We have had recent calls involving Carbon Monoxide (CO) alarms at residential properties. Upon investigation, we have determined the root cause to be linked to the charging of lead-acid batteries in an enclosed area and often when being trickle charged. Example: in a garage.
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 approximately 10% of the actual value. So, that 300ppm reading of CO was in most likelihood about 3000ppm of Hydrogen gas. A small confined space can create an explosive atmosphere if Hydrogen reaches its Lower Explosive Limit (LEL) of 4%.
Ensure adequate ventilation is provided while charging batteries. Besides the toxicity and flammability of these substances, it should be kept in mind that they will all also displace oxygen. Unconsciousness can result in as little as a few seconds' exposure to an oxygen deficient atmosphere and for that reason SCBA should be worn on ALL events with Carbon Monoxide alarms ringing, until the atmosphere is tested and proven not to be dangerous to life or health.