This post was on the Mediterranean Cruising Facebook Group and I thought it worth sharing here:
"I heard from someone quite a while back that Carbon Monoxide detectors are very sensitive to Hydrogen Sulphide.
I can now confirm from first hand experiences that this is true! So thought I would share this with the group for awareness.
Our bank of house batteries were not holding charge and a replacement set are currently in transit to us. When we got back to the boat late at night the CO alarm was going off. The boat was locked up but all main hatches were left open with a reasonable breeze blowing. No smell noticed inside the boat and the alarm would reset but then activate again in less than 1 minute. I checked the battery bank and one cell was getting pretty hot but no smell of rotten eggs in the battery compartment (located under starboard rear bunk). Once the cell had been taken out of circuit and cooled, the CO alarm would reset....no more issues overnight!
The CO detector is located in the galley area at least 3m from the house batteries. Despite open hatches, the battery compartment closed and under the mattress, the alarm activated. We were on shore supply, without the CO alarm the battery would have continued to have ‘cooked’ without us knowing and built up potentially dangerous levels of Hydrogen Sulphide. Very glad I installed it a few years ago!!"

The poster does not say what type of batteries but I assume wet lead acid.  I also assume it would not be a problem with gel or AGM batteries as they are supposed to be non-hazardous.  Is this so?