My brother visited recently, and a couple of the alarms chose that time to begin that awful chirping. The units are hard-wired into the electrical grid with battery back ups in case of power outages. Changing the batteries shut one of them up but not the other. I was under the impression that the basic hard-wired portions were good indefinitely, but apparently not so, according to this handyman article:
One of my cousins asked if I have carbon monoxide monitors. No, I don't; my house has no gas connections and no fireplace. But any house that has open flame should have the monitors, which should also be checked regularly.