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I recently found myself troubleshooting a CO alarm on a houseboat. Inside the cabin, a Fluke CO-220 was reading 40 ppm. In the generator engine compartment, the CO level was zero (while the generator was running). We eventually found the source of the CO to be a large lead-acid "house" battery which was warm and leaking:

Fluke CO-220 showing >1000 ppm CO near lead-acid battery

The OL on the display of the CO-220 indicates a value of >1000 ppm CO. I watched it count up through 600, 800, 900 before indicating OL.

Mechanics replaced the batteries, and ensured the battery box was properly sealed and vented. After this, the CO levels were at zero.

Can lead-acid batteries leak carbon monoxide?

This article discusses the chemical reactions of a lead-acid battery, and as far as I can tell there is no carbon involved whatsoever. So I believe the answer here to be "no".

Can a Fluke CO-220 detect gasses other than carbon monoxide?

The CO-220 manual says nothing about any gasses other than CO. In the specifications:

Sensor type | Stabilized electrochemical Gas-specific (CO)

I contacted Fluke and they responded with:

I do not know what gas batteries would put off and the CO-220 has a senor [sic] for CO only. Hope this helps.

I assume the answer here is also "no".

So then, what was going on?

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You're probably picking up hydrogen gas, which is produced when lead-acid batteries are overcharged at high charging voltages (a danger in its own right). This article details a situation similar to yours: charging a lead acid battery in a golf cart (in a confined space) sets off a $\ce{CO}$ alarm, and typical sensors are activated by $\ce{CO}$ at levels of 150 ppm for 30 minutes and/or 300 ppm $\ce{H2}$ for 30 minutes. Because $\ce{CO}$ is not produced by battery charging, the offending gas is thus identified as hydrogen. After venting, the levels recede.

Also see this post from Hawk Environmental, which details how hydrogen gas can create false positives for carbon monoxide. This post goes into a little more detail about the hydrogen/carbon monoxide sensor issue, and the author states that:

Lead-acid batteries produce Hydrogen when charging.

Carbon Monoxide detectors use something called a "Metal Oxide Semiconductor (MOS)" sensor, which detects a variety of gases including Hydrogen.

A MOS sensor calibrated for CO will give a false positive in the presence of Hydrogen gas at ~10% of the actual value. So, that ~85ppm reported of CO was in reality ~850ppm of Hydrogen gas.

Finally, in an article in the September 1996 issue of Fire Engineering, entitled "Other gases may set off $\ce{CO}$ detectors" (here), it is stated that

CO detectors are electrochemical sensors, which detect a variety of specific gases. In most cases, the gas of interest reacts with the chemical solution in the sensing electrode to produce ions (charged particles) that move through the solution to an electron-accepting electrode, completing an electric circuit. The number of ions produced and the magnitude of the current developed are proportional to the concentration of the gas being measured.

and that

Interfering gases of similar molecular size and chemical reactivity may produce false positive response. Major interferents that can affect $\ce{CO}$ detectors are acetylene, dimethyl sulfide, ethyl alcohol, ethylene, hydrogen cyanide, hydrogen sulfide, isopropyl alcohol, mercaptan, methyl alcohol, propane, nitrogen dioxide, and sulfur dioxide.

I suggest you confirm/deny the above by contacting Fluke and asking them specifically if the sensor does not detect hydrogen gas at all and, if so, at what level. I strongly suspect that the well-documented behavior I've found online and summarized here is the cause of what you have observed in this case.

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    $\begingroup$ Thank you for this excellent answer! Given the fact that the battery was quite warm, and the visible yellowish, cauliflower-looking growth on the negative terminal, I'm also wondering if it could have been hydrogen sulfide being detected; it is one of the interferents listed. The article I linked to says "hydrogen sulfide (H2S) is a possible by-product of over-charging and battery decomposition." I did smell a somewhat "rotten" smell in the vicinity, but coincidentally, trash bags were stored in an adjacent hatch of the boat, so I didn't suspect sulfur. $\endgroup$ – Jonathon Reinhart Aug 15 '17 at 20:41
  • $\begingroup$ @JonathonReinhart - Absolutely, hydrogen sulfide is a likely culprit as well. Feel free to "accept" my answer if it has satisfied your query. $\endgroup$ – Todd Minehardt Aug 15 '17 at 20:44
  • $\begingroup$ @JonathonReinhart might be interesting to do electrolysis on a bit of water. Collect a wee bit of H2 and release it near your Fluke. Or if you have access to gas cylinders and can spare a bit, could test it that way. $\endgroup$ – BowlOfRed Aug 16 '17 at 1:38
  • $\begingroup$ Hemoglobin, the active compound in red blood cells That carry oxygen to the cells through the circulatory system and remove CO2 from them has a higher affinity for carbon monoxide than for oxygen, or it binds with the CO so tightly that the hemoglobin can no longer absorb or release O2 or CO2. This is why CO is poisonous. I was wondering if this aspect of hemoglobin can be used for a more effective CO sensor? $\endgroup$ – Ed Kideys Feb 28 '18 at 13:31

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