"Shaun Eli" wrote in message
...
Yeah, but with no internal clock how do they know to always start
chirping at 3 AM?

It just seems that way because that's usually the coldest time of the day.
While the battery may have enough "oomph" to keep from chirping during the
day, when it gets "cold soaked" at night the voltage drops enough to let the
chirping begin. That's why I try to replace detector batteries when DST
begins because that's the beginning of winter weather and smoke detector
"chirping" season. I can get up to three years on a single 9V battery, but
I usually replace them after a year and move the batteries to other devices
like remote controls, meters, etc. that wouldn't be life threatening if the
battery suddenly dies.

I even keep those batteries if they show more than 7 volts because I snap
all the almost dead ones together to make dog trainers. (-: I connect a
piece of zip cord to 6 or 7 batteries and leave it where the dogs can chew
on the wire. Once they get a mouth zap of 50 volts or so from chewing on
the "trainer" they rarely show any interest in any other wires afterwards.
There's never enough current flow to do permanent harm, but enough to make
sure that they don't go chewing on wires again. It worked on all the dogs
we've rescued except for one that liked to chew on AAA batteries, even with
the wire trainer experience under her belt.

--
Bobby G.