I recently installed a Nest E which worked fine once the battery on the base unit had been charged up. However the thermostat has stopped working in that it's just dead. Its powered via USB and the charger is working as I've checked it with another device, but the cable is hardwired so cant tell if there is a fault with that. Could there be an issue with the battery that is causing it not to work? Perhaps the battery can't hold a charge so despite it being permanently plugged in it won't work? It's not helped by many of the references online to Nest E refer to a wall mounted, battery powered version which is different to the freestanding, USB powered version I have. Any suggestions gratefully received.