Smoke Alarm Batteries

About smoke alarm batteries

Rechargeables okay for smoke detectors?

I've been using rechargeables for smoke alarms successfully for years, and find it hard to believe that doing so really poses a real problem, but to be clear, the manufacturers absolutely recommend against doing so:

• First Alert says, "Never use rechargeable batteries because they may not always provide a consistent charge."  (That seems extremely ambiguous.  What exactly do they mean by a "consistent charge"?  Consistent voltage?  Because voltage isn't "consistent" in non-rechargeable batteries, either: it drops as the battery is drained.)
• Kidde recommends carbon-zinc, alkaline, and lithium, but doesn't specify whether rechargeable lithium is okay, and says nothing at all about LSD-NiMH.  
• Rayovac says, "Never use a rechargeable battery in a smoke detector as brands vary greatly on how fast they lose their charge (some lose it in just one month) and are not designed for usage in smoke detectors."  (That's silly.  It's like saying you should never drink liquids because some liquids are poison.  Sure, regular NiMH's are high self-discharge, but almost all NiMH's are the low self-discharge flavor; it's actually hard to find the high self-discharge flavor these days.)
• Wikipedia pooh-poohs NiMHs for smoke alarms, saying that when NiMH batteries die their voltage drops so far and so fast that you might miss hearing the low-battery warning chirp before the chirp dies completely from insufficient power.  However, (a) that statement is uncited, (b) my 9V rechargeables definitely chirp when they get low, and (c) one person tested the low-battery warning with NiMH AA's and found that it chirped at least three weeks (after three weeks he stopped his test).  This easily surpasses the one-week standard listed in smoke alarm manuals for how long the chirp should last.
• Others suggest that the charge might be too low to power the siren, but high enough that you don't get the low battery warning, though I'm skeptical whether that could really be the case.  I'll test that if I can ever find the time.

If you do try rechargeables (at your own risk) and you're using the 9V size, see my article on 9V batteries for your options.  FWIW, Li-Ion has greater capacity but a lower initial voltage (7.4V) than NiMH LSD (typically 8.4V).  In my own use, Li-Ion seem to last longer than NiMH LSD, but I haven't kept any records.  Smoke alarms reportedly start chirping around 7.6V (see again my 9V batteries page), but 7.4V Li-Ion seem to work anyway, probably because 7.4V is the nominal voltage, and actual voltage is a little higher for maybe 2/3 of a charge cycle.

For the AA size, rechargeable lithiums aren't available, so your option would be NiMH.   If you go with NiMH, then of course you'd use the LSD (low self-discharge) variety, since normal NiMH's will completely self-discharge after just a few months.

Either way, go with brand-name batteries, since you don't want to trust your family's safety to some off-brand.  See my page on the best NiMH batteries.

Whether to change your batteries twice a year or not

Some smoke alarm makers tell you to change your smoke alarm batteries twice a year, but I feel that's just a waste of batteries.  Here's why:

• There is not much point in throwing away a good battery.  The smoke alarm manufacturers certainly haven't explained any benefit to throwing away a battery before it's used up.
• Alkaline batteries last around five years in most smoke alarms, and even the "heavy duty" batteries (which are actually weaker than alkalines) typically last more than a year.  (Details below.)  One maker says, "A fresh battery should last for one year under normal operating conditions." (Kidde i9030 manual)
• The alarms in modern homes are "hardwired" (connected to AC wiring), and the battery is just a backup in case the AC power fails.  So batteries in those units will likely last far longer than battery-only units.
• You can consider using rechargeable 9V batteries (like I do), though know that the alarm manufacturers recommend against it.  (See the top of the page.)

How long do smoke alarm batteries last?

Alkalines typically last around five years in battery-only ionizing alarms.  One hero ran a long-term test and found that three alkalines all lasted 5 years plus 1-3 months. (source)  One alkaline did worse, the Rayovac at only 13 months.  The five-year runtime matches what's predicted by the math: 9V x 550mAh (source) = 4950 mwH, ÷ 0.1 mW (source, p. 9) ÷ 24h/d ÷ 365d/y = 5.7 years.

The "heavy duty" battery lasted 1.6 years in the test.  "Heavy duty" is a misnomer, those batteries are actually weaker than alkalines.  Even so, the heavy duty battery lasted more than a year and a half, making the "replace your batteries every six months" advice seem questionable.

