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DC IR VS AC 1kH IR measurements
#41
Looking at the spec sheet for the INA260 the accuracy is going to be an interesting problem because it's designed to work with relatively stable loading. I was hoping that the trigger mode would allow for an external trigger input (rising or falling edge of the PWM) but the trigger pin is driven from internal criteria only. Even thought the trigger pin could be used to create the PWM by a bit of hokey pokey, yes not in a manner that would make the results easy to interpret, deep rabbit hole. The Fig.13. Frequency Response plot is relatively trouble free upto about 200Hz.

The ADC does not seem to specify a sample and hold type ADC so switching transitions of the load will cause more detectable singular sample errors. This critically also limits the upper load frequency and may require a formula to convert the INA260 values to the "actual" values. This is because if the INA is sampling at say 1mS and the load is 1kHz (50/50) the load is only on for 0.5mS and the measured ADC value will be lower than the actual value by some function as to how the ADC works (e.g. successive approximation). With a rising edge the ADC may chase the measurement to the top but on a falling transition the ADC would stop at some random point and this random value would occur as a function of frequency. As the frequency rises the measured value will decrease more and more below the actual value on sample averaging approach.

I'm wondering if triggered sampling mode (write to register to trigger conversion of one sample only - no averaging) on the INA260 can then remove over 95% of detectable current measurement errors (switch off transitions) by filering with a maximum expected allowable deviation.

The ADS1115 does not have an extrernal trigger either, unfortunately. Searching web for parts...

So... low frequency testing may be better driven from a pin rather than the servo controller PWM output, but the frequency limit will be quite low. Higher frequency will have to average a lot more samples and also be limited below 1kHz without further interpretation of measuements.
If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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#42
CC

Got it.
I was thinking the same triggering the MOSFET from the ESP32 initially and also sticking to low f for now.

At least we can get the prototype functional initially, working out the details as they come along.
It's going to be a challenge as it is, putting something together with cheap off the shelf parts and experimental code for now.
Looking into the ledc channels for proper PWM and interrupts to accumulate data for a specific amount of time? Maybe?

Code:
sei();  //Enables interrupts
delay (ms);  //Wait for data (ms) determined by encoder position or some other means
cli();  //Disable interrupts
Also it looks like the ESP32 may have a problem with anything below 15Hz on the ledc channels? Don't know yet.
At first I think I'm just going to manually set up a 10s on/off and see what reaction I get from the sensors.
Pull out my math books, which I swore I wouldn't do, and see if the calculations will be right.
That will be step 1.
After that we can go to Step 2 and so on.

Wolf
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#43
(07-08-2020, 03:41 PM)completelycharged Wrote: Found the sheet, starting to play.... [...]

Iteresting falling off the cliff


It'd be interesting to see that with the DC IR plotted too. Below are results from a study of 800 cells that show analogous results. Note how the capacity uncertainty increases as the cells age, e.g. 130m could be 93-103%, but 160mcould be 5-45% capacity. Presumably that's due to the growth of the non-ohmic resistance - which cannot be seen by the 1kHz AC test. But they didn't do any DC IR tests so we can't be sure.

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#44
From that plot it looks like the charged vs discharged Ohm difference might widen with age/capacity as well. Individual cell data through a full cycle will get quite interesting.

The two discharge charts in the paper also show the degredation characteristics a bit better when shown together and the interesting 70-80% point of accelerated degredation.


Full cycle testing with enough random sample cells (from typical recycling) would allow for the voltage discharge curves to be derived without having to do 800 cycles on a single cell.... as long as the cells are correctly identified and not mixed up.

Within the bode diagram, thier test data also shows up a slight variation/pattern in the response around 100Hz ???

 

Would seem a bit strange and carless to have 50/60Hz harminics in the test setup if it is just the supply, otherwise could this small deviation actually be showing something very particular if the resolution was amplified ?

Compare Fig 9 and Fig 10 around the 100Hz point... phase angle indicates less capacitive impedance but Z increasing, so.... non-ohmic internal characteristic ????
If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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#45
Looking at the cell test data, there appears to be a bias between testers or just coincidence that a set of cells were tested on some with some testers.

XTAR reports lower capacity and higher IR than Foxnovo for example. <50mA bias and >5mOhm bias for XTAR.

Messing about with plot for Resting volts X, mAh Y and bubble as mOhm above sample minimum (rebased to normalise)


 

XTAR top

Based on the cell numbers it appears random..
If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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#46
CC
If you are talking about the LGGBM261865 sheet....
Oh yea definite bias.

