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Old Jul 03, 2014, 04:56 PM
Dont be divided from the truth
Dangaras's Avatar
Canada, ON, Toronto
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Ignorance is bliss.... How does the model fly? When cells start getting weak you can 'feel' the difference IR meter or not.

The real truth is found using telemetry. Since I put my first telemetry system in and the shock it gave me regarding the real performance of batteries during flight, I now have one in every model. The batteries with 'issues' very quickly expose themselves.

I like the IR meter as a basic ground based tool....
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Old Jul 05, 2014, 12:06 PM
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...and the huge benefit to the 'ground based tool' is that you can isolate potential problems with your packs prior to strapping them into an expensive aircraft rather than finding the issue with a huge drop in power and perhaps experience an unfortunate and unplanned encounter with mother earth. I prefer the former.
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Old Jul 05, 2014, 01:08 PM
3D? I only got two thumbs!
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Joined Jul 2012
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Totally agree that the ground based tool is a good thing.

My puzzlement is that the plane flies just fine with the batteries in question, they're nicely balanced both before and after use, they aren't any warmer after a flight than I'd expect. However the IR measurement with the ESR meter produces numbers that indicate there the batteries are tired. If they really are as tired as the numbers suggest I would have thought it would be obvious in flight. The complicating factor is that these batteries are my first 4 cell batteries for use in my first two balsa planes - maybe they really are tired and the planes are so much better than the EPP hybrids I've been flying that I don't realize they're not performing as well as they could.

I've decided I'm going to order a couple new batteries of exactly the same kind and see how they compare.

I wouldn't be surprised if these planes to experience an unfortunate and unplanned encounter with mother earth sooner or later, but if they do I hope it's because I get too ambitious and aggressive flying, not for a lame reason like tired batteries
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Old Jul 05, 2014, 05:24 PM
ancora imparo
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Joined Jul 2005
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Aero, the numbers you are showing look fairly logical to me.

1) The ESR/IR meter shows a max IR of 9.12 for a 2600 cell. This comes out at a recommended current draw of 41 but that's a conservative number and for average current. You say these packs are peaking at between 47 and 60 Amps (guestimate). So the average current draw is likely to be 40Amps or under - unless you drive around with the throttle wide open all the time which you clealy don't. The fact the packs deliver adequate power and come down mildly warm confirms this.

2) My PL8 always gives lower IR than the meter. It measures IR during a very brief resting pulse and we do not know what correction algorithm FMA have used to allow for voltage settling. So I would not expect the two methods to agree since they are measuring IR in different ways. My guess is that the PL8 is also more sensitive to variations in settling time between cells because of the very brief measurement window and a bit of variation in IR and inconsistancy is not really a surprise. I'm assuming the PL6 works the same.

Bottom line is don't get stressed about it but ignorance is not bliss.

The IR meter will accurately and consistantly track the decline of these packs and show up any potential problems arising. Don't sweat the absolute value it's the relative numbers over time and between cells that really matters. The LiPo tool is intended as an emperical guide, that's all.

You have the critical piece of information - these packs fly the plane just fine and do not get excessively hot. Just keep monitoring their health with the IR meter and observing flight performance.

I'm guessing given their age and your recorded 3.5V under load that they are actually average quality "20/25C" packs that are gradually increasing in IR as they get older. Probably good for quite a while yet, especially in your 700 Watt plane. 900 Watts is probably pushing them a bit.

John
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Originally Posted by AeroKen View Post
Totally agree that the ground based tool is a good thing.

My puzzlement is that the plane flies just fine with the batteries in question, they're nicely balanced both before and after use, they aren't any warmer after a flight than I'd expect. However the IR measurement with the ESR meter produces numbers that indicate there the batteries are tired. If they really are as tired as the numbers suggest I would have thought it would be obvious in flight. The complicating factor is that these batteries are my first 4 cell batteries for use in my first two balsa planes - maybe they really are tired and the planes are so much better than the EPP hybrids I've been flying that I don't realize they're not performing as well as they could.

I've decided I'm going to order a couple new batteries of exactly the same kind and see how they compare.

I wouldn't be surprised if these planes to experience an unfortunate and unplanned encounter with mother earth sooner or later, but if they do I hope it's because I get too ambitious and aggressive flying, not for a lame reason like tired batteries
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Old Jul 05, 2014, 05:38 PM
Dont be divided from the truth
Dangaras's Avatar
Canada, ON, Toronto
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Quote:
Originally Posted by mrforsyth View Post
...and the huge benefit to the 'ground based tool' is that you can isolate potential problems with your packs prior to strapping them into an expensive aircraft rather than finding the issue with a huge drop in power and perhaps experience an unfortunate and unplanned encounter with mother earth. I prefer the former.

