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?

  • "Lithium-Ion rechargeable 9V batteries are now readily available". If, by this, you mean the kind which has a little micro USB connector directly built-into the battery itself for charging, you should definitely not use those. They are essentially just a little USB power bank, just with a different output voltage. Just like power bank, the electronics inside will not perform very well at very low currents (i.e smoke alarm), and can fail, too. – JonasCz Jul 18 '18 at 6:06
  • @JonasCz That's good information. I was actually referring to the type that use a separate charger, but good information nonetheless. – RockPaperLizard Jul 18 '18 at 6:18
  • Have you researched lithium primaries? Long life at low drain sounds like a perfect use case. – Agent_L Jul 18 '18 at 11:51
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    Read NEC 110.3b. The instructions and labeling on the smoke detector must be followed. It is mandatory. You must use the type of battery specified in the instructions, and that is the end of the conversation. If you are not 100% clear on this, talk to your insurance company's claims department and your mortgage lender. – Harper Jul 18 '18 at 15:19

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.

  • +1 for "When you remove the "old" battery, that battery is still fine to use in other gadgets." – RockPaperLizard Jul 18 '18 at 4:13
  • This correlates to what I've read in reviews about "10 year" batteries. Some people complain that they won't work when taken out of the package. Perhaps that's because the voltage has already dropped a little by the time they use them. Also, there's a lot of misconception that those batteries are supposed to last 10 years once installed... from what I've learned, that's not the case... they have a shelf life of 10 years. – RockPaperLizard Jul 18 '18 at 4:16
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    There are 9V rechargeables which have high voltage for when a "fresh" battery is assumed, for example by using 7 cells instead of 6 for NiMH chemistry. An example is Powerex MHR9VI Imedion 9.6V. Though I think it's a dubious proposition to use rechargeable batteries in a low-drain application (even IF it works fine, is it really worth using rechargeable batteries if you're only going to recharge them once a year?) I just want to mention they exist. – Blake Walsh Jul 18 '18 at 10:41
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    Unless the device is designed to use rechargables I think the bigger issue is that the old battery check won't work right. Alkaline batteries drop at an approximately linear rate from 1.5v to 1v as they are drained. NiMH (and IIRC NiCd) batteries deliver a steady 1.2v until almost completely dead. – Dan Neely Jul 18 '18 at 14:20
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    NiMH and other rechargeable batteries often have a high self-discharge rate, in some cases up to 1% of their charge per day. An unconnected battery can become fully discharged in a few months. A smoke detector is a low current load application so that the self discharge characteristics will often dominate and limit their useful time between requiring a recharge. – uɐɪ Jul 18 '18 at 15:06

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:

enter image description here

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:

enter image description here

Note- not a specific brand/model/product recommendation

  • Thanks Jimmy. I'll have to see if I can find the manuals online somewhere for the hardwired units. I wonder why some don't want you to use rechargeables. Do you know? – RockPaperLizard Jul 18 '18 at 2:24
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    I would not be at all surprised if they don't want you to use rechargeables simply because some people would, incorrectly, assume that the smoke detector will actually do the recharging and think they never need to change (or recharge) the batteries. Of course for an extra $ they could build a recharging circuit in (like in a cordless phone or many other devices) and solve all the problems. But $1 for the circuit and $1 to include a rechargeable battery and now they are $2 more than the competition... – manassehkatz Jul 18 '18 at 2:28
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    I agree with @manassehkatz as for the reasoning against rechargeables but will add that anecdotal info I got from searching around seems to indicate that normal alkaline batteries have a slow tapering voltage drop curve, whereas NiMH and NiCd batteries have a rather steep drop, meaning that they go from functional to non-functional quickly, at end of life. – Jimmy Fix-it Jul 18 '18 at 3:09
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    There is also the factor that most rechargeables have excessive self-discharge for a low drain application such as a smoke detector -- it's just not worthwhile given how much life a modern alkaline gives in such an application. – ThreePhaseEel Jul 18 '18 at 3:52
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    @RockPaperLizard Because it hasn't been tested for that. This isn't China. First the manufacturer must do an engineering cycle or two with company testing. Then they must write up labeling and instructions and send it to Underwriter's Laboratories or similar lab, where it must survive third party testing. Why would they pay for all that just for your corner case, when their preliminary testing certainly showed what ThreePhaseEel is saying: it's a mistake and a waste of time. When the battery self-discharges in 2 months and starts beeping, who'll the consumer blame? – Harper Jul 18 '18 at 15:39

I would not recommend it for two reasons:

  • Cost

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.

  • Capacity

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.

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    The capacity issue needs to be explored further. Thanks for bringing it up. Regarding cost, I find that in hardwired smoke detectors, typical alkaline 9-volt batteries (Duracell, etc.) drain in about a year. That's why I'm considering switching everything to rechargeables. The initial cost is 3 times that of non-rechargeables, but they should last much longer than 3 years (with recharging). – RockPaperLizard Jul 18 '18 at 2:21
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    @RockPaperLizard 3 times cost difference doesn't sound right, are you comparing the most expensive alkalines with dirt-cheap rechargeables? If you compare products of similar quality (duracel vs duracell), you should get about 5-10x and at 5-10 years, aging of rechargeables becomes an issue. – Agent_L Jul 18 '18 at 11:50

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.

  • Two decades ago I would have agreed, but since 2005 rechargable batteries can be used for years. I use those almost everywhere, in clocks, remote controls, flashlights etc. – Martin Jul 20 '18 at 18:00
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    @Martin, even if there are specific rechargeables that meet all of the requirements and could work, think of the confusion and errors it would cause trying to advise the general public about the exceptions that are OK. Most people don't read instructions carefully. The guidelines need to be simple and idiotproof. And the type of batteries you linked to wouldn't be useful for a smoke detector. "Long life" in this case just means it doesn't suck as bad as a regular NiMH for self-discharge, but it has even lower capacity, and the voltages aren't high enough when charged, let alone at 70-85%. – fixer1234 Jul 24 '18 at 21:17
  • @fixer1234 I agree that noone should do safety-related work without sufficient knowledge. I just wanted to point out that using rechargable batteries seems to be possible. I don’t think that it’s risky to try them. Following the instructions you surely test the smoke detector to confirm the battery voltage is sufficient when charged. When it discharges, the detector makes noise to remind you to replace (recharge) the battery. This may or may not occur after a shorter period of time than with alkaline batteries. – Martin Jul 27 '18 at 7:34
  • @fixer1234 The capacity of current NiMH batteries is similar to alkaline. As for voltage, the 1.5 V of an alkaline cell is just the initial voltage which drops relatively linear when discharging. NiMH on the other hand stays relatively constant at about 1.2 V. This means after some amount of discharge the alkaline cell voltage actually drops below the NiMH cell voltage. Generally speaking, it depends on the device wether alkaline or NiMH last longer. – Martin Jul 27 '18 at 7:34
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    @Martin, see the explanation about smoke detectors and voltage in my answer. Smoke detectors are kind of unique in that they need close to the full voltage of an alkaline battery. At the drain of a smoke detector, an alkaline might operate for years before the voltage dropped to the NiMH level, but they need to be replaced before then. In this device, the total battery capacity isn't all that relevant. When the battery is too low for a smoke detector, you can put it in another device and get almost the whole life out of it in the second device. – fixer1234 Jul 27 '18 at 7:53

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