If your nest thermostat runs on 4 wires configuration , the charging will be only on steal mode, or said trickle charging , at up to 40 mA, which could be easily end up to low battery and shut off the nest thermostat . There many things could cause nest thermostat to consume more powers , like unable to connect to WIFI and cause continues re-try .
Add or connect C wire will improve charging greatly , make charging current up to 200mA , and this would enough to most of the cases . If in a situation still have charging problem , one solution is to install an extra 24v power adapter to supply 24v power on wire C and Rc .
