I will look into it more today. It is certainly not a cell issue as my Fluke meter shows all the cells at the correct voltage within 0.002 of what the BMS is showing when the buck converter is off. When I turn on the buck converter, the cell voltage readings all start to bounce a bit, but just the cell 12 dropped the 20 mv. I can't explain why just that one went more goofy than all of the 13 others.
These are LG Chem Chevy Bolt cells. They are pouch cells in proper cell holders. The outsides of the pouches are a type of plastic film. There is an aluminum sheet between ever second cell for cooling when used in the electric car. I do not measure any leakage current to the aluminum sheets. I did pull a 20 amps load and checked for any voltage drops at the buss bars. Most of the connections are still the factory spot welds. Everything measures fine. The pack is currently charging at 20.9 amps. About 1,200 watts, out of the 1,900 watts that are coming from my solar panels.
Here are the cell voltages with the system charging. The charge current does bounce from about 19 to 24 amps, the average charge current is 20.97 amps right now. That bouncing is caused by the XW-Pro charge current being a full wave rectified sine wave. This picture shows all of the cell voltages are between 3.825 and 3.828, just a 0.003 difference from top to bottom.
Now I turn on the DROK 12 volt Buck converter with a small load.
The total voltage reading dropped 0.16 volts. That is not real. I see no change in voltage at all with this small load. The battery is still chargin at over 20 amps. But now the cell balance is all over the place. The Cell 2 dropped to 3.814 an 11 mv change, but then look at cell 12 3.753 a drop of 73 mv, from a 0.4 amp change in current. Nope, this is not real. Cell voltage on the Fluke meter is still 3.825 volts at the balance leads. The reading at cell 10 actually went up to 3.829, but that is only 1 mv, well within the margin of error. It has to be something to do with the balance wires picking up the switching noise of the buck converter. It is just odd that it is hitting Cell 12 so much more than the rest.
When I shut the converter back off, it all returns to normal. All of the cels have climbed about 2 or 3 mv due to the 21 amp charge current.
Cell 10 is still highest and Cell 1 is the lowest, just like in the before picture.
I also have a 24 volt converter. Obviously from the same original manufacturer as the heat sink case is identical. These are the water tight sealed units they sell on Amazon for golf carts to step the voltage from the main battery down to run the lights and a radio etc. Here is the one causing the issue.
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Here is the 24 volt one.
I have not put a load on the 24 volt one yet. At no load it does not seem to cause an issue, but the 12 volt one does, even at no load. My first step will be adding a large filter cap on the input lines to the converter and see if it makes a difference.