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Lithium-Ion rechargeable 9V batteries are now readily available.
Are they appropriate for use with hardwired smoke detectors, which typically use 9V batteries as battery backups?
What about for use as primary power in battery-powered smoke detectors?
I was advised by one of the big smoke detector manufacturers that the real issue is voltage. Most battery-operated items will still run, more or less, on a much lower voltage than the nominal voltage on the battery label. That means that the battery can seriously run down and the gadget will still do something, even if it doesn't perform like with a fresh battery.
Smoke detectors aren't like that. They need the voltage of an almost new battery to operate properly. That's why they tell you to replace the battery at least once a year, even if a battery checker shows that it still has a lot of life. When you remove the "old" battery, that battery is still fine to use in other gadgets.
The battery doesn't sit idle in the detector if there isn't smoke. The detector constantly runs self checks, which use a little current. If you routinely test the alarm (as recommended), that also uses some current. So a year sitting in a smoke detector isn't like a year sitting on the shelf. The battery runs down a little and the voltage drops.
I've got to assume that the unit will still work if there's a fire and it hasn't produced a low battery warning. Fresh batteries typically don't produce a low battery warning in only a year, so there may be some PR compromise involved.
It's a pain in the butt to deal with a low battery alarm, especially if interconnected units all over the house decide to alert you in the middle of the night because one of the batteries dropped too low. So the one year time frame may be so that you can calmly replace the batteries at your convenience and avoid the "emergency" (and not hate the manufacturer). But what the unit considers a critically low voltage is still much higher than most other devices.
Which brings us to rechargeables. They typically have a fully charged voltage that is a little lower than the voltage of a fresh alkaline battery, or quickly drop below that level. The per-charge run time is also much shorter than the run time of a disposable alkaline battery. And as @ʎəʞo uɐɪ points out in a comment, rechargeables tend to have fast self-discharge rates and not hold their charge for a long time relative to smoke detector needs.
These characteristics aren't a problem in most gadgets. But it means that in a smoke detector, it may never be at the needed voltage even when fully charged, or will be above that level for a very short time. That's the main reason they aren't suitable.
You should use only the batteries recommended by the manufacturer of the detector, and all of the hard-wire w/battery backup models I have experience with expressly forbid the use of rechargeable batteries:
For "battery only" models, you might consider the sealed disposable "10 year compliant" models (some jurisdictions require these if you install "battery only" detectors) which include a guaranteed 10 year life lithium battery:
Note- not a specific brand/model/product recommendation
I would not recommend it for two reasons:
A disposable alkaline battery will almost certainly cost less than a rechargeable battery of any type. Since a smoke detector is, by its nature, a set-it-and-forget-it item, you won't want to bother taking out the battery to recharge it. So why bother to spend more if you aren't going to recharge it? Plus, for safety's sake you would actually need two rechargeable batteries if you actually did recharge it, so that you could use the "spare" while recharging the first one.
Look at 9V Battery specifications. The total power varies considerably by chemistry. An ordinary alkaline battery has more total capacity than most of the rechargeable batteries and only a little less than Lithium-Ion. In addition, the nominal voltage of an alkaline battery is 9 volts, where most rechargeable batteries provide less than 9V.
What would make sense - and there may be some out there that do this - is to have a hardwired smoke detector with a built-in rechargeable backup battery. But my hunch is that most of them are not designed that way because most hardwired smoke detector installations have relatively few power interruptions. In fact, many may have no power interruptions in their lifetime except when there is an actual fire, so no drain (or extremely minimal) on the battery of a hardwired smoke detector the vast majority of the time, so that a regular alkaline battery will last many years.
In my experience, there is no rechargeable battery that will hold a charge even completely unloaded for a year or more. Much less with a constant trickle load like you have in a smoke alarm. Even if it would work, you'd be looking at replacing (swapping out for recharging) the batteries every month or so rather than "once a year", with the pleasant chirping that comes with a failing smoke alarm battery every time.
Aside from that, as Harper commented, it's a code violation, so no, just no.
