne's above comments, one additional point that you should be aware of is the resolution of the iCharger vs. the ESR Meter. The iCharger has 1 mOhm resolution and, as such, will round to the nearest integer. Conversely, the ESR Meter has .04 mOhm resolution. With large capacity (>3000mAh) high performance packs, resolution becomes an important factor and the iCharger will not be able to resolve differences as accurately as the ESR Meter.

I have found that for packs that are ~3000mAh and smaller, the 1 mOhm resolution of the iCharger is perfectly adequate in providing an excellent performance prediction. If you're using 5000mAh lipolys in a high performance application and want to be able to quantify the performance of your packs, you will want the ESR Meter.

Quote:
Originally Posted by ParkJeff View Post
I look forward to the day where we can tabulate the IR's of popular packs as a sticky in this forum and have an easy cost vs performance comparison which cuts through all the "max C rate" hype that we have put up with for so long.
This is precisely what was envisioned with the database thread: http://www.rcgroups.com/forums/showthread.php?t=1578001

Whether this method it will prove to be useful to many modelers over the long term remains to be seen. It has sure been helpful to me personally so far and has enabled me to very quickly get an extremely accurate assessment of the real-world performance of my lipoly investment.

Mark
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