I've been using a First Alert carbon monoxide monitor model C0710. Has a guaranteed 10-year lithium battery life. Only $50 at: <https://www.firstalertstore.com/store/products/carbon-monoxide-alarm-digital-display-and-10-year-sealed-battery-co710.htm#:~:text=The%20First%20Alert%20CO710%20Carbon%20Monoxide%20Alarm%20uses,an%20alarm%20to%20warn%20you%20of%20the%20threat.> Chris Byer