Hi,
we see a lot of cables. More than I expect.
I can not see which is used for what.
Does the wire carry current ot not?
Does the battery supply any device or not?
So it´s not clear to me whether:
* the voltage fluctuation at the measurement nodes are true or not
* it´s a wiring problem
* it´s a power supply problem
..
Hard to give advice.
I recommend to keep the wiring as simple and as short as possible.
Then do new tests and tell us what happens.
Use a battery (3x AA or 3x AAA) to supply the voltmeter .. then tell us what happens.
Klaus