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Old Jan 27, 2011, 05:55 PM
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There is something wrong in those figures as they are all far too high.
eg I know from measurements that a Turnigy 2200 25C cell at room temp (20 - 25Deg cent is about 5 - 7 milliohms.
If you look at the 'worst' pack; the 4S 3700 20C Rhino, then the total resistance of the four cells is 446 milliohms ie 0.446 ohms. If this were correct putting a dead short on a fully charged pack would only produce a max current of 37.7A ie 10C, or put another way if you tried to take 10 C from the pack the voltage would fall to zero.
How does the meter take a reading - do you press an operating button?
Is it only connected to the pack by the balance connector when measuring IR.
I have looked at the instruction manual but cannot see how you are intended to take IR readings.

Wayne
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Old Jan 27, 2011, 06:36 PM
ancora imparo
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Melbourne, Australia
Joined Jul 2005
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Wayne, from the diagram in the manual, it looks like this is a simple two wire IR meter. Separate battery to power it and you just plug a two pin jumper into the balance connector. See attached. Explains the readings. It is measuring the resistance in the balance leads as well. Same issue we had with Paul Daniel's otherwise excellent IR meter (which I bet was more accurate than this one in any case!).

Looks like a useful all round meter though. Be interested when someone does a decent test and checks it for accuracy.

John
Quote:
Originally Posted by Wayne Giles View Post
There is something wrong in those figures as they are all far too high.
eg I know from measurements that a Turnigy 2200 25C cell at room temp (20 - 25Deg cent is about 5 - 7 milliohms.
If you look at the 'worst' pack; the 4S 3700 20C Rhino, then the total resistance of the four cells is 446 milliohms ie 0.446 ohms. If this were correct putting a dead short on a fully charged pack would only produce a max current of 37.7A ie 10C, or put another way if you tried to take 10 C from the pack the voltage would fall to zero.
How does the meter take a reading - do you press an operating button?
Is it only connected to the pack by the balance connector when measuring IR.
I have looked at the instruction manual but cannot see how you are intended to take IR readings.

Wayne
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Old Jan 27, 2011, 07:19 PM
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Originally Posted by jj604 View Post
Wayne, from the diagram in the manual, it looks like this is a simple two wire IR meter. It is measuring the resistance in the balance leads as well.
...which makes it functionally useless for high capacity and high performance cells, in my view.

Any IR measurement device must employ 4-wire measurement (main connector and balance connector) in order to serve any value other than mere curiosity.

Just my 2 cents, of course.

Mark
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Old Jan 27, 2011, 07:30 PM
Learning is worth the effort
United States, MI, Grand Rapids
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Quote:
Originally Posted by Wayne Giles View Post
There is something wrong in those figures as they are all far too high.
eg I know from measurements that a Turnigy 2200 25C cell at room temp (20 - 25Deg cent is about 5 - 7 milliohms.
If you look at the 'worst' pack; the 4S 3700 20C Rhino, then the total resistance of the four cells is 446 milliohms ie 0.446 ohms. If this were correct putting a dead short on a fully charged pack would only produce a max current of 37.7A ie 10C, or put another way if you tried to take 10 C from the pack the voltage would fall to zero.
How does the meter take a reading - do you press an operating button?
Is it only connected to the pack by the balance connector when measuring IR.
I have looked at the instruction manual but cannot see how you are intended to take IR readings.

Wayne
Thank you for your comments. Yes, I failed to mention an important parameter, the ambient temp of the batts. All batteries were at 70 degrees F for all measurements. I also noticed my readings were high compared to your meter. But I dismissed that because they appear to be approx one decimal point off. I thought maybe the designers of my meter might have used a load resister that was incorrect for milli-ohms. Maybe I'm wrong there but it seems logical considering the source (China).

The measurement is taken at the ballance connector by inserting only two pins, feeding the meter input, into the balance connector of the battery. I started from the most negative point moving up the balance connector. So, the black wire of the meter was inserted in the most negative pin of the balance connector of the battery with its associated red wire on the pin next to it. I moved up the balance connector, one pin at a time until all cells were measured. Also note that the meter is powered by another (separte) LiPo battery pluged into the meters power connector, independent of the battery under test.

What do you think about my theory that my readings are approx one decimal point off? I'm going to be really disappointed if that isn't the case. At least I am comfortable with that because we are really only looking for relative readings from cell to cell and battery to battery.

