Charging Batteries

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Hydrogen gas can trigger CO alarms

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. This charging process often leads to off gassing of Hydrogen through venting holes in the battery that are designed for this process.

CO car battery

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. A small confined space can create an explosive atmosphere if Hydrogen reaches its Lower Explosive Limit (LEL) of 4%.

CO alarm


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, you should evacuate the building immediately anytime Carbon Monoxide alarms are ringing. Call 911, get out and stay out until the atmosphere can be tested and proven safe.

Tips for charging batteries

  • Charge batteries in a well ventilated area
  • Follow manufacturer’s instructions
  • Be careful when attaching / detaching clamps and ensure correct polarity 
  • Clean battery terminals according to manufacturer’s instructions before recharging
  • Do not attempt to charge a frozen or damaged battery
  • Monitor the charging process
  • Turn off charger before disconnecting
  • Unplug the charging device when completed
  • Remember, don’t become a victim - Stay Safe!