Interesting! I understood that while charging a battery hydrogen and oxygen were produced....I wonder how your particular sensor senses CO levels. I checked wikipedia and came up with this about a type of CO monitors:
"Electrochemical
A type of fuel cell that instead of being designed to produce power, is designed to produce a current that is precisely related to the amount of the target gas (in this case carbon monoxide) in the atmosphere. Measurement of the current gives a measure of the concentration of carbon monoxide in the atmosphere. Essentially the electrochemical cell consists of a container, 2 electrodes, connection wires and an electrolyte - typically sulfuric acid. Carbon monoxide is oxidised at one electrode to carbon dioxide whilst oxygen is consumed at the other electrode. For carbon monoxide detection, the electrochemical cell has advantages over other technologies in that it has a highly accurate and linear output to carbon monoxide concentration, requires minimal power as it is operated at room temperature, and has a long lifetime (typically commercial available cells now have lifetimes of 5 years or greater). Until recently, the cost of these cells and concerns about their long term reliability had limited uptake of this technology in the marketplace, although these concerns are now largely overcome"
I wonder if the added presense of elevated hydrogen levels skewed how the sensor was sensing the CO levels....??????????