First Alert Smoke & CO: Battery level report very wrong...?! CHIRP! CHIRP!

So I had one of my First Alert Z-Wave ZCOMBO Smoke & CO Detectors go into the dreaded (though not as bad as fire! :fire:) “low battery chirping mode” this morning.

Exactly one of the situations I was hoping to avoid by using a “smart” device. :crying_cat_face:

According to the “Recently” tab, at 8:15am the battery was reported to be 77% (history shows a drop from 79% to 78% over the last 6 days … seems perfectly normal). In other words: *there was apparently sufficient battery and connectivity to a rather more than sufficient battery capacity at 8:15am& … I’m not sure what time the chirping started though!).

I took out the batteries and re-inserted. The device promptly reported battery level as 1%!

Of course, if the device were a little smarter, then it should offer an additional special “low battery alert” message that corresponds exactly with the start of chirping (or perhaps at a level that is typical of 24 hours before chirping?..), but absent that, I wonder what’s happening here.

This unit uses 2x1.5v AA batteries. Checking with a multimeter, the batteries have about 1v each. I understand that “low voltage conditions” are necessarily considered linear to “battery capacity”, so (1v+1v)/(1.5v+1.5v) = 66% normal voltage does not necessarily correspond to 66% battery capacity.

In other words, I could say the batteries are pretty close to the 77% reported, but I also could understand if the device thinks that 66% is too low and warrants chirping, but how can it report 77% one moment, and 1% after just inserting the batteries the next moment?

I’m going to put the same batteries in a second device and see if it also reports 1%; though I’m not sure that information will help contribute to this problem.

2016-11-30 11:39:22.097 AM PST
19 minutes ago	DEVICE		battery	1		Den Smoke CO Alarm battery is low!	true
2016-11-30 10:48:33.134 AM PST
1 hour ago	DEVICE		alarmState	clear		Den Smoke CO Alarm test cleared	true
2016-11-30 10:48:33.134 AM PST
1 hour ago	DEVICE		carbonMonoxide	clear		Den Smoke CO Alarm carbon monoxide is clear	true
2016-11-30 10:48:33.134 AM PST
1 hour ago	DEVICE		smoke	clear		Den Smoke CO Alarm test cleared	true
2016-11-30 10:48:32.226 AM PST
1 hour ago	DEVICE		alarmState	tested		Den Smoke CO Alarm was tested	true
2016-11-30 10:48:32.226 AM PST
1 hour ago	DEVICE		carbonMonoxide	tested		Den Smoke CO Alarm was tested	true
2016-11-30 10:48:32.226 AM PST
1 hour ago	DEVICE		smoke	tested		Den Smoke CO Alarm was tested	true
2016-11-30 10:48:26.854 AM PST
1 hour ago	DEVICE					Den Smoke CO Alarm code 13 is 0	true
2016-11-30 8:15:40.710 AM PST
4 hours ago	DEVICE		battery	77		Den Smoke CO Alarm battery is 77%	true
2016-11-30 7:25:17.971 AM PST
5 hours ago	DEVICE		battery	78		Den Smoke CO Alarm battery is 78%	true
2016-11-28 6:55:55.265 AM PST
2 days ago	DEVICE		battery	78		Den Smoke CO Alarm battery is 78%	true
2016-11-26 4:34:11.408 PM PST
4 days ago	DEVICE		battery	79		Den Smoke CO Alarm battery is 79%	true
2016-11-24 4:12:28.679 PM PST
6 days ago	DEVICE		battery	79		Den Smoke CO Alarm battery is 79%	true

Funny you posted this because mine has been stuck on 100% for months. I just now tried to reset it and then the low battery notification went off showing an update at 100%. Not sure whats going on with these.


Yah… that’s rather annoying. Of course, as long as actual Smoke & CO alerts get transmitted, that’s more than half the value.

But avoiding the chirp and proactively replacing batteries would sure be a nice benefit.

There are other brands to consider, but First Alert seems to be good quality in general (and a great bargain compared to Nest or whatnot!). An “alarm detector” wouldn’t have the battery reports anyway.


Be advised that current CO detectors have a five year end of life timer that forces you to replace them. This is from the date of manufacture and a replace by date is usually printed on the back. It wouldn’t surprise me if the warning was similar to a low battery chirp. Not saying that is your problem but it is something to consider.

My Insteon Smoke Bridge is supposed to alert to low battery as well, never has, not once, with 4 OneLink detectors. I think the devices just stink at reading the level.


Same issue here. Super annoying as they seem to hit the chirp warning level at 2am. I am going to proactively replace at 77% as 76% seems to be about where it starts to chirp. Another interesting thing, I hit the test button the detector on one that was around 77% and sure enough it jumped down to 1% and started chirping!


My First Alert Z-Wave smoke detectors start chirping at 77% – this has happened twice now. The device handler should send a notification when the battery level is at 78%. That would provide about one week of leeway time to change the battery before being woken up in the middle of the night. I am glad that SmartThings is open and provides an opportunity to fix the battery reporting problem.

The challenge? … Even though the battery level is shown in percent %, the actual resolution of the battery voltage meter in the smoke detector may be much lower.

It might, for example, show 88% for weeks and then drop directly to 77%.

Keep in mind when testing batteries with say a dumb tester (cheap) or even a DVM that the reading will be higher then what a device will report as there is no load on the battery when testing with a dumb tester (cheap) or even a DVM.

