Posted 3/12/2015 by Paul Basson/Foreign Affair:

I recently invested in a dual gas detector, which monitors LPG and Carbon Monoxide.

I was surprised to find that the Carbon Monoxide alarm would go off when the boat was unattended.

I suspected the battery charging and checked with the manufacturer, who confirmed that the Carbon Monoxide sensor will also detect Hydrogen which is generated during battery charging.

I checked my boat drawings and found that the Engine start batteries had a pipe to the service battery box, because Hydrogen is lighter than air and it escaped up to the Service battery box, which also has an escape pipe at the top forward end that feeds to the bottom of the mast. The idea being that it then rises up the mast and dissipates.

However in my case the pipe from the service batteries had been dislodged and it no longer fed the bottom of the mast.

Problem solved – Thank you Lars.

Comment from Lars: If the hose exits from the top of the service battery box it is suggested that you check if there is an oval hole for the hose in the mast at the same height. 

You are right that the hydrogen is expected to exit through the mast, when the sun shines there is a strong upward draft. On some models there is a small blower creating suction from the box when the batteries are charged. This blower must be of an explosion proof type.

When I moved my suede mast cover, the oval hole was there – of course