I noticed that after about 200 cell its seems the slots of each "testers" mAh results where consistently higher/lower.
Look at the OPUS 1 slot 4 it sticks out with always having a higher mAh result. the SKYRC cells 2 and 3 always just a bit higher.
Slot 2 on the XTAR always a little higher and so on.
Also tester finish voltage on the OPUSES (or is it OPI,or OPUSI : Tongue )
is always a bit lower on OPUS 3 1-4.
IR was not checked with the testers but obviously with the RC3563.

On another note I finally got a very stable V and accurate reading on the ESP32 with the ADC1115 after playing with divider resistor values and formulas. 
On to the next step Amp readings.
On the prototype board with cobbled together code though I was able to get  Voc , Load V and I and did a quick calculation for DC IR with a 10 second load. Preliminary results but encouraging for the first step.

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#47
Below is a graph that may help to make more concrete some of the things I discussed above. It shows a typical voltage response when the current is instantaneously changed from a higher to lower value (e.g. at the end of charge, when it changes from the termination current to zero). Below I briefly explain the various resistance components that contribute to the voltage decay and give some links for further reading.



RO = Ohmic resistance. The instantaneous voltage drop ?V1 is caused by RO, the purely ohmic component of the resistance (which corresponds closely to the resistance measured by AC 1kHz meters). This is the sum of the resistance from the cell's components that behave like a resistor, i.e. are governed by Ohm's law: ?V = R ?I, such as  internal wires(tabs) and electrolyte ionic resistance.

RCT = Charge Transfer resistance accounts for the voltage drop  ?V2 typically occurring in the first few seconds, due to charge transfer at the electrolyte/electrode interface (as well as double layer capacitance).

Rp =  polarisation or diffusion resistance corresponds to the shallow almost-linear voltage drop off that occurs after a few seconds, caused by ionic diffusion. This is the the main component of voltage drop observed when removing a cell from a charger, as the cell slowly asymptotically creeps to its steady-state resting voltage (it may also include some of the drop from the prior faster timescale components if you measure it just before removing from the charger, or very quickly thereafter).

DC IR measurements typically include all of these components (but only an initial part of the hours-long Rp voltage drop), depending on the length of the test pulse (and its amplitude). Taking all of this information into account may help to yield a better idea of how the cell's voltage drops under longer timescale DC loads - info that is much harder to glean from the short timescale 1kHz AC tests. This may in turn lead to more accurate measures of cell health.

There is a nice introduction to this and related matters in Barai et al. A study of the infuence of measurement timescale on internal resistance characterisation methodologies for lithium-ion cells, 2018. It also includes helpful links to prior related work, see esp. the paper Schweiger et al. Comparison of Several Methods for Determining the Internal Resistance of Lithium Ion Cells, which reviews and compares many of the common methods of testing IR = internal resistance.
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#48
I found this quote interesting from https://www.nature.com/articles/s41598-017-18424-5.pdf
from gauss163.  Nice read thanks.


"Depending on cell type and experimental setup (e.g. cable assembly), the 1 kHz resistance measurement can
be in the inductive or conductive region. Typically, for cells with large capacities (e.g. a 40 Ah pouch cell) the
resistance at 1 kHz is dominated by inductive behaviour. On the other hand, cells with relatively lower capacities
(e.g. a 3Ah 18650 cell) can have a 1 kHz resistance close to the point where Im(Z(ω)) = 0 29, and thus give reliable
and repeatable measurements."

So for single 18650 cells the 1kHz method is ok, but for larger cells or comparing packs it doesn't work well ... Is this conclusion correct?
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#49
Here is some initial data of a new US18650VT4 cell with  19.185 mΩ AC IR.

Resistor was 4Ω and time was 12 seconds

All preliminary. I'm going to go with 1 or 2 more decimal points on the current measurement for better resolution.

Here is the chart.

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#50
For me, there's two angles to this:
1) determining good/bad cells early with an easy test, eg the usual impedance meter 1kHz tests
2) what our packs are doing while in-use.
From the pdf study a few posts up from Bubba, I'm most interested to see the results around our "in-service" use eg figure 4 b) or c) where they show a cell already under some DC load & the load is increased. Or charging the same sort of step.
I don't see much value in charge to discharge steps or in "resting" to load/charge steps.
Also, like mentioned earlier, understanding what happens with the 50/60Hz half-sinewaves out inverters typically draw would be good to figure out!
Running off solar, DIY & electronics fan :-)
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