Nest Thermostat - Battery Issue

[Deanna_M]

I have a Nest Learning Thermostat that after 4 years no longer holds a charge. I swapped a second Nest from another zone to this base and it works fine. The broken one charges on the USB, but then dies when attached to the base (even though it is receiving power). 

Nothing to do about the old one since it is out of warranty, but I am trying to decide if I have a system issue before I get another one, to avoid having that one just die too. When I check the power readings, I noticed that my lin is low at 40 mA, even though I have a C wire connected (assumed it would be 100-200). Battery is over 3.8 V and Voc is 37 and Vin is 36, so those seem fine. The heating and cooling system works fine, but I wanted to figure out if I am overtaxing the Nest battery with a low power feed and that is why it died. 

Should I expect eventual issues if I install a new Nest? Thank you  

[CoolingWizard]

The typical power input from the HVAC system is 24VAC. It can be as low as 17 VAC or as high as 28 VAC and the HVAC control system will still operate safely. The total current of HVAC transformers is 40 Volt Amps.  

The internal battery Nest Thermostat has a life cycle rated in charge cycles. Actually all rechargeable batteries do.  When we see the very low charge current, we are typically seeing the battery near its charge limit. If the charging limit has shrunk do to repeated charging cycles, you will see lower total charge.  

The Nest battery is not replaceable sadly.  

[Patrick_Caezza]

Do you have a C wire installed?

Without a C wire installed, all Nest thermostats will have battery problems at some point. Install a C wire, and unless the Battery is completely dead, it should stop having low battery issues.

[Jeran]

Hey there Deanna_M,

I'm just checking up on this thread, did you have any more questions or need any additional help? 

Thank you for helping out on this thread, CoolingWizard and Patrick_Caezza!

Best regards,
Jeran
 

[Jeran]

Hey there Deanna_M,

Did you have any more questions or need any additional help? 

Thank you,
Jeran
 

[Jake]

Hey there,

I wanted to check in with you, and let you know that I will be locking the thread in 24 hours.

Best regards,

Jake