Third Gen thermostat refuses to charge

[Sendalac]

After a long hot summer with virtually Zero furnace use, our 2 year old G.N.L.T will not run or charge when either attached to the USB cable or attached to the base (that has a C-wire connected and a confirmed supply of 27 volts). The best that I can do is get it to start after hours on the USB charger. It may give 5 or 10 minutes of operation before I get a message stating "Your thermostat needs to shut down to recharge its battery"

Only the thermostat will not recharge until I use the USB cable

The cycle repeats over and over

I should say that prior to this episode, I only had a 2-wire setup on the W1 and RH connectors and it worked quite well until now. I installed a 5-wire setup in an effort to remedy my problem.

When it works it can identify wires on the W1, RH, C and G connectors.

I even connected the C-wire to the power supply and could confirm 27 volts at the base

I suspect that if the battery has not been charged in a long time, it develops a memory of low charge and no amount of charging will fix it. This interferes with any attempt to run on constant power.

This is the only explanation that makes sense, but I would like to know if anyone found out something that I am missing.

[philljones22]

It sounds like your thermostat’s internal battery has degraded after sitting through the summer with minimal charging cycles. Even though you’ve confirmed a solid 27V supply at the base and connected a proper C-wire, the fact that it only holds charge for a few minutes and requires hours on USB to even boot points strongly to a failing or “stuck” battery. These rechargeable lithium cells do lose capacity if they remain discharged for long periods, and in some cases they can no longer take or hold enough charge to run the device consistently, no matter how good the external power connection is. Since you’ve already verified wiring, power, and correct identification of the terminals, the missing piece is most likely the thermostat’s battery itself. Some people have managed to replace the internal battery, but most end up needing a replacement thermostat if the battery won’t recover.

[GabrielaG]

Hi @Sendalac,

Thank you for reaching out to the community. I'm sorry to hear you've been having problems with your Nest Learning Thermostat 3rd gen with the battery not charging. I understand how difficult this must be. I appreciate you've already taken the key steps of trying to charge the thermostat and checking the C wire connection with 27 volts. I'm glad to help.

Based on the wiring you described, your system appears to be a conventional heating setup. The presence of W1 and RH wires is a key indicator of this configuration. To troubleshoot further, could you please provide the following information?

• Are there any error codes on the thermostat's screen?
• Do you see any blinking lights on the Nest Thermostat display? If so, are they red, green, or yellow?
• Have you experienced any power outages in the last few days or weeks?
• Could you please send me a picture of the current Nest Thermostat's wiring?

Thanks, @philljones22, for all the details you've shared. We consider this information valuable. Just to let you know, we don't recommend replacing the internal battery yourself, as it could damage your device and may void your warranty.

I look forward to your response so we can find a solution for you soon.

Regards,

Gabriela

[Sendalac]

Hello GabrielaG, I appreciate the help. In answer to your questions:

No, I have not seen any error codes on the screen

Yes. I see a blinking red light as soon as I press the display. Before pressing it there is no light.

Yes, but back in June

Pictures are attached

[GabrielaG]

Hi @Sendalac,

Thank you for your reply. I appreciate the time you took to respond to my questions and the information you provided me about your Nest Learning Thermostat 3rd gen. I'm glad to continue assisting you.

With the information you sent us it seems the problem is the internal battery of your Nest Learning Thermostat 3rd gen. The team would like to know more about this behavior. When you get a chance, please fill out this form. Remember to include the link to this post. Let me know when you’re finished. From there, we'll contact you via email.

Regards,

Gabriela

[Sendalac]

Hello GabrielaG,

I filled out the form. I had to do it twice because I filled the first one out incorrectly.