I recently purchased a Nest thermostat. After couple weeks of usage, the Nest is constantly shut down due to low battery. I search online that adding a common wire is the go solution for this. However, I am not too familiar with electrical stuff, So I hope someone can help me on this. Unfortunately, I could not locate the circuit board, hopefully some of below pictures might be helpful.
Update: I have reorganize the picture to match the number with scheme. Please let me know where I can connect my unused red wire to pick up the 24volt and used as common wire.