One Link 2In1 Smart Smoke Carbon Monoxide, Alarm Alerts You To Both Types Of Danger In Your Home, Alarm Sends Emergency Notifications, Via Your Phone, While Voice Alerts Tell You The Type Location Of Danger, Interconnects With Other One Link Devices Sold Separately To Create A Smart Home Safety Network, So If One Alarm Sounds, All Will, Works With Amazon Alexa So You Can Ask Alexa To Check The Status Of Your One Link Alarm, Easy Access Battery Door, For Simple Battery Changes, Replaceable 5 Year Battery