Shelly H&T in a deep freeze
I've alluded to this experiment a few times here and in the Home Assistant sub, but I've never really written it up. So, here goes.
Back in January, a coworker who also uses Home Assistant posted in an internal Slack channel looking for options to monitor the temperature of a deep freeze (chest freezer, storage freezer, whatever you call it) because his mother-in-law had had a catastrophic event with hers. He was looking at an ESPHome-based device that has a flat cable that's supposed to go under the seal, allowing the electronics to live outside the freezer with the temperature sensor inside.
I suggested that a Shelly Blu H&T might do the trick. My only concern was that, according to the spec sheet, they're not designed to work below -5°F. Depending on where I put the sensor in my freezer, it can get a bit colder than that:
A related concern was that lithium button cells are also reputed to have pretty poor performance at low temperatures.
But I happened to have a spare Blu H&T just sitting on my desk doing nothing useful (the indoor unit for the weather station is also in my office, so I already know the humidity and temperature in here) so I took it and dropped it in my own freezer in the garage.
It is now May, a little over 4 months since I started the experiment. The battery level dropped precipitously over the first couple of weeks, but it leveled off at about 70% after a month or two, where it has pretty much stayed ever since:
A graph of battery level for the year 2026 so far
Another question I hear a lot is "isn't a freezer just a big Faraday cage?" and I'd say that while it may be to some extent, between the gap for the rubber seal and presumably other gaps, enough signal gets out that it seems pretty reliable. As I move things around in the freezer and the sensor gets moved from place to place, the signal strength varies a bit, but it's usually in the neighborhood of -80 dBm or so. If I wanted to improve it, I could probably move the Blu Gateway closer to the freezer; it's currently on the other side of the garage. But it seems sufficient as it is.
Anyway, based on my results so far, I'd feel pretty confident in saying that while it's technically not designed for those temperatures, the Blu H&T does a pretty darned good job at those temperatures.