Although with duty-cycled devices that are designed for low power operation, the voltage under test shouldn’t be too far away from open circuit voltage, and irregardless, the behavior with my alarms is repeatable, and that is why I posted to provide two data points. Both of my incidents involved First Alert Z-Wave alarms manufactured in Q’3 2015; one detects smoke, the other also detects CO. Both started chirping immediately after reporting a battery level that showed up in my SmartThings logs as 77%. Both of the previous battery levels were 78%, and had been at 78% for at least one week. So the resolution on the First Alert alarms is 1% decrements. Both were running on the original Duracell AA alkaline batteries included in the package. My multi-meter read both batteries at just above 1.3 Volts; 77% of 1.5V would have been 1.15V under load. I think the behavior for new alkaline batteries is predictable with a nice linear slope. I plan to change my device handler for the First Alert alarms to reinterpret the battery status message range so SmartThing’s Device Health will warn me with enough time to avoid the midnight chirping incidents.
I certainly don’t expect known, predictable behavior with other battery chemistries or unbalanced alkaline pairs, but I am okay with the extra cost of using new alkaline batteries once a year in smoke and CO detectors.
YMMV, but this is an easy problem to address with a couple lines of code.
I haven’t looked into finding or creating a tool that saves and analyzes logs to automatically generate corrective battery status indicator curves. I do use rechargeable batteries in other sensors that are non-essential, and in those I observe consistent and reliable underreported values, but I take care to match and group rechargeable batteries by writing on them. Thanks.

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The source code for the Z-Wave Smoke Alarm (including First Alert Combo) is available if anyone wants to modify it.

SmartThings will even often accept and merge changes into the official DTH, via a “Pull Request”, if they are minimal and well tested.


Is there an app out there that will alert when the battery reaches 78%? Most of my detectors update battery levels correctly but of course ST won’t alert me of a low battery condition until well after they hit 77% and start chirping at 3am.

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I just hit this issue a few nights ago at 4 am - I was not amused. I was hoping for much more than that from ZCombos, but unfortunately they seem pretty rudimentary in functionality…

That said, better to light the candle than to curse the dark, so I last night I threw together a quicky SmartApp to monitor and alert when battery goes too low. It can be found here:

(you can copy the code or install directly from GitHub)

The app allows basic monitoring one or more ZCombo detectors (as well as other battery powered devices). It was a quick hack, with basic testing (I have not tested SMS functionality), so please report any issues you may encounter. Enjoy.

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But what battery percent level do you select for a warning?

These detectors can run for months at ~70% or above, but unfortunately do not send a “last gasp” Battery Level value report at the next level down (whether it be 69% or 44%…).

It is a configurable parameter in the SmartApp, but I set the default to 77% - which is where I started hearing the chirps - and from what I read, it seems to be the common place. It may be wise to raise it higher - esp if you use rechargables.

Worth noting that the threshold is set to be “less than or equal” rather than “less than” - so the setting of 77 means it will start alerting as soon as it hits 77% rather than when it hits 76%

Right… but mine sit at 77% for many days (or weeks) before chirping, therefore I would be changing (and wasting) batteries much more frequently than actually necessary.

Rechargeables are not recommended for smoke detectors because their power drops off so rapidly that it may not even have any juice for the “chirping” stage, and thus be dead without any warning (no SmartThings warning, nor chirp warning).

While I guess getting alerted at 77% is better than nothing, I fear it isn’t very helpful, since I certainly don’t want to change batteries far too early.

If only these would send a report just above the level that triggers a chirp; I think it may be around 45%.

Just frustrating. Hmph.

Well, since as far as I know we have no way to change how firmware senses the battery level, not much we can do about it. For what it is worth, in my case it started chirping right after it hit 77%. But even if it takes a while between it hits 77% and the chirping - in the end, it is just an alert - it is up to you to actually change the battery. If in your case it will take a while, you can take your time changing it, but at least you’ll be ready. Personally I’d rather change batteries a little early, than have it chirp at 4 am.

I would be curious as to what the curve signature of rechargeables is - I bet you can probably detect them dying and alert ahead of time. While conventional wisdom about them not being safe may be right, it does not take into account being alerted when, or even before, they die - and even if you have to charge them twice as often, with smart notification reminders, that may be worth it.

In any case, my next purchase will be hardwired Nest Protect’s instead of these :frowning:

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As a bit of explanation, the 1% you see is generated by a special Z-Wave message. If the device sends us a battery value of 0-100% we just pass that along as given, but the reserved value 0xFF (255) is just “low battery” in the Z-Wave standard. We made a choice to translate this value to 1% so that it’s clear that the device itself thinks its battery needs to be changed. So it’s not so much that the actual voltage is dropping from 77% to 1%, it’s just that it goes below whatever the mfr.'s hardcoded threshold is for “low battery”.


Helpful answer, Steven… thank-you!

Is there someone on your side who could research why First Alert doesn’t send the “low battery 0xFF” message some time before entering “chirp mode”?

In fact, if I recall correctly, First Alert never sends the “0xFF” message. I presume it prioritizes saving the battery for chirping, but … gee, couldn’t they update their firmware to fire off a single Z-Wave message?

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Thanks for the explanation. I was always curious why they went from 80% to 1%