Accurate affordable RI meter for low impedance cells?

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Accurate affordable RI meter for low impedance cells?

Post by MitchJi » Oct 31 2011 3:52pm

Hi,
liveforphysics wrote:The iCharger sadly can't check Ri below about 50mOhm with even a shred of accuracy. I've got a $3500 Ri meter here at work, and the iCharger can read 15mOhm with the actual Ri is 3mOhm, or sometimes visa-versa.

So, if you want to check the Ri of a single cell, or likely even the stack of 6, it's not really the tool you want to use for these fairly low impedence cells.

I would check resistance by leaving them paralleled and floating a charge, so they are all at the identical state of charge, then 1 by 1, measure resting voltage, apply some known load (like a 10ohm resistor), and measure the voltage sag after 10 seconds, and record this.
fechter wrote:Sounds like the IR measurement on the iCharger is just like my Cadex. Useless in the range where these batteries run.

The 10 second drop measurement is what I proposed before. This will give you the best indication of a bad vs. healthy cell as far as I can tell. It will be essentially the same as a relative IR measurement. This is what the Prius guys do.
I think the iCharger does very accurate charging/balancing so I was surprised to hear its IR readings are essentially useless. Richard's Cadex analyzer is very expensive so that's even more surprising.

Its good to know that there is a work around but an absolute IR measurement would be more useful than a relative IR measurement and it would be nice not to need to put all the cells you want to check into an identical state of charge.

Are there any accurate affordable RI Meters (dedicated or in RC chargers) available?

Since affordable is relative a better way to phrase the question might be what is the least expensive piece of equipment that can measure IR accurately with low impedance Cells?

If there are not any accurate affordable RI Meters available new, how about used on Ebay? What should we look for?
Best Wishes!

Mitch


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Re: Accurate affordable RI meter for low impedance cells?

Post by liveforphysics » Oct 31 2011 6:24pm

A good precision resistor, and a good multi-meter IMHO will give you the best Ri reading that you can achieve. It's more accurate and more consistent than even the $3500 Ri meter that I use at work.

Put your multimeter leads right on the cell ends or tabs. Take the voltage reading. Have some silly 00awg cables just a few inches long on your precision resistor. Short it across the cell, take the voltage reading after about 2sec (to make sure capacative effects aren't holding up voltage), remove the resistor, record what the voltage bounces back up to in the following couple seconds. Find that delta-V, and use V=IR to calculate current with the resistor in the system at the voltage it was pulled down to, and then use V=IR to calculate the resistance of the cell.

If your multi-meter can do 5 digits of resolution (which decent cheap bench-top meters can do), then as long as your resistor value was enough to pull say ~20-30amps, then you've got the data to read down to under 1/10th of a mOhm with really good accuracy for under say ~$200 of equipment.
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Re: Accurate affordable RI meter for low impedance cells?

Post by zombiess » Nov 01 2011 12:53pm

regmeister wrote:this seems to work ok

http://www.ebay.ca/itm/20R-Internal-Bat ... 4aa02d2920
The lowest scale on that thing is 0.2ohms, that's 200 mOhm. It also only has 2 digits after the decimal so unless I'm missing something the lowest possible resistance that meter could read is 0.01 ohms which is 10 mOhm which isn't very low for measuring a lipo battery cell which could be well under 1 mOhm, 0.001 ohm per cell.

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Re: Accurate affordable RI meter for low impedance cells?

Post by webfootguy » Nov 01 2011 5:39pm

zombiess wrote:
regmeister wrote:this seems to work ok

http://www.ebay.ca/itm/20R-Internal-Bat ... 4aa02d2920
The lowest scale on that thing is 0.2ohms, that's 200 mOhm. It also only has 2 digits after the decimal so unless I'm missing something the lowest possible resistance that meter could read is 0.01 ohms which is 10 mOhm which isn't very low for measuring a lipo battery cell which could be well under 1 mOhm, 0.001 ohm per cell.

