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Old Jul 03, 2011, 09:52 AM
like2fly! is offline
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You can tell a weak cell from a spot IR reading alone
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Old Jul 03, 2011, 10:18 AM
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Snip from post #6

Quote:
It measures the effective resistance by applying a pulse load of controlled constant current load of 16A to the pack, measuring the voltage just before the start of the pulse and again just before the end of the pulse. It then does the maths of subtracting the last figure from the first figure, divides the answer by 16 to give result in ohms and divides that by 1000 to display the answer in milliohms.
A very simple means to spot a weak cell is to just connect a CellLog or other such device which reads and displays each cell's voltage by way of balancing connector and then run up your motor to provide a load.

Some chargers have the ability to monitor and graph each cell's voltage under a load which gives a very clear indication of a cells relative health.

https://www.rcgroups.com/forums/show...8#post18621408


FYI In the case of this LiPoly there really is not signaficant difference in the cell's IR.

https://www.rcgroups.com/forums/show...89&postcount=8

Charles
Last edited by everydayflyer; Jul 03, 2011 at 10:26 AM.
Old Jul 03, 2011, 06:26 PM
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Wayne is the expert on this but I think he is on hols so here is a quick answer.

Total "resistance" in an AC circuit is a combination of in phase and out of phase voltage/current interactions. It is called impedance and varies with frequency. ESR is commonly specified for capacitors and is the sum of in-phase AC resistance of the capacitor in an AC circuit. That is the "DC" component at a particular frequency. It includes resistance of the dielectric, plate material, electrolytic solution, and terminal leads at a particular frequency. ESR acts like a resistor in series with a pure non-resistive capacitor (thus the name Equivalent Series Resistance).

A LiPo is very similar to a large capacitor in many ways in regard to internal impedance effects. We generally ignore any AC (out of phase) effects since it is supplying DC current so ESR becomes the DC Internal Resistance of the battery -commonly called IR. Wayme used the technically correct term for the meter but it does in fact only measure resistance using DC. Some IR meters use a specific frequency as the standard to specify the resistance (1kHz from memory but I'm no expert on this).

The value of IR you get depends on the frequency you measure it at (and a host of other things). This meter measures it under the conditions that interest us. Supplying DC.
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Originally Posted by Aviefly View Post
A couple of questions. First is probably easy for most but what does ESR stand for?
Old Jul 03, 2011, 07:11 PM
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Wow thanks John. I think I got something wrong in my question as that isn't likely to be easy for most. I think I got the idea of it, thanks for taking the time.
Old Jul 03, 2011, 07:29 PM
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Don't worry about it! First time I had to try and understand this stuff and the lecturer said, "Now we are going to have to get an understanding of imaginary numbers" I was a bit stumped.

For us, for all practical purposes ESR and IR are interchangeable. AFAIK IR is the correct usage for batteries.

Just think of a battery as a container that includes a voltage source (that varies with state of charge) in series with a resistance (the IR of the battery). The more current you draw from the battery the lower the terminal voltage and the hotter it gets because of the power dissipated in the internal resistance.
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Originally Posted by Aviefly View Post
Wow thanks John. I think I got something wrong in my question as that isn't likely to be easy for most. I think I got the idea of it, thanks for taking the time.
Old Jul 04, 2011, 07:03 AM
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Quote:
Originally Posted by jj604 View Post
A LiPo is very similar to a large capacitor in many ways in regard to internal impedance effects.
Here we go again!
I've been trying to discuss this problem with Wayne...
Old Jul 04, 2011, 07:15 PM
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I know a LiPo is not "like" a capacitor. I said it is "very similar to a large capacitor in many ways". Those ways help understand some things about LiPo behavior. It is an analogy not a correspondence.

There is a fine line between making things accessible and absolute accuracy. All of our models of the physical world involve some sort of approximations. The level of approximation is chosen to be appropriate for the purpose.

We know Newtonian physics and classic atomic theory are not "accurate" pictures of the physical world but they work just fine for almost everything we need.

Most people don't have your detailed and deep knowledge of the technology. My simplification was an attempt to assist someone understand how a concept such as IR might help in better understanding everyday LiPo behavior, not a definitive statement.

Just to be clear I'm not arguing or disagreeing with you about the best and most accurate model of a LiPo equivalent circuit. But for the purposes of the exchange a simplified analogy was adequate.

