A 10-Year Battery for AirTag

That is not how batteries work. Batteries drain even under minimal usage.

My bet is that in 2/3 years this device will stop working already.

Just change the batteries if you AirTag once a year. Especially if you are using an AirTag to keep watch over 10.000 dollar equipment.

Lithium batteries (like the 1.5V energizer suggested) have extremely low self discharge. The reason why lithium batteries are used for long life is because you can actually reliably count on them to not self discharge after 20 years. As long as the mAh is 10x the coin cells they use, I’d totally believe the 10 year statement.

Here [0] is the datasheet for the batteries used. 25 year shelf life is spec'd.

I have personally run these cells buried underground, and gotten 4.5 years out of 4 of them, though my application is just for fun and likely not as power conservative as an AirTag.

[0] https://data.energizer.com/pdfs/l91.pdf

Lots of smoke alarms now have 10 year batteries.

Now mandated in California.

No more waking up in the middle of the night and crawling up a ladder to find out which of your smoke alarms decided it was too cold and decided to sing its low battery chirp.

Okay, now I'm really curious what application you have for a device buried underground.

Also in New York. Personally I'm all for this even though I think it does make detectors a bit more expensive. Not only because it avoids the need to change batteries, but because it's more likely that smoke detectors actually get replaced at their end of life.

Ooh what's the application underground?

Does that fix it or make it worse? If you have 10 smoke detectors and they end up randomly staggered so this happens once a year, and you can't replace the batteries anymore... that seems worse than the old system which completely eliminated this issue if you just replaced them all every year or two on a schedule.

Remember that you are supposed to replace the entire thing because the other components like the sensor or simply capacitors also age. It is a very cheap safety device and simply not worth taking any risks by stretching it to say 15 years instead. The proper way would be to replace them while they were all still fine by making a note in the calendar.

There are two cases:

Your products are faulty and at least one has not made their intended 10 year lifespan. I'd change them all for better ones.

Or

They have reached their lifespan and you only noticed because the first one failed. I'd replace them all.

So I attend this local annual event that has a time capsule. Each year, we bury one and dig up the one from 5 years ago. This has gone on for several decades, and people are always thinking of interesting things to put in there.

I made a little electronics project that is somewhere between "what happens inside a time capsule while it's buried?" and "what if you could wind a watch once and have it still be ticking in 5 years?"

I nearly forgot that I made a project page for it: https://hackaday.io/project/160740-low-power-environment-mon...

I dug up the first one last year and it had made it to 4.5 years, and I'm actually due to dig up the second one next week.

Fair point, although with a 400+ year half life in the americium source in the detector, I am skeptical that a new smoke detector would be any more reliable than a very old one.

I would think testing them regularly - especially with simulated smoke as done in professional situations, or in my case via bad cooking, is probably more effective than regular replacement on a schedule to ensure they are always working.

If dealing with something that follows a Poisson failure probability distribution with a fixed percentage probability of failure per year (as is the case with most electrical components), regular replacement only makes the system more reliable if you are unable to test it, otherwise it makes no difference.

With a few rare exceptions, is largely a myth that replacing machines or technology at regular intervals increases reliability- people incorrectly assume this to be true, based on observing that most failures happen to things that are old, but this is merely because they spend more time being old, not because the rate of failure per time increases with age (it almost never does). Testing and redundancy are more effective and cheaper.

Now, everything I am saying would be wrong if smoke detectors indeed have components besides the alpha source whose failure rates are known to increase with age, and actually age out within a decade or so. Like you mentioned, this can be the case with electrolytic capacitors as well as non solid state relays. However, I wouldn't be surprised if the lifespan of capacitors at the low temp and low voltages in a smoke detector wasn't 50+ years.