First Alert CO400 Battery Powered Carbon Monoxide Alarm for $17.95. Shipping is free on orders over $25. If needed, use this Filler Tool to pad the order to qualify for free shipping. Here is a screenshot fo your reference. Note that you can also get the 3-pack for slightly less on a per unit basis. This is a battery-operated carbon monoxide alarm that uses electrochemical carbon monoxide sensor. Two AA batteries are included.