Many thanks for your input.
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Old Jan 27, 2011, 07:40 PM
ancora imparo
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The readings are not out by a decimal point. Mark's comment is correct. this meter is useless for measuring IR. The IR of modern reasonable sized LiPo cells is significantly lower than the resistance of the two balance leads and the contact resistance of the connectors. The designers just didn't understand what they were doing.
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Thank you for your comments. Yes, I failed to mention an important parameter, the ambient temp of the batts. All batteries were at 70 degrees F for all measurements. I also noticed my readings were high compared to your meter. But I dismissed that because they appear to be approx one decimal point off. I thought maybe the designers of my meter might have used a load resister that was incorrect for milli-ohms. Maybe I'm wrong there but it seems logical considering the source (China).

The measurement is taken at the ballance connector by inserting only two pins, feeding the meter input, into the balance connector of the battery. I started from the most negative point moving up the balance connector. So, the black wire of the meter was inserted in the most negative pin of the balance connector of the battery with its associated red wire on the pin next to it. I moved up the balance connector, one pin at a time until all cells were measured. Also note that the meter is powered by another (separte) LiPo battery pluged into the meters power connector, independent of the battery under test.

What do you think about my theory that my readings are approx one decimal point off? I'm going to be really disappointed if that isn't the case. At least I am comfortable with that because we are really only looking for relative readings from cell to cell and battery to battery.

Many thanks for your input.
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Old Jan 27, 2011, 07:51 PM
Learning is worth the effort
United States, MI, Grand Rapids
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Originally Posted by mrforsyth View Post
...which makes it functionally useless for high capacity and high performance cells, in my view.

Any IR measurement device must employ 4-wire measurement (main connector and balance connector) in order to serve any value other than mere curiosity.

Just my 2 cents, of course.

Mark
Mark, I appreciate your thoughts. You are correct about how the measurements are made. It does not employ a 4-wire measurement. Again, you are also correct in my interest being more curiosity from the standpoint of knowing which of my batteries are better than others and how much better on a relative basis. Since there is nothing I can do to improve the IR, I wasn't really concerned about absolute accuracy....I just wanted to know how bad off my batteries were in relation to others. If I could do somehting to improve the IR then I might look at things differently. At least I know not to store my batteries charged. These forums taught me a valuable lesson there. I really appreciate everyones comments because I learn a lot from them.
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Old Jan 27, 2011, 08:32 PM
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Originally Posted by jj604 View Post
The readings are not out by a decimal point. Mark's comment is correct. this meter is useless for measuring IR. The IR of modern reasonable sized LiPo cells is significantly lower than the resistance of the two balance leads and the contact resistance of the connectors. The designers just didn't understand what they were doing.
I guess I am not real concerned about absoute, scientific accuracy. Please explain to me why real accurate absolute readings are important if there is nothing we can do to change (improve) the IR? If we are only looking to see relative condition of our batteries and degradation over time, why can't the readings from this (not so good) meter satisfy that? Isn't that all a "good meter" would do?

Please understand I am not defending this meter, I am only trying to say that I don't care if it mesures "apples" and not "oranges". Because as long as I continue to use this same meter it will always be measuring "apples to apples" in relation to previous readings. Maybe this isn't a good analogy but it makes sense to me.

My readings give me a baseline of new (unused) batteries from which I can compare batteries in various stages of age and use. That's really all I want since I can't do anything about it anyway, except throw away the worst ones. Oh, I guess I could consider buying more expensive, name brand, batteries like ThunderPower or PolyQuest batteries that may not degrade as fast. Another possibility is that Rhinos might not be a good value afterall. But my Rhinos have not been given proper care, so I will hold off on that conclusion. Thanks again for your comments.
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Old Jan 27, 2011, 08:45 PM
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In my view, a very inaccurate meter is of less use than putting a battery in a plane, running up the throttle, and checking voltage under load with a wattmeter.

If used as you indicate - to monitor packs over time, I concede that it can have some utility as long as you understand that a portion of the witnessed change in IR could be due to increase in balance connector contact resistance.

Personally, however, it would drive me bonkers as there's really no way of knowing whether the lipoly cell is deteriorating or the balance connector is deteriorating. But then I'm perhaps less tolerant of such things than most...

Mark
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Old Jan 27, 2011, 09:55 PM
ancora imparo
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Melbourne, Australia
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I second that. There are some things where relative accuracy is fine, some where approximate is good enough, and some where good accuracy is essential. And for the last, I am not meaning lab quality, just good enough for toy aeroplane purposes. Measuring cut off voltages for a LiPo charger or rpm is an example of the last one. You want to be as accurate as it is possible to be.