I have to disagree somewhat.

Having a tool and knowing how to use is the issue here. When two pieces of equipment that do the same thing give different results I will trust neither unless there is a very detailed document that explains why and how one works.

Volts and current are measured the same by all voltmeters and ammeters. The only difference is whether they were calibrated by a certified facility. Would you trust the voltage on a pack if the Meters you used told you you a reading in coulombs instead of volts and gave you different values?

Relative measurements also don't help if you have no empirical history or reference with which to base your measurements on...

So in this case it is not a good ground to until........ The user gets educated to the nature of what is being measured.
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Old Jul 05, 2014, 06:04 PM
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Originally Posted by Dangaras View Post
When two pieces of equipment that do the same thing give different results I will trust neither unless there is a very detailed document that explains why and how one works.
http://www.rcgroups.com/forums/showp...85&postcount=1

http://www.rcgroups.com/forums/showp...08&postcount=6
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Old Jul 05, 2014, 06:32 PM
Dont be divided from the truth
Dangaras's Avatar
Canada, ON, Toronto
Joined Mar 2010
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Thanks, good thing to post the documentation again but I already understand, I was referring to the people who barely understand volts and current and power.......

There has to be an absolute reference for all measurements. I will read the docs again for my own update.
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Old Jul 05, 2014, 09:37 PM
3D? I only got two thumbs!
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Quote:
Originally Posted by jj604 View Post
Aero, the numbers you are showing look fairly logical to me.

1) The ESR/IR meter shows a max IR of 9.12 for a 2600 cell. This comes out at a recommended current draw of 41 but that's a conservative number and for average current. You say these packs are peaking at between 47 and 60 Amps (guestimate). So the average current draw is likely to be 40Amps or under - unless you drive around with the throttle wide open all the time which you clealy don't. The fact the packs deliver adequate power and come down mildly warm confirms this.

2) My PL8 always gives lower IR than the meter. It measures IR during a very brief resting pulse and we do not know what correction algorithm FMA have used to allow for voltage settling. So I would not expect the two methods to agree since they are measuring IR in different ways. My guess is that the PL8 is also more sensitive to variations in settling time between cells because of the very brief measurement window and a bit of variation in IR and inconsistancy is not really a surprise. I'm assuming the PL6 works the same.

Bottom line is don't get stressed about it but ignorance is not bliss.

The IR meter will accurately and consistantly track the decline of these packs and show up any potential problems arising. Don't sweat the absolute value it's the relative numbers over time and between cells that really matters. The LiPo tool is intended as an emperical guide, that's all.

You have the critical piece of information - these packs fly the plane just fine and do not get excessively hot. Just keep monitoring their health with the IR meter and observing flight performance.

I'm guessing given their age and your recorded 3.5V under load that they are actually average quality "20/25C" packs that are gradually increasing in IR as they get older. Probably good for quite a while yet, especially in your 700 Watt plane. 900 Watts is probably pushing them a bit.

John
Thanks for the detailed response John. Your conclusion is especially helpful.
This was a bit of an odd situation since the packs had sat on the shelf for almost 2 years, I wasn't sure how much they had pre-aged before I got my first IR measurement and first flights with them. It will be very interesting to see what the IR is on the two new identical packs I ordered turns out to be.
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Old Jul 06, 2014, 02:21 PM
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Just a bit of background might help here.
As a electronics power engineer and modeller, it was natural, when I retired, to look at how to measure the performance of the battery packs I was using in my models.
When working, I often had to choose batteries for use in power system back up applications which in those days were usually nicads.
The manufacturers always quoted the impedance of the cells measured at 1Khz which I always assumed was a useful parameter to know for general use, but not so much help when selecting cells for high current applications. Lipo makers still use this technique ( possibly because the results look better ) but it measures IMPEDANCE not RESISTANCE.
Applying a heavy DC load is as near as we can get to our real life use of the pack to measure the cell performance under controlled conditions.
I therefore started by applying high current load pulses to the nicads I was using in models and noting their behaviour, specifically the voltage drop and recovery time with an oscilloscope.
As Lipos appeared I applied the same techniques and noted that the recovery time, as well as the volt drop, varied from one type to another.
I finally settled for a 15mS constant current load pulse of 16A as the best compromise. A 16A pulse load is a reasonably heavy current and the voltage of all but the very poorest lipos have recovered to steady state in less than 15mS.
The reason it is a compromise is that the powerfet in the unit is subject to 400Watts on a 6S pack during the measurement pulse and must therefore be a substantial (read expensive) device. Using a shorter pulse would save cost but would give an optimistic result on poorer cells. A shorter pulse may well be why the PL8 shows lower IR results
Using these values I built an analogue test rig and not being software capable, I asked Phil Green (phil_g on RCG) if he would design the operating system which he kindly did. We put the two together and the meter worked accurately and consistently immediately. I was never intending to sell them but made a few for friends and then, after interest on forums, decided to build a small batch to see if other modellers were interested. They were and still are, so I have carried on making them.


