I've had it running again for the past 2 days, and this time the graph looks fine (i was charging at the end).
Things i've changed:
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Moved the GND wire closer to the arduino. It was almost at last point before, after everything else (including the LEDs) -->Probably too much noise
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Set up a capacitor between AREF and GND.
I guess i will try again over the weekend, to see if it is staying stable.
Thanks you for the help.
Kind regards.