"HeyBub" wrote in message

Someday there will be a mains-powered smoke detector with a battery that
gets charged during normal operation, much like the button battery in a
computer.

Most PC's I've seen use a non-rechargeable lithium coin cell, usually a
CR2032. I haven't seen a rechargeable unit on a motherboard since my old
AMI 486-50. Laptops are a crapshoot. Some use rechargeables, some don't.
Not sure why.

I suspect part of the reason that alarms don't use rechargeables is that the
voltage drop at the end for NiCads/NiMH is far quicker than alkaline or
lithium cells. They might not "low power beep" long enough to meet safety
standards as in if you've away for the weekend, the low beep could have come
and gone in that time. I have seen rechargeable lithium cells in some
applications, but they do have a tendency to explode if overcharged. I
could see how a smoke/fire detector wouldn't want a recall risk for
explosive detectors.

Save the batteries you remove in a specially-marked container. Use these
used batteries in non-critical applications (radio, toys*, etc.).
Batteries
in smoke detectors draw a miniscule amount of current and smoke detector
batteries typically approach their shelf-life even when installed
(shelf-life is measured in years).

That's good advice. If you rotate them twice yearly, they'll have more than
enough power for a meter, radio, etc. Even once yearly on detectors that
don't use much current leaves you with a battery close to nine volts. We
have about 10 detectors and when our pup was young and chewed wires I
snapped about 90V worth batteries of them together connected to a short
length of zip cord hanging down next to her crate with a very low rated
glass fuse attached to one lead. One chew through and a loud yelp and she
never came near another wire.

There's 90VDC but very little current. More than enough to cure the dog's
wire chewing habit. It may sound cruel to some, but the choice was to have
her eventually chew through a 110VAC wire that could deliver 20A. By the
time I had to build the trainer, she had already developed a taste for CAT5
network cabling which was good for a half day lost trying to figure out the
failure messages.

I put a sticker on my alarm batteries with the date of installation.

Good idea, too. I've taken to using lithium batteries on the devices that
are really hard to reach as well as the outdoor temperature/humidity
transponder. I keep checking them for replacement but it's been two years
and they still read 1.7 volts for the AA cells and well over 9 volts for the
rectangular batteries.. That's much better performance than alkalines but
at a pretty high premium. Still, if it means not climbing a ladder as
often, I'm all for it. (Except, of course, that I ended up climbing up
anyway to measure them so I can get a handle on how long they'll last when
it *really* gets hard to go up and down a ladder.)

--
Bobby G.