Nest Thermostat Low Battery? Why It Happens and How to Fix It

The Short Answer

What to do depends on your model:

A low battery that keeps coming back is not a battery problem: the thermostat is not getting enough power from your heating and cooling system to stay charged. Google says repeated warnings can point to the C wire.

Nest Thermostat (2020): Replace the AAA Batteries

Google’s steps: pull the display off its base, remove the old batteries from the back, insert two new AAA batteries and press the display back until it clicks. Use “high-quality 1.5 V AAA alkaline batteries from a known manufacturer”.

The batteries are backup and supplement power the system cannot provide on its own, so if they drain quickly, check the wiring below.

Learning Thermostat and Thermostat E: Charge It

These charge their built-in battery from the HVAC wiring and cannot have the battery replaced. When it runs flat, Google’s fix is to pull off the display and charge it through the USB port on the back with a USB cable and wall charger; it takes several minutes to an hour depending on how empty it is.

A dark yellow or blinking red light means the battery is low but charging. Google suggests waiting up to an hour before assuming something else is wrong.

Why It Keeps Running Low

• No C wire. Without a common wire, the thermostat has to draw power from the other wires in short bursts, which is often not enough, especially in mild weather when the system rarely runs.
• The system keeps shutting itself off. Google notes that a clogged air filter can make the system overheat and shut off, and then the thermostat cannot draw power to charge. Replace the filter.
• Low battery causes other problems: Google says a thermostat with a low battery will not connect to Wi-Fi to save power, so “offline” and “low battery” often appear together.

For a problem at the Rh wire specifically, see our guide to the Nest E74 “no power to Rh” error.

The Lasting Fix: Steady Power

• Connect a C wire if there is an unused wire in the bundle at the thermostat and at the furnace or air handler. If you are not comfortable at the furnace end, have a technician do it.
• Use the Nest Power Connector, Google’s adapter for systems without a C wire, which is installed at the HVAC equipment.

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Frequently Asked Questions

Why does my Nest thermostat say low battery?

On the 2020 Nest Thermostat, the AAA backup batteries are low: replace them. On a Learning Thermostat or Thermostat E, the built-in battery is not getting enough power from your HVAC wiring, usually because there is no C wire or the system keeps shutting off.

Can I replace the battery in a Nest Learning Thermostat?

No. Google says you cannot replace the batteries in a Nest Learning Thermostat or Nest Thermostat E; charge it over USB and fix the power supply instead.

How long does a Nest take to charge over USB?

Google says several minutes to an hour, depending on how depleted the battery is.

What does a blinking red light on a Nest mean?

Google says a dark yellow or blinking red light means the battery is low but charging; wait up to an hour.

Related Guides

• Nest thermostat offline — low battery often causes it
• Nest E74: no power to Rh — a wiring power fault
• Best smart thermostat without a C wire — if you are replacing it

Sources

• Google Nest Help: Change Nest Thermostat batteries
• Google Nest Help: Thermostat won’t turn on
• Google Nest Help: Nest thermostat is offline