Wide Input-Range Connected Thermostat

A thermostat system for control of an HVAC (Heating, Ventilation and Air-Conditioning) system, the thermostat system comprising a power conversion module, the power conversion module further comprising a power input of either a direct-current voltage or an alternating-current voltage, a battery management system including a battery and a battery management circuit, the battery management circuit configured to select either the battery output voltage or the primary output voltage based on an operating condition, a communication module, a temperature monitor, one or more environmental sensors, a graphical user interface display, and a processor.