Mind blowing realization about IR

[ajw22]

I should have known this, and it's so blindingly obvious now, but I simply never questioned it. I apologize if this isn't news to anyone, but...
I always thought that the IR value of a cell is a stable fixed number that only changes with age. A convenient value that an IR tester spits out in seconds, from which the approximate age and health of a cell can be derived.

Not so fast... it also changes significantly with SoC and temperature!
All those times the camera refused to work in the cold? => Because the battery IR gets high when cold!
Camera coming back to life when you rub the battery warm? => Because the IR gets low when warm!
When the cordless drill loses that "Umpf" towards the end? => Because the battery IR gets high as SoC drops! (and the lower voltage)

So if any of you tested IR of nearly empty cells in a cold place... you might want to retest those cells after charging and warming up.


Here's a nice graph, link to paper below. Even a brand new cell could show 100+mOhm of resistance when at 10C and less than 10%SoC!

Figure 9 from The impact of battery operating temperature and state of charge on the lithium-ion battery internal resistance | Semantic Scholar

Figure 9: for different temperatures charging current of 1C [10]. - "The impact of battery operating temperature and state of charge on the lithium-ion battery internal resistance"

www.semanticscholar.org

 

[Bubba]

Based on the information I see, Fig 3 for example, SOC has no real effect on the Internal Resistance factor. SOC for 1kHz tester such as YR1035 or RC3563 is basically also irrelevant if done at room temperature.
I always measure my cells at room temperature.

Be careful reading that report, they gather the results while charging or discharging. The results can be quite a bit different from an idle cell.

 

[OffGridInTheCity]

I'm not educated/scientific on IR they way you guys are... but I did have an experience where I built some packs with very high IR - e.g. Green Sony G5/G7s with 200+ mOhm IR per OPUS - and put them in series with 'regular packs' of 60mOhm per OPUS in a 14s. Its was 5 Sony packs to 9 regular packs. The Sony packs would swing *wildly* up/down the voltage range when charging/discharging compared to the others. I'm talking an entire 1/2 volt higher and lower kind of thing. The ah/pack was the same and the in/out on each cell was 200mah'ish range. The ambient was steady - e.g. there was no heat/warm cells or anything like that.

I put this down to IR and have respected IR ever since. These days I make sure packs have similar IR and compatible discharge curves (as @Wolf has shown in many tests) and I've never had such a wild swing since. I put all the Sony's together into 1 battery in an APC and they balance OK. This doesn't prove IR was the cause - but its my working theory and was my 'ah ha' moment

 

[Bubba]

I'm not educated/scientific on IR they way you guys are... but I did have an experience where I built some packs with very high IR - e.g. Green Sony G5/G7s with 200+ mOhm IR per OPUS - and put them in series with 'regular packs' of 60mOhm per OPUS in a 14s. Its was 5 Sony packs to 9 regular packs. The Sony packs would swing *wildly* up/down the voltage range when charging/discharging compared to the others. I'm talking an entire 1/2 volt higher and lower kind of thing. The ah/pack was the same and the in/out on each cell was 200mah'ish range. The ambient was steady - e.g. there was no heat/warm cells or anything like that.

I put this down to IR and have respected IR ever since. These days I make sure packs have similar IR and I've never had such a wild swing since. I put all the Sony's together into 1 battery in an APC and they balance OK. This doesn't prove IR was the cause - but its my working theory andwas my 'ah ha' moment

Different chemistries at high current will cause issues like you describe. Typically chemistries that are able to supply higher instantaneous current have lower IR.
Does this fit?

 

[OffGridInTheCity]

Different chemistries at high current will cause issues like you describe. Typically chemistries that are able to supply higher instantaneous current have lower IR.
Does this fit?

I don't think high current was the issue - e.g. 200mah/cell for a Sony G5/G7 is 50% standard discharge and 15-20% of standard charge - https://secondlifestorage.com/index.php?threads/sony-us18650gr-g5-cell-specifications.2336/ and https://secondlifestorage.com/index.php?threads/sony-us18650gr-g7-cell-specifications.2320/ Also, its a mystery to me why these cells have a 200+ mOhm IR per OPUS - have not see this in the other cell types I've processed..

 

[Dr. Dickie]

From what I have seen (very limited experience--around a hundred or two) the IR of the cells I have tested (2 years old) goes from around 38-29mOhms at ~2 Volts, to 35-36 mOhms at 4.2V.
If I see a change much greater than that, goes in experimental pile. All this at room temp.
Now I have ONLY tested LG-26 cells.

 

[floydR]

My experience is the same as yours Dr.Dickie LG m26's 2 years old IR of 38- 39mOhms @~ 2v to 35-36mOhms@ ~4.18v after sitting for 15-30 days. Almost done capacity testing. I do have 30 or so +40 mOhms cells I will test later. Now comes the hard part building the packs.
Later floyd

 

[Dr. Dickie]

My experience is the same as yours Dr.Dickie LG m26's 2 years old IR of 38- 39mOhms @~ 2v to 35-36mOhms@ ~4.18v after sitting for 15-30 days. Almost done capacity testing. I do have 30 or so +40 mOhms cells I will test later. Now comes the hard part building the packs.
Later floyd

Almost there with you. I have about 160 more cells left to test. Of course, after the last batch gets capacity tested, then they have to sit for 25 days to test for self-discharge. So, I am about a month and a little over a week away from starting to make packs.
Best of luck.

 

[daromer]

Its correct. We talk about chemical stuff and that is highly affected by many factors like SOH, SOC, temperature and eventuall factors around it.

 

[ajw22]

Based on the information I see, Fig 3 for example, SOC has no real effect on the Internal Resistance factor. SOC for 1kHz tester such as YR1035 or RC3563 is basically also irrelevant if done at room temperature.
I always measure my cells at room temperature.

Be careful reading that report, they gather the results while charging or discharging. The results can be quite a bit different from an idle cell.

Note that Fig3 only shows down to 20%SoC. The IR starts to spike below around 10%.

Yes, the main takeaway is that IR should always be tested at room temperature... NOT in my shed that is now at near freezing ~3C.

Yes, tests done during charging/discharging vs idle changes the result eg. due to internal warming of the cell. Couldn't find any that tested idle cells.

 

[ajw22]

Let me add/rephrase...
Whether Lithium or NiMH or Lead acid... All the times I casually blamed a battery for being "weak" due to low temperature and/or low SoC, it meant that it was in a very much reversible high IR state.

 

[Dr. Dickie]

If, chemist speaking here, a significant portion of the internal resistance (impedance) is the result of the resistance due to the movement of the lithium ions from anode to cathode( which I would think it should be), then IR being very temperature dependent makes perfect sense (chemically speaking). The rule of thumb for non-catalytic chemical reactions is that they double in rate for each 10 degrees Celsius increase. Of course, how bright is your thumb?
Now of course a lot of that increase is in the form of the proportion of molecules with sufficient activation energy to get over the hump, but increased movement (both ions and solvent) is a part of that.

Even if the ions moved due to just diffusion (which they do not--there is electrostatic forces), the Einstein diffusion coefficient is straight temperature dependent.

 

[higher_wire]

IR being a function of temperature is mainly due to the electrolyte. Viscosity of the electrolyte increases as temperature decreases, which will reduce the ionic conductivity. The IR subsequently rises due to the increase in the impedance of the directional migration of ions. This is actually a topic we're working on for deep space applications (i.e., solid electrolyte with a vanadium-based cathode for significantly better low-temperature performance).