The way I read the specs, the lowest scale is 0 ~ 199.9 mOhm and each digit of the scale is 0.1 mOhm which should be plenty of resolution for any cell under 1 mOhm. I'm not confusing accuracy with resolution here (I have no idea how accurate it is but it appears to have the resolution needed). EDIT: manual says accuracy is ±(3%+1). See http://www.aidetek.com/New_products_inf ... manual.pdf

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Re: Accurate affordable RI meter for low impedance cells?

Post by 999zip999 » Nov 01 2011 9:25pm

So is it a buy ?

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Re: Accurate affordable RI meter for low impedance cells?

Post by regmeister » Nov 01 2011 9:41pm

I am happy with the purchase. It seems to work ok though I have nothing to compare it with.

I do remember complaining a bit about the lense as it did not have a protector and came to me a bit scratched up. Minor...

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Re: Accurate affordable RI meter for low impedance cells?

Post by liveforphysics » Nov 01 2011 10:08pm

A meter like that can be mildly useful for compareing cells to cull bad ones.

I'm afraid its usefullness for reading cells at a couple mOhm is likely to be super limited.
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Re: Accurate affordable RI meter for low impedance cells?

Post by pm_dawn » Nov 02 2011 2:32am

I would use the iCharger as a load and setup two different loads on the same cell.
for example. first load the cell with 10a and record the voltage at the tabs with a good meter.
And then do the same with a 20a load.
After that calculate the Ir from those two readings.

But that is pretty much what Luke said. But using the iCharge as a CC load instead of a resistor.
if you already got the iCharger it would be good to check.

I found that connectin the sense leads when doing the automatic Ir check in the iCharger made it much more accurate.
But I guess you all knew that already.

I haven't tested and compared the different values from the both processes.
I'll do that
Regards
/Per

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Re: Accurate affordable RI meter for low impedance cells?

Post by MitchJi » Nov 02 2011 6:34pm

Hi Luke,
liveforphysics wrote:A good precision resistor, and a good multi-meter IMHO will give you the best Ri reading that you can achieve. It's more accurate and more consistent than even the $3500 Ri meter that I use at work.

Put your multimeter leads right on the cell ends or tabs. Take the voltage reading (1). Have some silly 00awg cables just a few inches long on your precision resistor. Short it across the cell, take the voltage reading after about 2sec (2) (to make sure capacitive effects aren't holding up voltage), remove the resistor, record what the voltage bounces back up to (3) in the following couple seconds. Find that delta-V, and use V=IR to calculate current with the resistor in the system at the voltage it was pulled down to, and then use V=IR to calculate the resistance of the cell.

If your multi-meter can do 5 digits of resolution (which decent cheap bench-top meters can do), then as long as your resistor value was enough to pull say ~20-30amps, then you've got the data to read down to under 1/10th of a mOhm with really good accuracy for under say ~$200 of equipment.
Thanks! Exactly what I was looking for (other than a $200 IR Meter with similar accuracy :)). But would you (or someone else) clarify the information above, mostly the sentence I put in bold? What is the first voltage reading (1) used for? What does "V=IR" mean (used twice)?

Also do the capacity (ah) of the Cells impact the amps pulled/accuracy of the results? For example does pulling 20-30amps give the same accuracy with 20ah A123 pouches as with their 18650 1.1ah cells?
pm_dawn wrote:I would use the iCharger as a load and setup two different loads on the same cell. For example. first load the cell with 10a and record the voltage at the tabs with a good meter. And then do the same with a 20a load. After that calculate the Ir from those two readings.

But that is pretty much what Luke said. But using the iCharger as a CC load instead of a resistor.
if you already got the iCharger it would be good to check.
Sounds like a good idea. Thanks!
Best Wishes!

Mitch


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Re: Accurate affordable RI meter for low impedance cells?

Post by texaspyro » Nov 02 2011 10:26pm

I have one on they way... I know how to torture it...

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Re: Accurate affordable RI meter for low impedance cells?

Post by webfootguy » Nov 03 2011 2:06pm

texaspyro wrote:I have one on they way... I know how to torture it...
Looking forward to your analysis of how it did ;-)

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Re: Accurate affordable RI meter for low impedance cells?

