The Nest is the same. That's why I was asking about if it was a four wire system.Ecobees require a C wire to run at all, either a real one or one through the PEK, so there's no battery to run out. @Patrick if the Nest operates your AC/heater fine and it just runs out of battery, your Nest may be bad, or you should just run a new C wire (which should be a lot cheaper than replacing the entire AC/heating system). Although I'm not sure if you've already tried that in your troubleshooting steps.