Battery for thermostat not charging properly

[NicksNest]

Some general info:

• Purchased in 2020
• Installed in 2020 - I'm in the Netherlands, so the Heat Link is on 230V and the T1 and T2 out is 12V to the thermostat.
• All worked fine since 2020
• Now the thermostat has a low charge. It shuts down to charge. It doesn't (or veeeery slowly maybe).
• Charged it with USB for a whole night. Doesn't stay connected with the Heat Link btw.
• In the morning, battery is charged, it wants to be connected to the base on the wall. Temperature is 13°C now.
• It works again and starts heating my home! ...until the battery is low again (within 1 or 2 days). Rinse and repeat.

I've checked the voltage the Heat Link supplies over the T1 and T2 lines, it's 11.86V. Is that enough? I'm guessing either the battery is not okay anymore, or the Heat Link does not provide enough power anymore.
I've tried to contact Google with the form, but it says "Please try again during support hours." - which are from 12 a.m. - 11:59 p.m., each day; what am I missing?

[catheryn]

Hello @NicksNest! 

Thanks for bringing this to our attention and posting it in the community. I’m really sorry to hear about the inconvenience you're experiencing with your Nest Thermostat’s battery.

Before troubleshooting, I’d like to confirm a few things:

• When did the issue first start?
• Are the power breakers on, and is your home receiving power?
• Is the thermostat’s display securely seated?
• Are all the wires firmly connected to the thermostat’s terminals (no loose or exposed wires)?

Once we have this information, we can diagnose the issue and provide a solution.

Thank you,

Catheryn

[NicksNest]

Hi Catheryn,

Thanks for your response. My answers:

1. About 10 days ago

2. Yes it is, only the thermostat dial has the issue.

3. It is very securely seated.

4. They were. Currently I've disconnected them and replaced them with a (temporary) USB cable to keep it on/(semi-)charged.

I got in contact with someone from Google customer service, who couldn't do anything because out of warranty.

It won't charge past 3.9V, whereas 3.7V lithium batteries normally charge to about 4.2V maximum. So I suspect the battery is the culprit and doesn't hold enough charge from the 12V supply in the Heat Link. Or that supply might not be sufficient (anymore).