Approximate is fine for some things. kV for ordinary modelers is a reasonable example. Excepting the gurus and motor designers for most of us a kV reading within 100-200 is perfectly close enough for practical purposes (like figuring out what prop might be appropriate). If you are serious, then further measurement with a wattmeter is going to be involved anyway.

Relative accuracy is OK if you wanted (for example) to keep track of your IR over time or compare two similar batteries. Problem here is there are TWO sources of error. The intrinsic measurement accuracy of the meter - which I suspect is probably OK- and the error due to the method of measurement which is not. IMO this meter is WORSE than no meter for measuring IR and I agree with Mark. It is giving you a totally false sense of what you are measuring. You are measuring a small (~ few mOhm) cell resistance in series with a larger (~10's mOhm) lead and contact resistance which will vary from cell to cell and over time. Problem is in this age of digital readouts we all get caught up in the apparent authority of the number on the screen and lose sight of how it got there. The vast majority of modellers would quite reasonably buy one of these, take a reading and expect (say) the difference between 23mOhm and 25mOhm on two cells to mean something. It doesn't. It is much more likely to be the variation in the connection resistance. Any real change in IR of a new good cell will be of the order of 1or2 mOhm and will be swamped by any variation in lead resistance between the balance connections.

IMO, the only realistic way to measure IR for our purposes, which is to assess performance under heavy DC loads, is the way Wayne's meter and some chargers do it. Using 4 wires - one set to carry the fairly heavy current load needed and the other to measure only voltage with negligible current flow and hence the resistance of the leads has no effect on the reading.

Rant mode OFF

John
Quote:
Originally Posted by mrforsyth View Post
In my view, a very inaccurate meter is of less use than putting a battery in a plane, running up the throttle, and checking voltage under load with a wattmeter.

If used as you indicate - to monitor packs over time, I concede that it can have some utility as long as you understand that a portion of the witnessed change in IR could be due to increase in balance connector contact resistance.

Personally, however, it would drive me bonkers as there's really no way of knowing whether the lipoly cell is deteriorating or the balance connector is deteriorating. But then I'm perhaps less tolerant of such things than most...

Mark
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Old Jan 28, 2011, 06:01 AM
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Rugby, UK
Joined Feb 2007
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Ibrinson,
I'm sure Mark and John are correct in saying that the readings you are getting are meaningless.
It occured to me that they could be a decade out but they would then be too low. The problem of trying to read anything useful into them is that they are so wildly out that the real figure you are trying to read, say 6 milliohms in the Turnigy 2200 25C case is less than 20% of the reading so it is swamped by the error itself. Therefore they are no use even as a comparitive tool.

Just to carry on John's rant, as he ran out of steam (!), there is a problem with a lot of cheap 'battery measuring and monitoring devices' which we now see at such bargain prices.
I have a small cheap wattmeter which reads cell voltages with errors of up to 9%, a little cell monitor which plugs into a balance connector and reads up to 17% error and have just bought a GT8 charger which shows charging and discharging errors in current (and hence capacity) of 14%.
John is cright in pointing out that when we read a voltage of say 3.732V on a digital display, we assume by implication that it is within 1mV as it has a resolution of 1mV.
We do need to be aware and question readings of many of these cheap instruments.

A UK Lipo supplier who uses a Robbe IR measuring device (not cheap) has just asked why my IR meter reads higher than the Robbe unit. It read 7 milliohms where the Robbe read 4 milliohms.
It turns out that the Robbe meter measures IMPEDANCE, ie it applies a 1kHz AC load and measures the ripple to take a reading.
The instruction book says that it measures RESISTANCE of a battery, which is just not correct.
The difference is that a lipo is a very capacitive component so it produces much lower readings. We are only interested in the resistance ie the volt drop on load.
I am surprised at such a basic error. As an old work colleague said to me; "Just because people manufacture equipment, it does not mean that they either know or care what they are doing!"

End of my rant which has not answered your question of how can you use IR readings.
The answer is that it allows you to compare different makes and it is IR which dictates how much power a pack can deliver and how hot it will get and hence probably, how long it will last.
Individual cell readings will also tell you if you have a weak cell which is usually a pre-warning of the end of life for that cell. A voltage reading will not tell you that.
To quantify the power bit, if you double the percentage voltage drop of a pack under load it will give an approximation of the percentage power loss.

