A couple of weeks ago I arrived back at my boat after an evening in town.
An irritating chirping was emanating from my cabin - could be heard from the parking lot.
My CO detector was going off. I opened all my hatches and reset the detector.
I was confused about what might have set off the alarm as I understand that carbon monoxide is produced by combustion. I had not been running the engine, cooking, using my paraffin lamp or doing anything that I thought should produce CO.
About half an hour later the alarm went off again. This led me to believe that there was a real issue.
I went over all my electronics, wiring etc. to ensure nothing was shorting out. Everything seemed fine.
The only thing that was going on that was not usual was that my battery charger was working hard to charge my batteries up from dead flat. This was a result of a switch being switched off inadvertently and the batteries being drained by the fridge.
I looked on-line to see if I could find a relationship between battery charging and carbon monoxide.
It turns out that there is a link. The batteries (it seems to be the 6-volt golf-cart batteries are the most affected) off-gas hydrogen at high levels when being charged. Although hydrogen isn't toxic it is recognized as CO by many CO detectors (at 10% of it's actual level).
Once the batteries were fully charged the CO detector did not activate.
I found information on this topic here: The Unexpected Golf Cart Hazard