Impedance Graphs

[wptski]

I'm posting this series of impedance graphs of various AA/AAA cells. I used a Duratrax ICE charger and a Extech BT100 to perform these tests. The ICE only allows up to thirty minutes rest between a charge and discharge cycle. Most of the tests were done with discharged cells but a few weren't but the graph has a note. The graphs on the left are impedance and voltage but since it didn't provide any real grid to calculate the level of impedance very close, I redid the impedance values in Excel which is on the right of each.

Impedance in milliohms is a very small value and very sensitive. A cell might read 30 mOhms by itself but when you try to clamp it for logging, you always gain a few mOhms! So the graphs tend to read a few mOhms higher than actual cell impedance. Since I was just trying to see what cells do while charging/discahrging, a few mOhms doesn't matter.

What you'll notice here is that some cells increase at times while others decrease. I you search the net, you'll find statements where impedance increases when a cell is discharged and increases when it's charged! Well some do and some don't. The lower the impedance the better and the lowest that I've seen was PowerEx 2Ah AA at about 15-17mOhms.

There's been talk here about using spring type holders for charging multiple cells. I have several kinds, four, six and eight cells for AA's. I took four AA's in series while held in a piece of PVC pipe and check the total impedance and then check it while in these various holders. The Radio Shack eight cell holder had half the impedance of the others!

If it looks confusing, it is!

 

[SilverFox]

Hello Bill,

Now... what does it all mean? :devil:

It is interesting that there are some sudden increases and then some sudden drops.

Am I correct it observing that the impedance increases suddenly at the end of the charge?

Tom

 

[Doug S]

Bill, I note your sigline but I'll ask anyway. There seems to be a industry quasi-standard of measuring cell impedence at 1kHz. Are the values you have provided in fact AC impedences, and if so are they at 1 kHz, or are they actually cell DC resistances? BTW, the later value is more useful for our purposes.

 

[wptski]

Tom:

Yeah, some go up and some go down too!

Doug S:

Yes, they are the standard 1kHz AC impedance non-destructive values. From what Tom and I have read, DC resistance is about twice the impedance value. Energizer gives some values for impedance and DC resistance on cells which show about twice the impedance equals the DC resistance.

 

[SilverFox]

Hello Bill,

It would be interesting to take a Duracell 2300 mAh cell and do a DC internal resistance check on it.

Tom

 

[wptski]

Tom:

Well, the Duratrax ICE does give internal resistance on a discharge cycle but do you mean a graph? At that Maxim chip site they used V drop in mv/500(ma) for 500ms to calculate internal resistance. I wonder how that would graph out in continuous mode? I don't have anything that will log at 500ms interval but 1000ms is no problem, in fact the tester used above does go down to 1000ms. Or do we need to use the method described for the CBA where two different loads are used? I guess the question is how should internal resistance be calculated??

EDIT: I did a search and found a few instances where internal resistance was calculated with no mention of time at all. If so, I could use the same data points from a Duracell plots above, use Excel to calculate the voltage drop per interval, divide by the load and plot that. What you think?

 

[SilverFox]

Hello Bill,

I was thinking in terms of the CBA test. However, you could probably get an idea just by observing the reading when you stick the cell in the C9000.

I don't think a graph is necessary, but it would be interesting to look at...

It looks like the Duracell 2300 has an impedance of around 0.040 ohms, until a charging or discharging current is applied. Then it seems to drop to around 0.025 ohms. I am trying to figure out where the DC internal resistance value comes in at.

Tom

 

[wptski]

Tom:

I'll see what the ICE shows for internal resistance on the Duracells. I also used a DMM which has a zero or relative function. Connect to a cell, zero the meter and put a 500ma load from the ICE. Note voltage drop right as the load is first applied and use Ohms Law. It compared to ICE fairly close.

 

[wptski]

Tom:

Two 2.3Ah Duracells showed 1.43V/1.45V, 33/43mOhms on my tester and 68/75mOhms on the ICE charger.

 

[SilverFox]

Hello Bill,

Hmm, that doesn't exactly fit my definition of "crap" cells.

I would suggest that you contact Maha and tell them your cells are getting very hot.

You might also try charging them on the C808M.

Tom