Hi Peter,
the water level, usually, should be around 1\” below the tops of the tubes. See this picture.
http://www.thelastfourbooks.com/Kabbalah/Joe_Cell/Joe_Cell_Stage_3c.jpg
This is from my Joe cell archives page.
http://www.david-house-productions.com/House-of-David/alternative_energy/joe_cell.html#Joe_Pics
You\’ll notice that the water level is around even with the tops of the tubes, but NOTE: this is because it is a water charging kit and not the actual joe cell that is installed in the car.