How to check internal resistence??

1806 Views 6 Replies 4 Participants Last post by spdas, Feb 7, 2011

spdas

Discussion Starter · Feb 6, 2011

I googled and here is what I found. Am I doing it right?

Connect a load drawing 1A (12W) from your battery.
Record the current from and the voltage at the battery

Connect a second load drawing a total of 2A (24W) from your battery
Record the current from and the voltage at the battery

Now calculate the difference of the two voltages under the two different loads (= delta U)
And calculate the difference of the two different currents (= delta I)

Now divide delta U by delta I and you get the internal resistance of your battery.

Ri = ΔU / ΔI

MY TEST = (on 3.69v LiColbalt)

3.62 Volts on a 10 amp draw
3.56 Volts on a 20 amp draw

so 3.62-3.56 = .06 v difference
and 10A-20a =-10a difference

.06/10= 6 milli ohms? Is this right?

thanks
francis

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Tesseract

· #2 · Feb 6, 2011

spdas said:
I googled and here is what I found.
...
Ri = ΔU / ΔI
...
.06/10= 6 milli ohms? Is this right?

You got it, Francis...

Oh, and thanks for doing a search first then asking if your understanding is correct.

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major

· #3 · Feb 6, 2011

Tesseract said:
You got it, Francis...

Oh, and thanks for doing a search first then asking if your understanding is correct.

I'll second what Tesser says.

But Fran, 6 mΩ seems too high unless it was a very small cell. What cell did you measure?

However your method looks correct. I use it. Typically I use data points near 1C and 2C. Also, I use V and I values with Fluke grade instruments, maybe more precise than your values.

Regards,

major

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spdas

Discussion Starter · #4 · Feb 6, 2011 (Edited)

major said:
I'll second what Tesser says.

But Fran, 6 mΩ seems too high unless it was a very small cell. What cell did you measure?

However your method looks correct. I use it. Typically I use data points near 1C and 2C. Also, I use V and I values with Fluke grade instruments, maybe more precise than your values.

Regards,

major

Just getting my decimal points correct.... The used 160TS cell tested above is at 6mOhms but new cells are about .3mOhms (point 3) or is it 3mOhms?

Anyway for my used TS 160 cells that have resting voltage at 3.35 and deliver 161ah, how much life is left in them?

thanks Francis

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Tesseract

· #5 · Feb 7, 2011

Yeah, 6mΩ is a bit high for a TS 160Ah cell. I have some old style 200Ah cells - moderately abused (okay, seriously abused...) - which measure out at 1.4mΩ with a dI of 100A. Are these cells of yours swollen at all, Francis?

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peggus

· #6 · Feb 7, 2011

If you didn't already do so, try sanding the terminals a bit and doing the test again, especially the positive terminal. If the cell has a bulge, try compressing it.

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spdas

Discussion Starter · #7 · Feb 7, 2011 (Edited)

Tesseract said:
Yeah, 6mΩ is a bit high for a TS 160Ah cell. I have some old style 200Ah cells - moderately abused (okay, seriously abused...) - which measure out at 1.4mΩ with a dI of 100A. Are these cells of yours swollen at all, Francis?

Yes they are swollen a "bit", since i have no experience I do not know what amount of swelling is a lot. Here is a photo of the worst swollen one. And I need to make a better connection to the battery....only using small alligator clips.
(Photo is hard to make out, but the steel ruler makes contact with the cell in the center and has a 1/8"to 3/16" gap on either end.)

Francis

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How to check internal resistence??

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