Power Profiler Kit 2 measures exponentially decreasing current at constant current

[lukas76352]

Hi there,

I am trying to monitor the charging current of a battery cell. For this I am using the PPK2 in ampere mode, a bench power supply at 4V as source which limits the current to 0.8A. I am measuring the current with the Power Profiler 3.1.3 through the PPK2.

The issue I have is that  the Power Profiler shows an exponentially decreasing current (as shown in the screenshot below) while the current should be constant (as measured by the bench power supply and an external ampere meter). The current measurement then settels at 50mA while the actual current is at 0.8A. What could be the cause of this?

My set up is as the following picture indicates:

I am on a MacBook Pro running macOS High Sierra 10.13.5.

I am using nRF Connect v3.7.2 running Power Profiler v3.1.3

On the PPK2 it says: PCA63100, 1.1.0, 2023.12, DB2E2D24

Any help is greatly appreciated!

best,

Lukas

[hmolesworth]

Incorrect connections, remove and connect as follows:

PSU GND do not connect or optionally connect to PSU '-'

PSU '-' connect to both PPK-II GND and nRF52 under test GND

PSU '+' connect to PPK-II VIN

PPK-II VOUT connect to nRF52 under test VCC

Note GND on the Bench Power Supply (PSU) is not necessarily the same as GND anywhere else. If using a grounded MacBook Pro then USB GND may be the same as PSU GND, but when the MacBook Pro is not connected to a power supply its GND is floating with respect to PSU GND. For now just ignore GND on the PSU.

Note also the PPK-II may have been damaged by the connections used in the original post; fingers crossed that it's not ..

[mahboud]

Lukas is trying to measure the current that the battery is drawing. There is no nRF52.

The wiring I'd use is
PSU + to Vin
PSU - to GND
Battery - to GND
Battery + to Vout

That's very similar to what Lukas has except he uses the PSU GND instead of the PSU -. 

Lukas, can you try changing the connection to PSU GND to be to the PSU - instead?

Also, why do you think you should have 800mA of current? That's the maximum that the PSU allows, but a somewhat charged battery could only take a trickle of a current or a fraction of the 800mA.

[lukas76352]

Hi,

Thank you both for your replies.

@mahboud: This is the wiring I initially tried, however, I got the same result. I then changed to the wiring I described above, because it seemed closer to what is written in the PPK 2 manual.

Regarding the 800mA: The battery cell is empty, hence I am expecting a large current and to not damage the PPK, I limited the current to 800mA, which is what the PSU is outputting and also what I measured with an additional multimeter.

@hmolesworth
I don’t quite understand how the set up could have damaged the PPK. Could there have been such a large potential difference between the USB GND and the PSU GND that a large current balanced the two potentials?
Anyways, I hope it didn’t damage the PPK.


In any case, I will check if the PPK still works as expected in other situations and I am also going to retry the wiring that you suggested.

Thank you for your help and I’ll get back to you sometime next week.