Hi Kumaran
Thank you for your suggestions.
The electrolyte was new with about 15 - 20% KOH. I submerged the cell under water and connected to the power. There were some small bubbles from the electrolysis outside the cell, but you can easily tell the difference if the bubbles are from the electrolysis or if they are from the leakage in the cell. There was some minor leakage in the refilling part and that appeared only at greater pressures, but then I screwed the cap tighter and there was no leakage anymore. Then I tested the LPM. I took a 5l bollte and filled it with water, then submerged it and let it fill with the gas. It filled only in 3 minutes 30 seconds. That is 1.43 LPM. The amps were the same - 20A and the voltage across each 6-cell section was exactly 12V (I measured this with a voltmeter, the power supply voltage was 12.7). This all gives an efficiency of 84%. Too bad. I don't get it, how could that be? The connectors and the wires are not getting warm. Could it maybe be that the plate spacing is too close? So that the bubbles have a hard time climbing up to the electrolyte surface? Maybe I need to try 4mm vinyl as a spacer? I am confused