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

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.