Nest thermostat low battery warning

Which Nest thermostat do you have?

The answer to a low battery warning on a Nest depends almost entirely on the model, because Google uses two different battery designs. One model takes ordinary AAA batteries that you swap yourself. The others have a sealed rechargeable battery that the thermostat keeps topped up from your heating and cooling system.

Battery design and very low battery signs on each Nest thermostat

Model	Battery	Can you replace it?	What you see when it runs very low
Nest Thermostat (2020)	2 AAA alkaline, 1.5 V, used as backup	Yes	An alert on the thermostat and in the Google Home app; Wi-Fi and motion sensing switch off
Nest Thermostat E	Internal rechargeable	No	Blinking red light while it charges
Nest Learning Thermostat (3rd gen or earlier)	Internal rechargeable	No	Blinking red light while it charges
Nest Learning Thermostat (4th gen)	Internal rechargeable	No	Dark yellow light while it charges

If the box or the app calls your thermostat simply "Nest Thermostat", it is the model with AAA batteries. The rest of this page covers that model first, then the rechargeable ones.

What does the low battery alert mean on the Nest Thermostat (2020)?

This model does not run on its batteries day to day. Google says it gets its power from your heating and cooling system and uses the 2 AAA batteries as a backup, or to top up the supply when the system cannot deliver enough. So when the batteries get low, it usually means the thermostat has been leaning on them.

Google says you get a notification on the thermostat and in the Google Home app when they start to run down, and you can check the battery level in either place. In Google's list of help codes, M19 is the code for "Change Nest thermostat batteries".

As the charge drops further, the thermostat saves power by switching off features such as Wi-Fi and motion sensing. With Wi-Fi off, the app loses touch with it; see thermostat offline. Google also says you might need to tap the right side of the thermostat to wake the screen.

How do you change the batteries?

Google's steps take a minute and stay on the thermostat itself:

1. Pull the thermostat display straight off its base.
2. Take the old batteries out of the back of the display.
3. Put in 2 new AAA alkaline batteries, matching the markings for which way each one faces.
4. Push the display back onto the base until it clicks into place.

Use high-quality 1.5 V AAA alkaline batteries from a known manufacturer, which is what Google asks for.

Why does it say low battery right after setup or new batteries?

Google gives a simple cause for an alert during setup: the batteries are loose, or a piece of the pull tab that ships with them is still stuck to the contacts. Take the display off, remove every bit of tab, put the batteries back the right way and reattach the display. If the message stays, Google says to fit new AAA alkaline batteries.

If you changed them recently and the alert has come back, Google suggests taking them out and putting them back in, making sure the contacts are clean and each battery sits firmly.

Why does the warning keep coming back?

Repeated low battery warnings, or other power messages, are Google's sign that the thermostat is not getting steady power from the system. Its battery article says the backup batteries can drain when there is no C-wire (common wire), and that heating-only, cooling-only, zone-controlled and heat pump systems may need a C-wire or the Nest Power Connector.

Google's order of work is to replace the batteries first, so the thermostat can keep running your system, and deal with the power supply after. That second part is wiring work. A licensed HVAC technician should handle it; nothing in the thermostat menus replaces it.

Google's C-wire page lists other signs of the same power shortfall: shorter battery life, a display that will not turn on, and dropped Wi-Fi connections. If you also see a countdown before heating or cooling starts, read Nest thermostat says "Delayed".

What does low battery mean on a Nest Learning Thermostat or Thermostat E?

These models have no battery door. Google says the internal battery is not user replaceable and was designed to charge and discharge as needed for the life of the thermostat. When one of them has power trouble, Google says it is usually because it is not getting enough power from the household wiring.

You can read the battery voltage in the thermostat's own menu, under Technical Info and then Power. Google's information menu page gives the thresholds: a healthy reading while charging is above 3.6V. Under that, the thermostat shuts off Wi-Fi, puts software updates on hold and turns off its screen, and it needs 3.7V before it will install an update.

When the charge gets very low, a light comes on while it charges. That light, and how long charging takes, is covered on Nest thermostat flashing red light.

What should you do now?

Is it the Nest Thermostat (2020), the model with AAA batteries?

• Yes

  Did the alert appear during setup or soon after new batteries?

  • Yes

    Reseat the batteries, remove any leftover pull tab and check the contacts are clean. If it stays, fit new AAA alkaline batteries.

  • No

    Replace both AAA batteries. If the alert returns within weeks, Google points to the power supply, which a technician should look at.

• No

  Is a red or yellow light on, or is the screen dark?

  • Yes

    The built-in battery is charging or drained. See the flashing red light page for charging steps.

  • No

    Check the battery voltage under Technical Info, then Power. A reading that stays below 3.6V points to a power supply problem for a technician.

A black display is not always the battery either. On the Nest Thermostat E and Learning Thermostat, Google says the screen background turns black when the thermostat is in Off mode or in Eco, which uses its Eco temperatures. Both are settings you can change on the spot. For thermostats from other brands that run on AA or AAA batteries, see thermostat low battery message. For other Nest messages, see the Google Nest hub.