Photoelectric alarms use twice as much energy for monitoring as ionizing alarms. (0.2mW vs 0.1mW, p. 9)  So expect batteries in that flavor of alarm to last half as long in battery-only models.  I'm guessing this isn't an issue with alarms hardwired to AC electricity.

9V vs. AA

Smoke detectors used 9V batteries for decades, but starting around 2014 the trend has been to use two or three AA's instead.  I have a separate page about 9V batteries.  I'm guessing that newer alarms are more efficient and don't need as much voltage.  I've written to the alarm manufacturers for a definitive answer but they never replied.  By the way, three AA's have about three times as much energy as a single 9V, so perhaps part of the idea is that the batteries might not have to be replaced as often.

Lithium: pros and cons

• For some inexplicable reason, some alarm manufacturers say not to use lithium batteries in certain alarms.  (I inquired to Kidde about this, but they sent a B.S. reply that didn't answer my question.)  So, check the manual for your particular alarm, and if there's a prohibition against lithiums, then either don't use them, or use them at your own risk.
• Consumer Reports says not to use lithiums, because supposedly when they get low, they get very low very quickly, and won't give the "low battery" chirp for long enough.  I don't believe it.  As per below, the low-voltage threshold seems to usually be around 7.6V, and even with a cutoff of 7.0V, a lithium would still have plenty of juice left for the chirp, as you can see by looking at a discharge curve. (Battery University; double the voltage to get 7.4V since 9V lithiums use two 3.7-4.2V cells)  CR actually publishes some bad information fairly regularly; they're not a gold-standard type of source.  Also, whenever I write to them to ask for their sources, or to show their work (the math calculations) for spurious conclusions, they never respond.  Finally, lithiums are mandated in certain jurisdictions, and manufacturers sell some models with lithiums preinstalled.  That makes CR's advice especially suspect.  However, I would be wary about using lithiums in a smoke alarm if the cutoff voltage is abnormally low, like one model I found by Allegro which says its cutoff is only 6V.
• Not all lithiums are the same.  The special "10-year" lithium 9v batteries made for smoke alarms often don't last a full ten years (I'd hope for 7), but regular 9v lithium batteries have a shelf life of only about 3 years.  That's less than the typical five years for alkalines in a battery-only ionizing alarm. (See section above.)  So, if you have your heart set on lithium, get a ten-year brand, not a normal lithium.
• Some areas are mandating 10-year lithium batteries for smoke alarms.  For example, Oregon, Louisiana, Michigan, and Madison, WI, (source) as well as for battery-only units in NYC apartments.(source) If you're in an area that requires 10-year lithiums, then at least you don't have to ponder what kind of battery to use; the choice has been made for you.
• "Ten-year" lithiums (e.g., Ultralife) often don't last a full ten years, but they usually run for at least a few years, outlasting alkalines quite easily.  And you can always take advantage of the ten-year warranty if yours die early.  A 1998-2001 study of homes that started with 10-year lithiums showed that 8-10 years later, only 38% of homes still had at least one functioning alarm, indicating that the residents removed the alarm or batteries because the "10-year" lithiums failed early.  Indeed, several of the alarms were found to have non-lithium batteries. (NFPA)   That's my experience as well:  years ago I bought several first-generation Ultralife "10-year" lithium batteries for smoke alarms, but I think the best one lasted only seven years, the rest even fewer.  Around 2004 the next-generation Ultralifes came out which are supposedly better (Ultralife), but you know, fool me once....  Also, the reviews for the newer Ultralifes show that many customers are getting far less than ten years of runtime.
  
• Avoid alarms that come with a sealed "10-year" battery.  The battery will probably die sooner than 10 years, and since it can't be replaced, you'll be faced with the cost of replacing your alarm early, too.  I prefer alarms with a replaceable battery.
• Capacity of 9V lithiums.  Ultralife says that their 9V is 1200mAh, and that no competitor's lithium is more than 800mAh.

Smoke alarms themselves

Photoelectric vs. Ionizing vs. Dual-Sensor vs. One of Each

1. Use photoelectric alarms in/near kitchens & bathrooms, because ionizing detectors give annoying false alarms from cooking and steam.
2. Everywhere else, use dual-sensing alarms (photoelectric/ionizing combos), because that kind can sense both kinds of fires well (flaming & smoldering).  I like the First Alert 3120B ($32), because Consumer Reports gave it the highest rating, and tested it as "Excellent" for both kinds of fires, and because the customer reviews on Amazon are overwhelmingly positive.  The Kidde PI2010 was rated just as high by CR but 20% of the reviews on Amazon are 1-star, mostly because of frequent false alarms.

