Earlier quoted context omitted.
I wish those mains-powered ones would just shriek continually if the battery was low AND mains power failed; then you could kill the breaker and find it easily.
I wish they just had a steady LED from the mains power on them when the battery was dead. Seems much easier to me.
This alarm has a low/missing battery monitor circuit which will cause the alarm to “chirp’” approximately every 30 - 40 seconds for a minimum of seven (7) days when the battery gets low. The chirp will be accompanied by a flash of the Safety Light. Replace both bat- teries when this condition occurs.
Some of the newer ones come with a built-in Lithium battery that supposedly is good for 10 years.