John

Quote:
Originally Posted by LeszekJ View Post
Here we go again!
I've been trying to discuss this problem with Wayne...
Old Jul 24, 2011, 05:28 AM
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AGENT IN USA

I have been off-line for four weeks thanks to British telecom., but finally now back on.
In the meantime I finished a batch of units which were sold to David Gray of ProgressiveRC in Seattle. David is enthusiastic about the unit and has them for sale at a very modest mark up. Customers in the US will save a siginificant portion of that in shipping costs.
Any further units I make will have to be at an increased price as it only started as a hobby and I was prepared to break even or even lose money on a few units but more units have been sold than I estimated. Obviously I can't lose money on any future units.

Link to David's site at : http://www.progressiverc.com/esr-meter.html

Wayne
Old Jul 24, 2011, 09:17 AM
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Wayne,
I showed the meter to my club and they want some. I have about 6 orders coming. I am also taking my meter to the US Nationals next week, and there are some big electric guys there. Get busy!!
Old Jul 25, 2011, 09:27 AM
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Wayne, glad to hear that you joined with the best vendor in the US for these!

I ordered mine and received it quite promptly along with a string of other items from progressive. So far I really like the ESR - it sorted all my packs out in short order!
Old Jul 25, 2011, 01:42 PM
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In post #158 RUMPLE says the following;

Quote:
I can see how this meter might be useful to sort the good from the bad packs at the hobby shop before purchasing! I.e., confirm (or more likely, refute!) the C-ratings on the packs.
So if I wanted to confirm or refute the "C" ratings on a pack, what would I do?

Would I use the 12000 divided by the capacity of the pack to get the estimated IR reading, and compare it to the actual?

How far from the actual is considered acceptable, and how can I calculate the "C" rating from the IR reading on Wayne's meter?

Whew, so many questions...

Thanks
Bob
Old Jul 25, 2011, 02:32 PM
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Thanks Rumple.

Bob,

The 12000 divided by capacity is Mark Forsyths's estimate and I agree although only really good packs meet his criteria.
You should get a feel for what is a good IR value although the above figure is a good starting point.
I did a thread on Turnigy nanotechs as I bought two 3S2200 25c packs and they both showed very high IR values which immediately said to me that they could not meet the 25C claim.
Thread at https://www.rcgroups.com/forums/show....php?t=1384403

When I tested the packs on a real continuous discharge it became obvious that the packs were only capable of 15 - 18c discharge rates dependant on what temperature rise the user is prepared to risk.
Results at https://www.rcgroups.com/forums/show....php?t=1406364

Look at the difference in the IR values for the Loong max, Tipple and older Turnigy packs and you will see that they meet Mark's criteria whilst the Nanotechs are far short. So your suggested calculation is broadly correct, although the 12000 figure is always arguable.

Wayne
Old Jul 26, 2011, 04:39 AM
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@Wayne

Did you get already GensAce? For testing?

They emailed me...but I am not a tester and I did advice to send some packs to you.
It seems that Gerd Giese also will do a test of GensAce, altough I did not see them at the German market yet.

The IR you have on the 25C Nano are indeed high. I have 9mOhm per cell (2200mAh), which is equal to my Rhino 30C (which are two seasons old).
Seems that Nano is a good marketing story. Still I like these packs, but regular Turnigy also work perfect. I have several 40C 1800mAh with 5mOhm per cell, which is IMO good.

The GensAce are 3mOhm per cell (2200mAh)
Old Jul 26, 2011, 07:31 AM
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[QUOTE=TreeDiver;18865086]@Wayne

Did you get already GensAce? For testing?


No unfortunately I have been off line for nearly 4 weeks due to British Telecomm's incompetence - it is so bad that it is hard to believe that anyone could be doing it accidentally - it looks deliberate, but can't be.
I hope to get one later and will post the results.

Wayne
Old Jul 26, 2011, 08:30 AM
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@Wayne

That is indeed odd! We have some bad providers here as well, but 4 weeks...that is shameful.

Maybe they have to much IR on their lines and pull too much amps!

Good luck and we see the results coming. Good news is that GensAce are available in Europe now
My Rhino 30C for the funjet are still in great condition... but it is very tempting to purchase GensAce 4s 2200mAh for them.


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