Mains Powered with Replaceable 9V Battery Back-up. The range of mains smoke, heat and CO alarms with 9V battery back-up, features unique Push2Fit bases for quick and easy installation to save you time and money on installations. Fitted with advanced electrochemical sensing technology, an accurate and proven method of sensing carbon monoxide. These CO alarms are calibrated and tested in CO gas, designed to accurately measure low levels of CO to provide the earliest warning.