Carbon Monoxide Alarm


 Our fourth season using the True Grit brought about an unusual problem. When not in use it is parked on the trailer under roof at my home, which is located in a heavily wooded area.  Southern Illinois has hot humid summers and the wooded location helps with the heat but increases the humidity. To prevent mildew I keep it closed up and run a small dehumidifier inside – an action that has successfully kept the fungi at bay. Because I have a gas heater and gas refrigerator installed, I also have a Carbon Monoxide (CO) monitor/alarm mounted in the cabin. 

    The problem presented itself a couple days after a four-day outing. I climbed up in the boat and the CO alarm was screaming and it’s LED readout showed a fairly high reading. This was curious because there was no source of CO in operation. I opened it up the cabin, aired it out and shut it up again. I reset the unit but the next day the alarm was again sounding.  I assumed the alarm was faulty so I installed a new one but the result was the same – the next day a high level of carbon monoxide was detected. 

Now I am totally confused.  How can there be CO with no source? The only items running in the boat were the dehumidifier and the battery charger. At a loss for an answer I decided to do an Internet search for other possible origins of CO that might be the cause. A google search yielded no other CO sources that made sense but I did find an interesting article about CO detectors reading the presence of hydrogen - . Further reading disclosed that most all electrochemical CO sensors will show a degree of cross sensitivity to hydrogen. Additional searches discovered numerous articles where CO detectors were being triggered by the charging of lead acid batteries in the near vicinity.

To test this theory I placed the detector in my house battery bank enclosure (two 6 volt golf cart batteries) and with in a minute it was sounding with a high reading.  I took the detector outside for a while until it read zero and brought it back into the cabin. Again it sounded.   I replaced the battery box lid and then I opened just the hatch in the saloon roof.  In a few minutes it cleared and the detector read zero.  To me this proved the gas was lighter than air and self exhausted through the upper hatch. To me this more or less confirmed it was indeed a hydrogen problem. 

But why now?  The first three seasons I had all the same equipment installed and had no problems with this.  My understanding is high hydrogen output from lead acid batteries is usually caused by overcharging as the result of a defective charger or a cell going bad. But charger was performing properly. In fact, I was getting this excessive hydrogen when the charger was in just the float or maintenance mode and showing a voltage about 13.3.   I then used a hydrometer to check the charge status on all the individual cells and found nothing unusual. I check with both the charger manufacturer (Xantrex) and the battery maker and neither offered an advice or explanation for the excessive hydrogen off gassing.

I will say the highest reading I obtained in the cabin was about 80 ppm. If the detector measures hydrogen accurately and I am interpreting 
the data correctly it does not appear this is anywhere near 4% level (40,000 ppm) to be an explosion risk.  But I could never find information on the safe level to inhale lower amounts over a long period of time. I decided the best course of action was to improve my battery box seal and venting system to eliminate the unwanted gas.

When I built the enclosure I did include a single 7/8" hole near the top of the box. On the outside it is covered with a small stainless steel clamshell and behind it a piece of screen wire to keep out the wasp.  I also was careful to seal the enclosure’s edges/corners and any penetrations. The top of the box was simply a piece of 1/2” plywood set into a recess to hold it in place.  There was no gasket or seal just a nice fit with gravity holding it down. This is actually the deck when sitting at the helm.

My first attempt at a solution was simply to add a foam weather strip type gasket to improve the seal.  Testing found it might have slowed down the process a little but still the hydrogen was escaping. Next, I decided to try to improve my passive vent system.  I added a second hole through the side of the hull - one would serve as an intake the other an exhaust.  Since I wanted both of these to remain high on the hull to prevent water intrusion I added a 1” plastic tube to serve as a duct routing the intake air to the bottom of the box. The new hole was added for the exhaust and was placed as high in the enclosure as possible.  The idea was the lighter than air hydrogen would escape the upper hole creating a slight vacuum that would pull fresh air down through the tube to the bottom. In theory this should work but reality often tends to ignore good theories. The gas buildup continued. 

Finally, I decided to abandon the passive vent idea and go with power. I honestly didn’t want to do this as it adds additional draw on the battery (however small) and provides another device which I know will eventually fail. But I wanted the hydrogen eliminated, as I just didn’t like the idea of it being present even in low level amounts. 

 Since I knew the amount of air transfer needed to keep the box clear would be minimal I decided on the smallest 12v fan that was easily obtainable – 40mm.  These are very inexpensive and since they are brushless motors there is no chance of causing an explosion. An added benefit was my composting head uses the same fan for ventilation so I always carry a couple onboard as a backup.  The current draw is negligible at about .9 watt or .075 amp. 

I simply made a mounting plate out of 3/4"  plywood with an 1 5/8" hole for the fan.  It was glued to the interior of the hull, centered over the existing hole with two screws then hold the fan to it. This allows easy removal and replacement when needed.

The biggest hassle was the wiring, as I wanted a separate fuse and switch to control the fan.  Plus, I did not want an inline fuse but rather utilize and existing open slot in the 12v fuse block. Not only does it make a cleaner install but moves the fuse outside the battery enclosure. I am fairly sure the ATC fuses I am using are not sealed and the arc created when one blows could have serious consequences in a possibly high hydrogen environment inside the battery box. The switch was located below the helm seat, which is also outside the battery box and has an indicator light to note the fan is in operation.

To date the fan seems to have solved the problem as in storage I have yet to see even a small reading on the CO detector. When the boat is in use we normally have windows and hatches open so I don’t run the fan as the gas is readily dispersed.  If the weather was colder and we have things closed tight I will simply run the fan to keep the fumes at bay.   In storage I allow it to run continuously. I honestly wish I had been able to get a passive system to work but this solution is frugal in both cost and energy so it is a compromise I can accept.

Still I am curious as to why this problem didn’t present itself until the fourth season of use. If anyone has a though or knowledge on this, please drop me an email. I really would like to better understand
the situation.


During the 2014 season this situation only presented itself once. And that was during hot weather. Other that that I have not been running the fan during storage and have not been seeing any hydrogen buildup.  Still totally baffled as to what sets this off at times.