Major setback. I tried to do a full “function test” at the kitchen sink with all parts mounted and the pump connected to the relay and the batteries – and it failed miserably!
The pump never started and the relay just “buzzed” for a second. It shows that the pump draws so much current that the battery voltage drops way too low for the relay and the rest of the circuits. It did not help that I tried chargeable NiMH-cells.
It’s back to the drawing board. Either the electronics and the relay has to have some power regulator with a capacitor that can hold the voltage or they simply have to have separate power (batteries).