How long will the battery last in my First Alert Onelink Environment Monitor?

Regardless of the manufacturer’s suggested battery life, you MUST replace the battery immediately once the unit starts “chirping” (the “low battery warning”).
The battery back-up is designed to provide emergency power to the CO Alarm for 7 days. Actual back-up time depends on the strength (freshness) of the battery. The battery back-up will not work unless a good battery is properly installed.