About every 2-3 minutes I hear this beep upstairs. I recognize it as a smoke alarm with a dying battery. I go upstairs, and stand in the middle of the hall. I hear a beep down the hall. I pull off the alarm and replace the battery.
In the middle of replacing the battery, I hear a beep. I go back downstairs, get another battery, and replace it in the alarm further down the hall.
While replacing that battery, I hear a beep. I express my displeasure verbally, go downstairs, get 3 more batteries, and decide to replace ALL the smoke alarm batteries, since I'm supposed to do that once a year anyway.
All the while I'm replacing, I hear more beeps.
Maybe it's the batteries.
So I go downstairs, get one more new 9V battery. I then go down the basement and put the new battery in an old video game (Mattell baseball). The battery is fine. I play 5 innings just to be sure.
I go back upstairs, and start taking the alarms back down to make sure I plugged them back in correctly.
While I'm doing that I hear a beep down by my feet. And I see the carbon monoxide detector. (With a "LO BAT" error message on the screen no less).