Post by pm_dawn » Nov 04 2011 1:15am

MitchJi wrote:Hi Luke,
liveforphysics wrote:A good precision resistor, and a good multi-meter IMHO will give you the best Ri reading that you can achieve. It's more accurate and more consistent than even the $3500 Ri meter that I use at work.

Put your multimeter leads right on the cell ends or tabs. Take the voltage reading (1). Have some silly 00awg cables just a few inches long on your precision resistor. Short it across the cell, take the voltage reading after about 2sec (2) (to make sure capacitive effects aren't holding up voltage), remove the resistor, record what the voltage bounces back up to (3) in the following couple seconds. Find that delta-V, and use V=IR to calculate current with the resistor in the system at the voltage it was pulled down to, and then use V=IR to calculate the resistance of the cell.

If your multi-meter can do 5 digits of resolution (which decent cheap bench-top meters can do), then as long as your resistor value was enough to pull say ~20-30amps, then you've got the data to read down to under 1/10th of a mOhm with really good accuracy for under say ~$200 of equipment.
Thanks! Exactly what I was looking for (other than a $200 IR Meter with similar accuracy :)). But would you (or someone else) clarify the information above, mostly the sentence I put in bold? What is the first voltage reading (1) used for? What does "V=IR" mean (used twice)?

Also do the capacity (ah) of the Cells impact the amps pulled/accuracy of the results? For example does pulling 20-30amps give the same accuracy with 20ah A123 pouches as with their 18650 1.1ah cells?
pm_dawn wrote:I would use the iCharger as a load and setup two different loads on the same cell. For example. first load the cell with 10a and record the voltage at the tabs with a good meter. And then do the same with a 20a load. After that calculate the Ir from those two readings.

But that is pretty much what Luke said. But using the iCharger as a CC load instead of a resistor.
if you already got the iCharger it would be good to check.
Sounds like a good idea. Thanks!
V=IR is also known as Ohms Law
V = voltage
I = current
R= resistance

And Voltage = Current x Resistance.

So to figure out the Ir of the battery you load it with a know current say 10A and record the voltage of the cel tabs after stabilizing. lets say 3.18v and then you do the same with a higher load 30A for ex. The Cell will show the voltage of 3.08v.
You then get the deltaV which is 3.18V - 3.08V = 0.1V
You then get the deltaI which is 30A - 10A = 20A

if V= IR then that also gives that R= V/I
So then put deltaV and deltaI in that and you get: R = 0.1V/20A = 0.005ohms = 5mOhms


This made up battery measurement gives that the cell has 5mOhms of resistans.

Regards
/Per

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Re: Accurate affordable RI meter for low impedance cells?

Post by MitchJi » Nov 04 2011 3:30am

Hi Per,

Thank you very much! I think I'm close.
Per wrote:So to figure out the Ir of the battery you load it with a know current say 10A and record the voltage of the cel tabs after stabilizing. lets say 3.18v and then you do the same with a higher load 30A for ex.
But in Luke's example why did he only load the cell once?

Thanks Again!
Best Wishes!

Mitch


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Re: Accurate affordable RI meter for low impedance cells?

Post by pm_dawn » Nov 04 2011 4:18am

Well its two different methods of getting the DeltaV.

I cannot say for sure which is the best method.
Luke has tons of experiense, I think he has to make the call on that.

Regards
/Per

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Re: Accurate affordable RI meter for low impedance cells?

Post by liveforphysics » Nov 04 2011 11:17am

MitchJi wrote:Hi Per,

Thank you very much! I think I'm close.
Per wrote:So to figure out the Ir of the battery you load it with a know current say 10A and record the voltage of the cel tabs after stabilizing. lets say 3.18v and then you do the same with a higher load 30A for ex.
But in Luke's example why did he only load the cell once?

Thanks Again!

Taking two points as Per suggested is a better way. If you're dealing with a non-buffered chemistry, either one will give a roughly identical resistance value. If you're dealing with a wonky chemistry, Per's method should give better results. It's just a bit more complex procedure for gaining an additional percent or two of accuracy on some chemistry (like lead, which should always be done with the 2 reading method).
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Re: Accurate affordable RI meter for low impedance cells?