1. There are two kinds of detection technologies: photoelectric (PE) and ionizing (ION).
2. PE models are better at detecting smoldering fires (slow & smoky), ION models are better at detecting fast, flaming fires.
3. If you have only PE or ION, the alarms might not go off (or not go off in time to save everyone) if you experience the "wrong" kind of fire.
4. The solution is dual-sensor alarms which use both PE and ION technology, so they detect both kinds of fires well. (NIST 2008 p. 234, 2009 Table 2; Consumer Reports, 2016)
5. Multi-criteria alarms are the next generation, better than either photoelectric and ionizing, but as I write this they're not widely available yet.

Installing one of each kind of alarm is impractical.  Let's say you're paranoid and want to install both PE's and ION's instead of dual-sensor models.  If your alarms are hard-wired into your house's electrical system, then you've got the hassle or expense of installing work boxes to put the second alarms into.  And if your home is pre-1978, then there's probably asbestos in the drywall joint compound which would be released when cutting into the ceiling.  If instead you install battery-operated alarms, you'll find that the new PE alarms you're putting in (because your existing alarms are almost certainly ION) are battery hungry, and the "10-year" batteries will probably last only 1.5 to 2 years in them.  If you go this route, then for PE alarms, you might as well use alkaline batteries, since they'll last about a year, saving the added expense of the lithiums, or consider using rechargeable batteries, even though the manufacturers advise against it.  (See the warning above.)  (source for battery longevity)

Interestingly, while flaming fires are far more common than smoldering fires, smoldering fires account for the greatest number of deaths.  That could be due to the fact that most homes use ION alarms which aren't as good at detecting fires as PE alarms. (NIST 2008, p. 66)

Disposal of alarms with radioactive materials.  ION-type alarms have radioactive materials, so some state and local codes prohibit throwing them in the trash, instead requiring them be brought to the municipal hazmat facility.  For older alarms with >1 µCi of Americum-241, federal regulations kick in.  For old alarms with more than 5 µCi, they must be returned to the manufacturer for disposal, by ground mail, not airmail, with special packaging. (FEMA, p. 5) 

PE alarms use twice as much energy as ION alarms for monitoring. (Australian Govt., p. 9)  That means that in battery-only alarms, batteries will last half as long in PE as in ION.  In hardwired alarms, that means about an extra 13¢/year, sucks to be you.

Replace smoke alarms every 10 years or not?

Standard advice is to replace smoke alarms every ten years, because they supposedly get less sensitive as they age or completely fail over time, even if they respond positively to the test button (which tests the electronics but not how well they sense smoke).  I've been skeptical and wondered whether this was just a ploy for manufacturers to sell more alarms.  Searching for evidence to back up the 10-year replacement advice, I wasn't surprised to find that the recommendation is based on old studies from the 1970s and early 80s, forty-odd years ago, when smoke alarms were less advanced. (CPSC)  And even according to the ancient data, only 27% alarms actually failed after ten years.  So for me, when my alarms hit the ten-year mark, I plan to test them yearly with a can of aerosol smoke.

Cutoff voltage for 9V alarms is 6.0-7.6V.

1. In technical specs I found for an alarm made by Allegro, the low-voltage chirp starts at 6.0V.
2. A 2002 government report says the low-voltage threshold is 7.6V, but notes that it varies by manufacturer. (source, PDF)  Nevertheless, I've had success with Li-Ion batteries which have a nominal voltage of 7.4V, probably because a fully-charged unit starts at a little above 8.0V and stays above 7.6 for maybe half its charge cycle.
3. I couldn't easily find cutoff voltages for other brands. If you can find any published specs, I'm all ears!

Smoke alarm tips

1. Figuring out which alarm set off a bunch of interconnected alarms.  First off, if you're just trying to shut them up, pressing the hush button on any alarm is supposed to silence them all.  If you're getting lots of false positives and you want to replace just that alarm (it's happened to me), the alarm that triggered all the others is supposed to flash red every few seconds.  Another option are "smart" alarms, which interface with a mobile app: you give each one a name (e.g., "Living Room"), and then the app tells you which one was the culprit.  The downside is that they're pricey, often around $100, and customer reviews on the first generation are underwhelming.  An in-between option are alarms with an LCD panel which tell you the status.
2. If replacing the battery in a wired detector doesn't stop the chirping, then follow these steps to reset the low-battery warning.
3. Consumer Reports says to position them at least 4" from walls, and well away from ceiling fans and heating/cooling registers.