Often cell datasheets are limited in data, particularly when it comes to internal resistance. So how do we go about estimating cell DCIR?
Although cell datasheets don’t explicitly give the DC internal resistance of the cell, they do often show the voltage versus SOC discharge curves.
Within this data is the DCIR value for the continuous discharge condition of the cell.
Here is the voltage versus SOC curves for the SES 50Ah lithium metal cell.
If we section this graph at 50% SOC and extract the values we can then plot voltage versus current.
I use a simple piece of code that allows me to calibrate the graph and screen, two red crosses.
I then select the points and this gives me the values for each.
As you can see the values are all at 50.3 to 50.5% SOC.
C-rate values can be turned into currents by multiplying them by the cell capacity, in this case 50.7Ah
Thus C/10 = 5.07A and 3C = 152.1A, then we can plot the voltage versus current. Fitting a straight line trend to this allows us to check this is linear and sensible.
The equation on the chart is of the form Y = mX + c where m is the slope and c is the constant. As we know Ohm’s Law is V = I x R the slope of the line is V / I and hence equal to R.
Thus we have the internal resistance as 1.134mΩ
OK, is this data extraction process repeatable?
Calibrating the graph and extracting the data points on two separate instances shows that the DCIR is 1.134 to 1.140mΩ
I think this is ok for this initial level of data analysis based on cell specification datasheets. I would expect these values to only be used to check and scope out options. Swapping to your own test data from the cells for more detailed confirmation of performance and design.