In case you canít be bothered to read all the above nausea, then in a nutshell, the meter comes as close to measuring and quantifying the voltage drop under real operating conditions as we can simulate.
Full discharge plots are better but take expensive equipment, knowledge of what you are doing and a lot of patience. I used to do a lot of such testing but the meter results are instant and correlate full discharge test results closely.

We all know that heat dissipated in a pack is bad news and often judge our lipos on how hot they are following a flight. The Lipotool is based on a maximum heat dissipation within a cell using the IR value and therefore gives a maximum recommended steady discharge current to avoid exceeding this limit which might otherwise produce excessive temperature rise.
This again correlates with full discharge test results where tests at higher than the recommended max current produced excessive temperature rise and other signs of stress.

Wayne
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Old Jul 07, 2014, 08:11 PM
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I cannot help but wonder if there is a way to modify an ESC to work with 3 passive loads to simulate high power conditions that occur in flight. Or, I suppose that a motor with a load other than a flight propeller might be used as a load.
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Old Jul 08, 2014, 08:03 AM
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Originally Posted by chuck75 View Post
I cannot help but wonder if there is a way to modify an ESC to work with 3 passive loads to simulate high power conditions that occur in flight. Or, I suppose that a motor with a load other than a flight propeller might be used as a load.
I've heard of light bulbs being used to simulate a load
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Old Jul 08, 2014, 09:32 AM
ancora imparo
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Melbourne, Australia
Joined Jul 2005
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I don't believe there is any simple way to load an ESC with a passive load since the ESC relies on the back EMF from the rotating motor to synchronise the drive pulses.

You can however use a motor driving another motor with shorted windings (ie as a generator) as the load. It's no longer simple, and approaching a true dynamometer test.

Simpler to use a direct passive load on the battery. This can be plain resistive, like light globes or power resistors, or active such as a controlled voltage regulator into a resistive load or a controlled FET Load. If you want a known constant current or power load then it has to be actively controlled.
Quote:
Originally Posted by chuck75 View Post
I cannot help but wonder if there is a way to modify an ESC to work with 3 passive loads to simulate high power conditions that occur in flight. Or, I suppose that a motor with a load other than a flight propeller might be used as a load.
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Old Jul 08, 2014, 10:36 AM
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There are also commercial off the shelf products available. Sometimes surplus equipment can be found for reasonable prices.
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Old Jul 11, 2014, 01:31 AM
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Originally Posted by Ohmic View Post
There are also commercial off the shelf products available. Sometimes surplus equipment can be found for reasonable prices.
Here is my super simple and adjustable battery load gadget, each bulb is 50watt (5amps at 10v) I use a little desk fan to cool the bulbs and the whole setup because at 25amp draw there is a lot of heat there.

I have since added an extra mount to give a total adjustable load up to 30amps, works really well.

My ESR meter arrived last week, I really love it, it along with the Emeter PC suite, is a great setup to test and log battery reliability.
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Old Jul 11, 2014, 01:41 PM
3D? I only got two thumbs!
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The two new batteries I ordered arrived - these are identical to the four I was asking about the IR of above. I checked the IR of the new batteries and one of the old ones after letting them sit overnight with the other ones so they'd all be at the same temperature (~75 F, been hot here recently, even the basement floor has warmed up). The highest cell IR in the new ones measures less than half of the lowest cell IR in the old ones. Much more inline with what I was expecting, I'm guessing I'm in for a real pleasant surprise this weekend when I throttle up with a new battery.

I guess the higher IR of the old ones is just what comes of a couple years storage at storage charge in the 60-75+ temps in my basement.
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