- Sep 25, 2018
Yes high discharge or high drain cells such as IMRINR NCA chemistry cells tend to have a low AC or DC IR to begin with.Redpacket said:That's some awesome work there.
I guessing the DC IR results also followed the discharge current rating for the cell?
Ie it would confirm high current cells would have a lower DC IR than say your average laptop cell?
A 1ohm load would be quite a high load for some cells right?
ICR chemistry is of course low drain and henceforth inherently has a higher AC and DC IR.
Preliminary results show with my rudimentary tester based on ohms law that the difference between AC and DC IR on a good cell is about 1 to 2.
Meaning that if a cell has 12m? of AC IR it will more than likely have a 25m? DC IRfor a high drain cell.
The same holds true for low drain with an IR of say 50m? will have a DC IR of 100m?.
A constant 1? load would behigh for most cells yes but in my case it is only applied for 1 second to get a stable vdrop Voltage.
I have tried 100ms 500ms and all kinds of variables. but the 1 second load seems to be the sweet spot to get a repeatable result.
I can do this test 3 or 4 times on a fully charged good cell and get the same results within a few m? every time.
My tester has a 1? 100W resistor load that gets put on the battery at 4.1 or so V.Technically that should produce a 4.1 Amp currentbut it does not as you can see by my discharge chart it is in the neighborhood of 2 .xxxAmps. ( I have verified that with my Fluke meters) The load wire is 10AWG very capable of handlingthat current. The sense wires are 20AWG although that really doesn't matter as there is no current flow through them.
So the calculation is V(open) - V(load) = V(drop)/mA(draw) = R
Unfortunately as the second paper I linked to basically says that there are no DC IR standards set so how do we know we are on a the right track?
Article publishedAPR 15, 2020.https://www.electronicdesign.com/te...e/21128843/measuring-dcir-of-lithiumion-cells
So I took an educated guess and these are my results. Are they right? I guess so. We are in somewhat uncharted territory and till there is a standard set we just don't know.
One encouraging spec sheet for the 18650-30Q does give us a DC IR value of:
Type Spec. (Tentative) Typical INR18650-30Q
Initial IR (m? DC (10A-1A)) ? 30 19.94 2
But how was this determined?
My tester seems to hit that nail on the head does that mean it is right?