It is an off the shelf part. It is (at least in the models I have worked with) possible, but not easy to replace, as the holder is blocked buy other components and the thermostat is not designed to be disassembled. The existence of the battery is undocumented.

Since we had over a hundred of these deployed in an apartment complex, when they started failing I carefully characterized their behavior. It just makes them go wonky when it is removed or dies. The at328p functions fine without it, there is no RTC or anything that needs a battery., and the chip is not powered by the battery when power is removed. In fact, there is no discharge from the battery when power is off. The at328p has nonvolatile eeprom and flash storage.

Interestingly, the wonky behavior it exhibits is the same (but doesn’t go below 40 degrees (f) ) as an old electromechanical one failing lol.

I'm curious too, but even if something is fairly easy to replace or repair a significant amount of people may still just buy new and not bother. (My own landlord is making this choice over a $100 logic board replacement for the garage door opener.)

I think the point is that you (or most people) will replace the entire unit because it's beyond warranty, stops functioning correctly, and a soldered (probably special) battery would be difficult to replace.

There's no such thing as a special battery. No manufacturer is going to have some kind of custom battery manufactured just for one part; they always come in standard sizes and voltages. At the very worst, you could hack in a similar-sized battery with the right voltage.

My bicycle light begs to differ. It’s a standard 18650 cell with a protection PCB slapped on top, and both poles on the same end of the battery.

This is exotic enough that replacements are very hard to find. I can either get the 2 cells needed from the manufacturer at about 70% the cost of a new light, or get a new light. It’s technically repairable/user serviceable, it’s just not practical to do. Apple is playing a similar trick with their 80lbs iPhone repair kit to change the battery yourself: technically user repairable, practically not.

>My bicycle light begs to differ. It’s a standard 18650 cell with a protection PCB slapped on top, and both poles on the same end of the battery.

Huh? You seem to be contradicting yourself: you say it's a standard 18650 cell. That's not something custom, that's something you can buy from an electronics supplier quite easily. And no, having both poles on the same end isn't something custom either, that's just a variant you can buy. You should be able to find this exact part on digikey.com.

>This is exotic enough that replacements are very hard to find.

If you can buy something on digikey.com, it's not "exotic". They even ship small orders by USPS for cheap. It might be "exotic" to a typical consumer, but those people don't know how to use a soldering iron.

This entire thread is people talking about planned obsolescence for _consumer_ devices and you keep making the same rebuttals. Typical home owners who buy a thermostat at Home Depot or cyclists who buy a tail light at a bike shop are unlikely to even know what digikey is, let alone order a replacement (especially if you have to buy in bulk) and do their own soldering to replace it if it's soldered in.

We all know the typical HN reader thinks a fun weekend would be spending 6 hours researching part numbers and getting out the soldering iron, but we are atypical. Most people will assume their product is at the end of its life because the warranty has lapsed and it doesn't work anymore. Companies are apparently betting on that and it's an environmental travesty.

>Typical home owners who buy a thermostat at Home Depot or cyclists who buy a tail light at a bike shop are unlikely to even know what digikey is, let alone order a replacement

Yes, I know. I said this exact thing above.

The parent said these devices used "custom batteries", implying that Honeywell et al were custom-designing and manufacturing their own special nonstandard batteries just for these applications. This is incorrect, and needed to be corrected.

It’s standard in the sense that a 18650 with a protection PCB on top and both poles on the same end isn’t exclusive to this manufacturer and others use it. There is nothing proprietary about it. But it’s not run of the mill 18650 either.

But it’s rare enough that it’s hard to find. So much so that it actually doesn’t make economical sense to find this (rare, at a premium) variant because it would cost about as much or more than a new light (which includes new batteries, ironically)

There are batteries special enough that I can't pick one up at Walmart, and for the general public that may render the device kaput. I didn't say it's an unhackable design.

>There are batteries special enough that I can't pick one up at Walmart

You can't pick up a standard 18650 battery at Walmart either, but that doesn't make it some kind of special battery. It's bog-standard. Availability at Walmart has nothing to do with something is standard or even common. Brake pads for some random mass-market 2015 model year car (GM, Toyota, etc.) are nothing special or difficult to find either, but you're not going to find them at Walmart.

But perhaps BMS expect specific battery product. I don't want to take risk by replacing to random Li-ion that not matching original BMS.

The question is how much hacking would be required.

Probably not much, but that's still far more than the average consumer is willing or able to do.

Yes, a soldered or special battery would be (weak) evidence of the tactic of planned obsolescence. A standard battery that you can just pop in and out, would not be.