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.
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.
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.
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)
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.