Post by MitchJi » Nov 04 2011 3:33pm

Hi,

Thanks Luke and Per!
Best Wishes!

Mitch


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Re: Accurate affordable RI meter for low impedance cells?

Post by MitchJi » Nov 06 2011 10:21pm

Hi,
MitchJi wrote:I think the iCharger does very accurate charging/balancing so I was surprised to hear its IR readings are essentially useless. Richard's Cadex analyzer is very expensive so that's even more surprising.
Now that I understand the process It's not surprising at all. We should be able to deduce it from the specifications of the devices. The Cadex has a maximum discharge rate of 4A. For our current project (6s NiMH ) most iChargers (20 or 30 watts except the 30 series) are worse . For 6s NiMH 7.2V that's only about 2.8A or 4.2A! For a single 3.7V Lipo Cell that's still only 5.4 or 8A.

I think the iCharger questions are:
1. Do they calculate IR correctly in regen mode (250W - 1000W)?
2. Do the 30 series (80 watts) calculate IR correctly with 1S or 2S Lipo or 6S NiMH (11.2A)?

For the amp draw does the size of the Cell's matter (ie. 20ah A123 vs 2.3ah A123)?
liveforphysics wrote: Taking two points as Per suggested is a better way. If you're dealing with a non-buffered chemistry, either one will give a roughly identical resistance value. If you're dealing with a wonky chemistry, Per's method should give better results. It's just a bit more complex procedure for gaining an additional percent or two of accuracy on some chemistry (like lead, which should always be done with the 2 reading method).
Is NiMH a non-buffered chemistry or for the purposes of IR calculation a wonky chemistry?
Best Wishes!

Mitch


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Re: Accurate affordable RI meter for low impedance cells?

Post by texaspyro » Nov 10 2011 4:54pm

The meter came in today... it's actually not half bad. I measured the impedance of some A123 cells and some LiPO cells and it agreed with "real" measurements... provided that you first short the leads of the meter to get it's zero reading (around 3 milliohms) and subtract that reading from the battery readings.

It does have 4-wire Kelvin connections to the probes, but you need to subtract out the residual value since the 4-wire connection is not at the very end of the probe tips.

Also, be aware that between volts mode and ohms mode, the meter scale switch is reversed (high volts range position is low ohms) On the lowest ohms range, the last digit is 0.1 milliohm. Lowest volt range it is 1 millivolt. The voltage readings were accurate.

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Re: Accurate affordable RI meter for low impedance cells?

Post by whatever » Nov 13 2011 10:43am

just reading this thread and thought I would try something :

charging up some cells with mobile phone charger ( instead of using resistor to find deltaV, using charger)
charger gives constant 0.38amp current
as example one cell I get deltaV = 0.03V (from volts when charger on 4.03 - start volts 4.00 )
using R=V/I gives R= 0.079ohms
anyone care to interpret what this R value is I'm measuring?
( i tested 7 different cells and got similar results)

one other thing, I thought that a cells internal resistance was a function of its state of charge? You guys are suggesting test is done when cells are fully charged?

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Re: Accurate affordable RI meter for low impedance cells?

Post by texaspyro » Nov 13 2011 12:53pm

The problem with this is one does not know what the circuitry in the charger is doing... the voltage change is most likely not an indication of the cell impedance. An 80 milliohm cell is useless for any kind of power application...

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Re: Accurate affordable RI meter for low impedance cells?

Post by whatever » Nov 14 2011 7:03am

yes its a weird result, cells I know are ok, charger puts out 6v and 390mA, I think both of values remain constant,
voltage comes down due to cell, but amps remains constant.

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Re: Accurate affordable RI meter for low impedance cells?

Post by busted_bike » Nov 15 2011 11:44am

LFP is pretty much spot on - I uploaded a simulation image that I created previously to explain why it's necessary to use the rebound voltage delta for calculating Ri, rather than droop. If you use droop rather than rebound, there is some voltage sag due to normal discharge effects that muddles up the calculation.

