Internal Resistance Meter

[Walter Clark]

Does anyone have experience with internal resistance meters?
I bought this one (about $45) and it makes no sense. It seems to read the same value 1.6 ohms no matter what the charge state of my LiPo battery. And batteries of different sizes read about the same.

I know how to measure it with a current meter, a fixed load and two voltage readings and I get .33 ohms with a fresh charge and 1.0 ohms when nearly depleted.

[Webmaster of Maglab]

Hi Walter.

If I remember correctly, the IR of a lithium ion battery doesn't change depending on SoC.  This is different behavior from NiMH batteries.

Thanks,

Brandon

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[Ryan Hass]

The IR for LiPo batteries will increase as the cells deteriorate or if they become damaged. I don't know about charge levels changing the IR, I think your findings are probably correct in that it doesn't change much. 

I am not sure what your goal is right now. If you intend to determine the overall health/safety of a LiPo with the meter I don't think there is a specific resistance value which you can use to determine when a LiPo is good or gone bad. I believe the IR varies by batter. You might be able to do this if you have initial resistance from when the cells are brand new and measure it over time to be able to figure out when the health of a LiPo is beginning to deteriorate. 

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[Alex Whittemore]

Walt - could you elaborate on the test procedure you used that yielded two different values?

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[Walter Clark]

Walt - could you elaborate on the test procedure you used that yielded two different values?

Alex Whittemore

Yes, thanks for asking Alex...

I discharged a .95 amp-hr 2 cell LiPo in my amp-hr meter and made internal resistance measurements periodically as the battery was depleted.

Unloaded voltage was 8.36 V and the loaded voltages are the first column and the difference
(the internal voltage drop) is the second column. The next column is the current being drawn by the amp-hr meter. I = E/R turned around is R = E/I. That's column 4.

7.8    0.56    1.70 A    .33 Ω

7.4    0.96    1.65 A    .60 Ω

7.2    1.16    1.60 A    .72 Ω

7.0    1.31    1.58 A    .83 Ω

6.8    1.54    1.54 A    1.0 Ω

The new commercial internal resistance meter reads a value of 1.6 Ω for both fully charged and depleted.  Dang. And unlike the above measurements, the meter value fluctuates from 1.5 Ω to 1.8 Ω. And as if that wasn't bad enough a battery of about four times the size of the above is also about 1.6 (1.5 to 1.8) ohms. Shit.

The whole point of this meter is to determine when a battery is no good and I can throw it out.

Walt

[Alex Whittemore]

Ah, I think that might be it. I don't think that test is valid. Take the last measurement, 6.8V. You measured V_open_circut to be 8.36V, i.e. this is a 2S pack with ~4.2V on each cell when fully charged (which is a normal lithium ion "full" cell voltage). As the pack discharges, the unloaded open circuit voltage will ALSO change, but you never measured that. In other words, the voltage drop across the internal resistance in that last measurement isn't 1.54V, because the unloaded open circuit voltage at that point is not 8.36V anymore. 

A valid test would involve discharging the battery to whatever degree, UNLOADING it and measuring open circuit, THEN re-loading it and measuring the new loaded voltage and current. 

What your fancy meter is probably doing is switching between two loads and measuring the voltage difference, then calculating Vdiff/Idiff = Rint, so that it never needs to measure an open circuit voltage. Or perhaps the two loads its measuring are simply "zero" and "something."

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[Walter Clark]

Alex,

> because the unloaded open circuit voltage at that point is not 8.36V
> anymore.

Oh, I see. I think you nailed it.

Thanks for writing the explanation.

[Alex Whittemore]

Of course! I should add that I’m now very curious and I’d love to see the results using your test setup, but with an unloaded Voc measurement at each test point. 

My intuition is that the internal resistance won’t change much but I still wonder if it’s measurable. 

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Alex Whittemore
617-213-0890

[Walter Clark]

Alex Whittemore,

You expressed interest in my broken All-Sun Internal Resistance Meter and gave me some advice on measuring internal resistance by hand.

I jumped through some hoops and the maker of the meter gave a new one that works.
I think.

It is consistent in that changing scales gives similar numbers but of course with different precision. It also measures batteries of different sizes in what sort of seems to make sense: D-cell is .078 Ω, the C-cell .124 Ω. The higher voltage cells, however, have much lower internal resistance which doesn't make sense to me.

I put a current meter to see what the "test" current is in determining the internal resistance. Well shoot. It's zero. But then I remember somewhere it said it uses an AC voltage to measure it. And sure enough the meter probes put out a 4 volt 1,000 Hz AC signal.

When comparing it to the multi-step paper and pencil with calculator method, the LiPo batteries measured on the new meter are a lot lower; .002 Ω vs .2 Ω.

By the way thanks for suggesting referencing the "discharged" open voltage rather than the "fully charged" open circuit voltage.

Walt

[Alex Whittemore]

How interesting! Yeah, I still find that meter pretty suspicious. I think I'd have to do a lot of A/B testing with it before I'd be convinced to trust it.

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Alex Whittemore
617-213-0890

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