… smoke and CO alarms alert you to a low battery starting in the middle of the night? 
What’s even more frustrating is my ('67) house was re-built inside the shell before I bought it, and they are all wired to the AC power, batteries are just for backup.
This!
26 years in this house and every. single. time. they chirp first in the early morning. 
be thankful, otherwise you’d probably get the chirping more often because you’d need to replace the battery more often.
i didn’t even know mine could be hardwired until i replaced one about 7-8 years ago and found the wiring underneath. sent the battery one i bought back to amazon and replaced both of mine with wired smoke alarms. made me very happy.
I could identify with this. Several weeks ago we had nighttime temps in the low 20s…and some snow. At 4:30 on morning I was awakened by battery chirp. I was successful for only about a half an hour ignoring it, then couldn’t stand it anymore…so I got up to deal with it. It was then that I noticed both of our dogs were nowhere to be found. (they can enter and exit at will through a dog-door)…so I called to them outside. Silence…the unknown was scaring me. I called again and they finally came in…shivering in cold and in fear…terrified of that strange chirping sound in the house. They had been outside in the cold and snow for at least a half an hour and were shaking and soaking wet.
I have a small house and I have 7 or 8 smoke detectors, and two CO detectors. Code here apparently requires one in every bedroom (that’s 4), plus there’s one in upstairs landing (plus one CO as there is a HVAC unit in attic), one in the narrow room near my gas water heater (plus CO), and two more in large rooms that also adjoin the kitchen.
Nuts, I tell you.
A CO detector was chirping when I got out of hospital in Feb., I was too weak to go upstairs and find it, let alone risk using a step stool to pull it down, so my ex did. It’s still sitting down here where I opened it to shut the darned thing up. I have batteries, just sick of dealing with these things. Staring at me. “Change me, Change me, you know you should”… 
crazy. we have a 1500sf house (1000sf downstairs and 480 upstairs in the finished cape cod attic), and we have two. one in the center hallway in the house, which is right next to HVAC and hot water heater), and one at the top of the stairs toward one end of the attic (conveniently not far from my GF). i can’t imaging we need more than that. the one in the hallway downstairs is 25 ft, thru a door and partway down the hall, from my stove and if i pull something smoking out of there (like when i’m pan searing steaks and i’m heating the pan up to 500 and finish off in the oven), with the vent off (i’m forgetful), it takes about 60 seconds and that alarm goes off (and the dog has a conniption fit when it does).
I stock up on 9v batteries in early September each year and change all the batteries on the equinox. Used to do it in both spring and fall, but testing shows that the backup batteries can last at least a year in our wired system. The only one I really hate is the one in our MBR – near the apex of the cathedral ceiling. That requires digging out the big ladder, man-handling it upstairs, negotiating a tight corner turn, etc. Having to do all that in the wee hours a couple of times made me much more diligent about the “equinox swap.”
i always have a pile of 9v batteries because of musical instruments. guitars, pedals, etc. all love to eat 9v batteries.
My home is 2150, almost 1/2 & 1/2 except an odd, narrow room leading to the water heater that also has two redundant doors to garage and back yard. 4 bed, 2 bath up. They combined 4 small rooms downstairs into two very nice large rooms, one is my living room and the other my workshop. Very spacious galley-style kitchen in-between with french doors that open to a ground-level deck and 1/2 acre 6’ fenced yard for my dog (was two previous dogS when I bought it), with a 1/2 bath, nice closet for all my bike gear, and stairs up. I live alone! One bedroom is my gun workshop and sewing room, another my electronics and servers (for when I was working), a spare for a guest, and my bedroom.
I love the layout but you can tell its age from the exterior design.
I recall a story of a guy who changed all the batteries put them in a plastic bag and tossed it onto a shelf. Almost burnt his house down when the batteries touched terminals. I have gotten more than 2 years before the chirp, so there’s enough charge left in a year old to do damage.
I probably shouldn’t complain, then. I have not needed to change the 9V in several of my detectors, and they were all the same when installed around 2015.
It always seems to be the same ones that need changing every 2-3 years.
My ex hasn’t needed to change any since I moved out in 2016. I asked her when she pulled down the CO one I mentioned above…
That was one reason I wanted to get Nest smoke alarms. You use the app or just wave at them to shut them up. Until I saw the price tag.
According to the Red Cross, you should replace the whole unit every 10 years. They install ones with a non-removable, non-wired 10 year battery.
I have been reluctant to replace all my wired ones with “disposables”, because I am convinced they will not have the same mounts.
That’s the current tech. No more needing to change batteries.
The sensor has a finite lifespan wired or not, so it’s good to replace them every 10 years.
For many people the sensors are t effective well before then though, dust buildup that’s not cleaned/vacuumed out makes it unlikely that smoke will trigger it. 
I believe this happens because the battery’s power output drops when the temperature goes down. Thus when your battery is right on the edge of being too old, that slight change in battery performance that comes in the middle of the night when your house cools off is likely to push it over the edge and cause the device to complain loudly that its battery is no longer performing adequately.
They almost certainly won’t. Nearly every time I replace one I have to replace the plastic bracket because even when you’re careful to buy the same brand they’ll have changed it. If we could get a mounting standard for these things it would go a long way to making it less annoying to replace them.
I dread this in my new house, because the ceilings are 10 feet upstairs and down, and one of them is at the very top of a vaulted ceiling 20 feet in the air. I don’t have a ladder tall enough to deal with it, and I’m still paying hospital bills from a ladder accident we had shortly after moving in.
In a prior rental, i heard a person talking in the middle of the day when i was home alone. We lived on the second floor, and i swear i heard someone in the attic. Called my husband home, called the landlord panicking, asking if there was any way someone could get into the attic or maybe they left something up there.
Her dad came to check it out and nothing. The next day, same thing, but couldn’t find anyone.
Husband thought i was hearing someone outside,
until it started in the middle of the night, a woman’s voice-much easier to discern what she was saying in the quiet of the night … landlord had installed talking smoke alarms without realizing, and it was installed on one of the high ceilings far above my reach.
It was scary and funny. I would’ve preferred the annoying beep.
I had no idea there were talking smoke alarms. I would have first thought my house was haunted before I thought of that, and I don’t believe in ghosts.
may i have your attention please… may i have your attention please… there is a fire emergency in the building. please evacuate.
I have ten Nest smoke/CO alarms. They have been great (to the extent that something which ideally is never used can be great). But they are starting to show low battery warnings, and since they were all installed around the same time, I’m going to have to replace them all.
They each require 6 Energizer Ultimate Lithium AA batteries. No others will do. Lithium primary batteries have a higher voltage than alkaline, so if you put regular ones in, it will immediately think they’re low. Nobody else makes lithium AAs (without resorting to sketchy AliExpress brands, and I prefer not to play games with a life-safety device). Due to various world events and presumably also corporate greed, the price of these has gone through the roof, to as much as $4 per battery. Even in suspicious bulk “my batteries came loose in a plastic baggie” packaging, it will still cost me over $150 to replace them.