Wayne
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Old Jan 28, 2011, 06:29 AM
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Fully charged.....
Well, most will agree that this does harm your LiPoly packs. Best is to store them at 50% of capacity. So arround 3.85 volt per cell.

Most likely, most of your packs would have been in much better state, if you did store them at 50% capacity!!!!!!!!

http://www.hobby-lobby.com/i_meter_7...34262_prd1.htm

Looks like an interesting device!
Especially for messuring the IR

Some of your packs will lack punch while flying. So the voltage will drop heavily under load.
Wise to change the habbit and keep your newer packs stored at 50%

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Old Jan 28, 2011, 11:37 AM
Learning is worth the effort
United States, MI, Grand Rapids
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Wayne, Mark and John,

Many thanks for your inputs and education. They did not appear to be rants to me, they were received as calm and rational explanations. Since I seem to be outnumbered as to the "usefulness" of my readings, I guess I need to reevaluate my opinion.

I certainly understand the benefit of testing the cells under load; however, I am having a difficult time getting through my thick skull why my "useless" readings seem to be giving me the results I expected to see. For example, I expected that my new unused batteries would have the least internal resistance...and they did. I also exected the oldest batteries with the most cycles to give the highest readings...and they did. So I am finding the readings useful from that standpoint and that is all I was looking for. Now I have a baseline form which to track their aging process and one battery to another.

I guess if I wanted to quantify the voltage drop of each cell/battery to determine power loss or C rating or something, then I would want accurate readings to work with. But right now, I am not concerned with an error of even 25% because all of my readings should have the same error and therefore I am getting the relative readings I wanted. Maybe I should be looking for more than that...I don't know.

We have beat this dead horse enough. Thanks again for your inputs and thoughts. They are much appreciated and I learned something.
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Old Jan 28, 2011, 12:25 PM
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Just thinking out loud here...

While I still maintain that a 2 wire device has very limited utility for measuring individual cell IR due to balance lead and connector contact resistance, it can be significantly more useful when measuring total pack internal resistance. The reasons why this is the case is very clearly explained by Wayne in post #1 of this thread (and precisely how his wonderfully elegant device works when in 'pack' mode).

Whether this device is capable of handling higher voltage on the input port when in 'internal resistance' mode is unknown and my quick skim of the manual did not provide any clarity. Were it me, I would fabricate a very short lead with heavy gauge wire that connects from the input port directly to the main pack connector (short charge lead) and give it a whirl. If doing so does not smoke the device and precipitates a reasonable and consistent reading, I would agree that this device is more useful for IR measurements than my initial thoughts.

Wayne's is MUCH better though.

Mark
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Old Jan 28, 2011, 08:18 PM
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Originally Posted by TreeDiver View Post
Fully charged.....
Well, most will agree that this does harm your LiPoly packs. Best is to store them at 50% of capacity. So arround 3.85 volt per cell.

Most likely, most of your packs would have been in much better state, if you did store them at 50% capacity!!!!!!!!

http://www.hobby-lobby.com/i_meter_7...34262_prd1.htm

Looks like an interesting device!
Especially for messuring the IR

Some of your packs will lack punch while flying. So the voltage will drop heavily under load.
Wise to change the habbit and keep your newer packs stored at 50%

Oh yea!! I have seen the light about not storing LiPos charged. I now store them somewhere between 3.8 and 3.85 V per cell. Although I knew there were problems with storing batteries charged, I didn't realize the drastic impact it had. I purposely traded off the convenience of having charged batteries "ready to go" at a moments notice. I didn't realize though how fast LiPos can deteriorate if stored charged. Now I know.

By the way...the thing that really proved it to me was the readings I got from my "cheapo" meter that some say is "useless". Sorry for the sarcasim, but the meter is doing exactly what I expected it to do. Granted, it isn't lab equipment but I knew that when I bought it.
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Old Jan 28, 2011, 09:34 PM
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Originally Posted by lbrinson View Post
Oh yea!! I have seen the light about not storing LiPos charged. I now store them somewhere between 3.8 and 3.85 V per cell. Although I knew there were problems with storing batteries charged, I didn't realize the drastic impact it had. I purposely traded off the convenience of having charged batteries "ready to go" at a moments notice. I didn't realize though how fast LiPos can deteriorate if stored charged. Now I know.

By the way...the thing that really proved it to me was the readings I got from my "cheapo" meter that everyone says is "useless". Sorry for the sarcasim, but the meter is doing exactly what I expected it to do.
Have you tried the tachometer in your meter? Is it really an optical tachometer?
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