In the attached image, the red line shows the discharge effect of a pulsed load (which can't be observed directly) and the blue line shows the actual cell voltage.
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Re: Accurate affordable RI meter for low impedance cells?

Post by pm_dawn » Nov 16 2011 2:41am

MitchJi wrote:Hi,

Now that I understand the process It's not surprising at all. We should be able to deduce it from the specifications of the devices. The Cadex has a maximum discharge rate of 4A. For our current project (6s NiMH ) most iChargers (20 or 30 watts except the 30 series) are worse . For 6s NiMH 7.2V that's only about 2.8A or 4.2A! For a single 3.7V Lipo Cell that's still only 5.4 or 8A.

I think the iCharger questions are:
1. Do they calculate IR correctly in regen mode (250W - 1000W)?
2. Do the 30 series (80 watts) calculate IR correctly with 1S or 2S Lipo or 6S NiMH (11.2A)?

For the amp draw does the size of the Cell's matter (ie. 20ah A123 vs 2.3ah A123)?
I would not trust the IR calculation done in the iCharger without checking it first.
But I think your mixing things up.
My suggestion was that you use the iCharger as a Load to perform the Ir calculation manually.
In that way you should be able to use the full 30A of the 30 series for doing calculations.
So with the iCharger 30 series you can use regen mode to do full 30 A discharge on series strings up to the max power of the charger. And they do it very well. I have tested a lot of thundersag 90ah cells this way. it only gives 0.3C load, but it was for capacity check of used cells.

Yes the size of the cell matters when you try to get good measurements.
in your example: testing the 20ah A123 with the same load as the 2.3ah cell will only test the 20ah cell to 0.1 factor of the 2.3ah cell.

You should strive to test the Ir in the C range that you expect to use the cells.
On the other hand, getting the Ir of the cell will pretty much show you how the cell will perform in any given C rate!


Regards
/Per

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Re: Accurate affordable RI meter for low impedance cells?

Post by MitchJi » Nov 16 2011 3:30am

Hi Per,

From my first post, starting this thread:
MitchJi wrote:I think the iCharger does very accurate charging/balancing so I was surprised to hear its IR readings are essentially useless. Richard's Cadex analyzer is very expensive so that's even more surprising.
MitchJi wrote: Now that I understand the process It's not surprising at all. We should be able to deduce it from the specifications of the devices. The Cadex has a maximum discharge rate of 4A. For our current project (6s NiMH ) most iChargers (20 or 30 watts except the 30 series) are worse . For 6s NiMH 7.2V that's only about 2.8A or 4.2A! For a single 3.7V Lipo Cell that's still only 5.4 or 8A.

I think the iCharger questions are:
1. Do they calculate IR correctly in regen mode (250W - 1000W)?
2. Do the 30 series (80 watts) calculate IR correctly with 1S or 2S Lipo or 6S NiMH (11.2A)?

For the amp draw does the size of the Cell's matter (ie. 20ah A123 vs 2.3ah A123)?
pm_dawn wrote: I would not trust the IR calculation done in the iCharger without checking it first.

But I think your mixing things up.
My suggestion was that you use the iCharger as a Load to perform the Ir calculation manually.
In that way you should be able to use the full 30A of the 30 series for doing calculations.

So with the iCharger 30 series you can use regen mode to do full 30 A discharge on series strings up to the max power of the charger. And they do it very well. I have tested a lot of thundersag 90ah cells this way. it only gives 0.3C load, but it was for capacity check of used cells.
I understood that you meant using it for a load. I was referring back to my original post and a lot of what you are saying is actually repeating what I said. For example in the section of my post in bold I was suggesting testing the iCharger under those conditions, when its possible for it to calculate IR correctly.
pm_dawn wrote: Yes the size of the cell matters when you try to get good measurements.
in your example: testing the 20ah A123 with the same load as the 2.3ah cell will only test the 20ah cell to 0.1 factor of the 2.3ah cell.

You should strive to test the Ir in the C range that you expect to use the cells.
On the other hand, getting the Ir of the cell will pretty much show you how the cell will perform in any given C rate!
Thanks again, very useful!
Best Wishes!

Mitch


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