After completing our single cell testing, we have a lot of data to go through. Hopefully this page will help guide people in going through the data. If you have any questions, contact Micah Black. The tests chosen were guided by the Individual Cell Testing Evaluation page.
...
Open Circuit Voltage
| Priority | 1 |
|---|---|
| File Name | cell#-OcvTestScenario.csv |
| Results Status | Completed |
| Visualization Status | Completed |
...
DC Internal Resistance Pulses
| Priority | 4 |
|---|---|
| File Name | cell#-DcCurrentPulseScenario.csv |
| Results Status | Not Completed |
| Visualization Status | Not Completed |
The DC Internal Resistance Pulse test is very common as it is super easy to do (maybe not so accurately) with a multimeter and a power resistor. However, there are many complications with this test.
...
Note on this graph (the third graph), the bottom axis should read pulse number, and not amplitude of current. These pulses were all done at the same current, and not amplitude of current. These pulses were all done at the same current.
More guidance for the DC Internal Resistance Pulses:
The voltage-current graph will show the pulses, in sequence and alternating from positive to negative as in the graph on the left. The DC internal resistance values calculated for each pulse are shown on the right.
For each pulse, we want to linearly extrapolate the voltage (using the last half of the voltage measurements for each pulse - green line) back to the start of the current pulse (black line). The black dot indicates where the voltage would have been at the start of the current pulse is parasitic transients are ignored.
The internal resistance in calculated using the following equation:
Internal Resistance (Ohms) = (V pulse - V rest) / (I pulse - I rest)
Where:
| Value | Meaning |
|---|---|
| V pulse | The steady voltage change, extrapolated to the start of the pulse - where the green line intersects the black line at the start of the pulse |
| V rest | The voltage of the cell before the pulse was applied. For our calculations, this will be the open circuit voltage of the cell measured from the OCV test |
| I pulse | This is the set current for the pulse. This can be taken from one of the current values during the pulse. Again, we can ignore the uA changes that may be happening, but a better approach would be to take the average of all of the points. |
| I rest | We will assume this to be 0A, as that is what we set the DC load to during the rest times. It may be somewhere in the uA range due to regulation errors, but we will ignore those. |
As can be seen, the internal resistance changes as more pulses are applied. This is an effect of the ions inside the cells moving around in and out of the separator, causing the resistance to decrease. As I understand it, this is purely a function of the number of pulses applied, and not a function of the amplitude of the current pulse.
When you plot the internal resistance for each amplitude of current pulse, then we get the graph above. As you can see from this graph and from the second graph from the first set of three, the internal resistance eventually settles out to a constant value. The blue lines show the internal resistance settling. The green line shows the constant value of internal resistance for both charge and discharge pulses. We want to have both of these values saved to the csv output.
DC Internal Resistance Slope
| Priority | 3 |
|---|---|
| File Name | cell#-DcLinearSweepScenario.csv |
| Results Status | Completed |
| Visualization Status | Completed |
This measurement is similar to determining the resistivity of a sample of material, but applied to a battery cells. We applied a changing current from -3A all the way to 3A and measured the voltage of the cell. This gives a fairly linear response, but would maybe be better split up in to charge and discharge sections, or possibly a quadratic would fit the data better.
...
AC Internal Impedance (1kHz)
| Priority | 2 |
|---|---|
| File Name | cell#-AcInternalScenario.csv |
| Results Status | Completed |
| Visualization Status | Not Completed |
The AC Internal Impedance test at 1kHz is an industry standard test for measuring the internal resistance of cells, and is usually the value that manufacturers put an upper limit on. (LG says <40mOhm no the MJ1 datasheet I believe). The AC test produces different results than the DC methods as the AC method also takes into account the inductance and capacitance of the cell, and some of the other complexities of the equivalent circuit model. Essentially, the voltage never has time to fully settle out.
...
Capacity Ration
| Priority | 6 |
|---|---|
| File Name | cell#-TenSecondsDischargeScenario.csv |
| Results Status | Not Completed |
| Visualization Status | Not Completed |
I think that this test is super cool and has the possibility to give some super interesting results.
...
Weight (Possible Indication of Capacity)
| Priority | 5 |
|---|---|
| File Name | cell#-weight.csv |
| Results Status | Completed |
| Visualization Status | Completed |
Some of the papers linked on the Individual Cell Testing Evaluation page found a correlation between weight and capacity, while others did not. For us, this data is just another source of outlier identification.
...
| View file | ||||
|---|---|---|---|---|
|
| View file | ||||
|---|---|---|---|---|
|
The graphs used for testing were made using this script:
| View file | ||||
|---|---|---|---|---|
|