Arduino relays are a thing that exist and "analog input" is just voltage 1 to 5.
Edit... maybe not the one in this picture, I don't see an analog input port. Velleman VMA406/Adafruit trinket.
Therefore all you need to do is run cell #1 on your lipo balance lead to the analog input port. You will have to connect your Arduino to your computer and program it
it to "activate relay" at the right voltage (for me this is about 4.15 volts).
You will connect your solar panel to the "NO port" (normally open) and when the relay triggers it will cut power from your solar panel to your bike.