>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.