What home thermostat do you have?

I need to install one of these. Current thermostat is in an upstairs hallway with no vent. We have to keep those doors closed because my two nieces live with me. So what I really need is the remote sensor to put it somewhere else. Problem is that the old thermostat is battery operated and has no C wire going to it. Are there any smart thermostats that are completely battery operated? I haven't found one. We ordered an Ecobee that has the PEK (basically just repurposes an existing wire to the unit into a C wire )converter for the H-Vac but the panel isn't easy to access and my confidence in the installer is very low.