gauss163 said:
There is no (single) DC IR measurement standard just like there is no single number for measuring human health, or a car's mpg (depends on your particular mix of city and highway driving, etc). Similarly the type of AC/DC IR measurement that is most pertinent will depend on battery usage conditions, viz.the type of load (e.g. constant current vs. short large pulses as in vaping/jump starting,or some specific mixture), and ontemperature ranges, etc. Only by knowing
all of the components of the DC IR will will have any hope of derivinggood estimates of how abattery performs under diverse conditions. Armed with such we have a much betterchanceof achieving near-optimal battery performance and lifetime - whether in powerwalls or other devices.
Generally DC IR is near constant vs. SOC except at low & high SOC, so it's essential to measure it in this constant region, typically between 80%-30% SOC, e.g. see the
graphs I posted here. Otherwise the measurements will not be comparable so will be of little use.
Interesting.
So if we do not have a (single) DC IR standard we don't have a chance in getting it right. We may have an idea where it should be but with temperature, SOC and many other factors we are always trying to hit a moving target.
We do have numbers that measure human health. Blood pressure, body mass index, cholesterol levels, an array of blood chemistry tests, ocularpressure test for glaucoma, bone density, alsoof late body temperatureand many more. Some are more subjective to interpretation others are not. Most of these tests have a low limit, a high limit and of course an average that has been determined by experience. Fuel mpg is a guess at the average also whetherit is city or highway or both.
So for DC IR we are left with a guess. How close can we get to the guess being right? I don't know There has to be a standard to go by.
I.e. Cell at 20 C, 3.6V, SOC 50%, 1? load, 1s pulse, and for ICR chemistry DC IR should be ?120m? and ?80m?.
It could even include the same parameters as above and change the ? load value and or the time the load is applied.
Looking at one of the graphs out of
A study of the infuence of measurement timescale on internal resistance characterisation methodologies for lithium-ion cells
the graphs that my system produces is incredibly similar. Am I on the right track? Maybe.
Posthttps://secondlifestorage.com/showthread.php?tid=9374&pid=64352#pid64352
You wrote
"Second, be aware that DC IR depends on SOC (State Of Charge) and temperature - at extreme SOC and temp it can increase by 10x or more, e.g. see the graphs below. So don't measure it in these extreme ranges (unless testingthoseextremes)."
In that statement I am not sure what you mean "at extreme SOC and temp" (in my minds eye) is fully charged. DC IR if anything is lower according to my tests, an average of 24% lower on all my cells good or bad. If you meant a extreme SOC (fully charged) and high temp,with thisother variable then I do not know as my ambient temp is (house temp) 20C to 25C. Additionally if you meant SOC (meaning discharged) then yes the DC IR would definitely be higheras the V drop would be significantly increased. Temperature is again a variable I have not tested with.
The chart you are referring to does indicate DOD of 80% to100% for those high readings if I am not mistaken.
As with1kHz AC measurements we do have a clue as to what it should be. Mostmanufacturers actually publish the 1kHz AC IR measurements as being
? some value. At least we have something to go by. I understand that it is a value assigned to a new cell and it is just to quickly verify that the cell after production is valid. Nevertheless it is a value even if it only represents a small portion of the SOH of a cell. The rest of the SOH of a cell I believe can be judged byhow well it performs in a C/D/C test.
One more thing.
In the conclusion of the
Comparison of Several Methods for Determining the Internal Resistance of Lithium Ion Cells
paper it states:
(Just an excerpt)
The method of measuring AC resistance at fixed frequency gives fast results. Because each cell type shows individual frequency dependence of impedance, AC resistance measured at different type of cells cannot be generally used for cell comparison and benchmarking. Therefore this method is only suitable for measuring and comparing internal resistance of the same type of cell, e.g., for quality screening.However, at frequencies about 1 kHz AC-resistance for different cells (which have comparable size) has very similar frequency dependence. It also could be used for comparing different cells (relatively). It may be expected that cells with higher small signal resistance also have higher large signal resistance. Despite the fact that the impedance spectrum of the cell was recorded with higher damping than the measurement at fixed frequency, the same result for 1 kHz was found.
I have read this over and over again and it certainly does not say that it is the wrong way but just another way.I am not a scientist or claim to be.
What I do know, is that through experimentationyou can find things out and if the experiment is repeatable every time there must be something to it.
I will continue to play around with DC IR as I find it a fascinatingstudy. Maybe I can find some correlation with the randomness of it all.
As far as my cell testing is concerned I will stick to AC IR for now as I know it has proven itself time and time again with the outcome shown on the testers mAH results.
Wolf
