Nest 3rd gen Thermostat low battery issue after several years

[AleNapoli]

Hello,

I have seen this come up in the past. I’d like some input on what are considered normal power values. 
I’m getting Vin 30V and  current 40mA when furnace is off. But Vin is dropping to 6.5V and current 20mA when furnace is on. 
Is this normal? Should the voltage drop that much? This could be the reason why the battery is not charging any longer?  Thanks

[Dodel]

There seems to be an issue with the batteries on the 3rd gen thermostats that Nest does not have a solution for. There are lots of people in this forum who have experienced this issue. I have seven of these in my house and one failed last week after 2 years. Went through a long call with tech support to tell me in the end the battery is defective and the unit would need to be replaced, at my expense since the warranty ran out 2 months ago. Over the last week it has gotten progressively worse to the point where it won’t stay charged for more than a minute. Nest will tell you to try charging the battery by using a USB cable, which works for about a minute. If you are still under warranty, act fast.

[AleNapoli]

Thank you so much for your reply. Yeah, I feared that the battery could be coming close to an end, given all the people with similar issues. I did try charging the battery via USB today, let's see how long it lasts. I also ordered a replacement battery from ifix.com, no way I'm buying another one of these. They should be guaranteed to last at least ten years before running into battery problems. These thermostats are not cheap to buy.

[zoeuvre]

Hi folks, 

Thanks for chiming in, @Dodel.

@AleNapoli, uh-oh! I'm sorry to hear that you are experiencing low battery issue with your thermostat after several years of using it. The voltage drop you are experiencing when the furnace is on is quite significant, and it could potentially be the reason why the battery in your Nest Thermostat is not charging. A drop in voltage from 30V to 6.5V indicates that there is a substantial power draw when the furnace is operating. Here's what you can consider: 

1. Verify that your Nest Thermostat is compatible with the power supply provided by your HVAC system. The Nest Thermostat typically requires a stable and sufficient power supply to operate correctly. Some HVAC systems may not provide enough power to simultaneously operate the furnace and charge the thermostat's battery. In such cases, the power may be prioritized for the furnace operation, resulting in a drop in voltage and limited charging for the thermostat.
2. Check the wiring connections between the HVAC system and the Nest Thermostat. Ensure that the wires are properly connected and securely inserted into the correct terminals. Loose or faulty connections can lead to voltage drops and poor charging performance.
3. If you suspect that the voltage drop is abnormal or causing issues with your Nest Thermostat's battery charging, it's advisable to have a professional HVAC technician evaluate your system. They can assess the power supply, wiring, and overall compatibility to determine if any adjustments or repairs are needed.

In summary, a drop in voltage from 30V to 6.5V when the furnace is on is not typical and may indicate a problem with the power supply or wiring. It's recommended to seek professional assistance to ensure that your HVAC system is providing adequate power to the Nest Thermostat and to address any potential issues that may be affecting battery charging.

Let us know if that helps.

Best,

Zoe