When I installed my Nest originally I ended up having to change the multi-conductor cable from a 4 to a 5 to get a common for proper battery charging.
Most people don't realize the other problem is that the integrated relays for the (call for heat &2nd stage/cool &2nd stage/humidifier/ & other extra controls) are not very tolerant to voltage drops or spikes. My furnace would try starting & sometimes fail giving a look as if it was cycling, but it was the Nest thermostats integrated relay briefly opening the circuit causing the furnace to restart its start-up sequence. I found this by connecting 2 Fluke multimeters, one to the incoming power & common wires, & the 2nd to the call for heat & common wires. My HVAC uses a 24vac transformer for its control voltage. The incoming voltage shows a steady 27.34vac then the call for heat shows the same but then drops out 25.47vac on the RMS, but when looked at on an oscilloscope shows a complete dropout. To fix this I purchased some high tolerance Finder brand relays for about $50 & controlled them with the Nest & controlled the HVAC via the Finder relays.
