Asklemmy

Different chemistries produce different voltages per cell. Alkaline produces 1.5v, NiMH produces 1.2v, Li-Ion produces 3.6v. These are averages, the actual voltage varies over the current charge level of the cell. This variation in voltage is how the low battery alarm actually works, although Alkaline cells produce 1.5v initially, once they are nearly empty they are producing 1.1-1.2v. Your thermostats will likely work fine on NiMH batteries, if you can live with them continually complaining about the batteries being low.

There are, or at least were, rechargeable alkaline batteries, but they don't last many cycles.

The 1.5v Li-Ions have a tiny circuit board on them that regulates the voltage down to 1.5v, which takes up space so the capacity is reduced. You could do that with NiMH, but it would have less capacity than the Li-Ion version, so there's little point.

It boils down to chemistry. The nickle-metal-hydride chemistry used in rechargeable batteries just don't produce 1.5V (explaining why is a lot more complicated), while the alkaline chemistry in non-rechargable batteries produces slightly more volts.In the same vein, lithium-ion batteries produce around 3.6V, while lead-acid produce about 12.4V.

To answer your other question, they should be able to power most things that aren't an analog clock (which might run slightly slow).

my understanding is that the nominal voltage you get out of a cell is always dependent on the chemistry it's using. alkaline battery cells are always 1.5V until they run down. even a 9V battery is actually six little 1.5 V cells in series in its little rectangular box. NiMH cells are 1.2V. Lithium batteries are actually a variety of different chemistries, but i think all of them have upwards of 3V nominal voltage, so they wouldn't make a good drop in replacement either without some kind of voltage regulation circuitry.

Eneloops are about the best rechargable AA and AAAs you can buy, they last close to forever. I've got some that are a decade old and still in use after thousands of uses and still about 80% of original. You chose well.

And it's fine. I used to use them a lot for GPSrs. A pair of eneloops lasts about 8 hours in an Oregon. A pair of decent branded Alkaline AAs lasts about 7-8.

The voltage is not an indicator of charge life when you're comparing different chemistries. The reason you're seeing low charge is that your device is not calibrated for rechargables. In things that are, they have settings for both so you can select which you use. If your thermostats don't, then they'll always removed about anything other than Alkalines.

The voltage of a battery is dependent on what materials it's made of and their electrical properties. So because of quantum physics and shit, a cell made of different materials will have a different voltage. The only real ways to change the voltage is to put multiple batteries in series like how there's six little batteries in a nine-volt battery (but two NiMH cells would give 2.4 volts and overvolting can cause even more problems than undervolting), or have some kind of active circuitry to adjust the voltage (which adds a significant amount of inefficiency and therefore reduces the effective capacity of your battery, not to mention the cost of having a circuit board with many components embedded in the battery).

It really came as a surprise, is there a catch? Are they only good for low power stuff like remote controls?

Most AA battery powered devices are designed to accept a fairly wide range of voltages because the raw voltage from a battery is inherently variable, and 1.2 is almost certainly within tolerance (depending on how reputable the manufacturer is, there's a good chance the engineers specifically tested it with rechargeable batteries to ensure they work, since it would be a really bad customer experience if they didn't work) Besides, for high power stuff, usually it's the current rating that matters, specifically, something called equivalent series resistance AKA how much the voltage drops when under load. High quality NiMH batteries can deliver a lot more current than alkalines so depending on what kind of load you have on it, the voltage of a NiMH might even be higher than alkaline when the voltage drop is factored in. I've personally never had any issues with NiMH's in high draw devices.

There are rechargeables with integrated buck converters that can provide 1.5V, but they have their own issues.

But there are 1.5v rechargeable AA batteries out there.

classic rechargeable batteries are always 1.2V. That's due to the chemistry.

The "low power" warning is the reason why a lot of devices will tell you not to use rechargeable batteries, as they will trigger the low battery warning earlier, and the cutoff voltage might be reached earlier.

I think there are modern rechargable lithium batteries with usb ports that will emulate 1.5V batteries and their voltage curve. But they have their own quirks as well

The problem with non-rechargeable batteries is how they can easily become e-waste. Yes, there are recycling programs, but I once heard rumors about fake recycling programs that simply dispatches the e-waste to landfills across poor countries, polluting their soils. Rechargeable batteries will eventually become e-waste, too, but it seems to take longer than non-rechargeable batteries... of course it depends on how frequently they are charged and discharged. I have rechargeable batteries (which I use in my wireless keyboard/mouse set, digital hygrometer and a Bluetooth joystick) that have been lasting for years, I even don't remember when I bought them. It's worth mentioning that the battery brand I use is known by their poor quality (Multilaser, a Brazilian company that sells white-label electronics under their brand), and yet they've been holding charge.

I've reached the point of absolutely hating rechargable batteries. Between their ghost discharge, and the fact that I don't go through as many batteries like I used, using normal alkaline batteries seems to be the best for me anyway.

The light coloured Eneloop ones seem to have the lowest self discharge. I use them in Arlec PIR sensors without issue.

I've bought a few of these before (no affiliation) https://www.amazon.com/dp/B0CCL7TJ48

Edit: this is a link to 1.5V rechargeable batteries, which I commented before OP's edit acknowledging them.