In the models here, a battery thermostat takes 2 AAA, 2 AA or 3 AA batteries, and some take none because they run on the house wiring or a built-in battery. Room sensors use a CR2, a CR2477 coin cell or AAA batteries.
Which thermostat or sensor?
The battery it takes
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Thermostat and sensor batteries by model
Thermostat and sensor batteries by maker and model
Device
Battery
Google Nest Thermostat
2 AAA alkaline
Nest Learning Thermostat, Nest Thermostat E
Built-in rechargeable
Nest Temperature Sensor
One CR2
Honeywell Home RTH2300, RTH221, RTH111
2 AAA alkaline
Honeywell Home T6 Pro Z-Wave
3 AA, or a C wire
Sensi smart thermostat
2 AA alkaline
Sensi Lite
2 AAA alkaline
Sensi Touch, Touch 2
None
Sensi room sensor
2 AAA
ecobee Smart Thermostat Essential
None; 24VAC through the C wire
ecobee SmartSensor, rooms or doors and windows
CR2477
From each maker’s help pages, manuals or product pages.
Changing the Nest Thermostat’s batteries
Pull the thermostat display off the base.
Take the old batteries out of the back of the display.
Put in 2 new AAA alkaline batteries.
Push the display back onto the base until it clicks into place.
Why a Nest Thermostat has batteries at all
Google says the Nest Thermostat gets its power from the heating and cooling system. Its AAA batteries are a backup, and they top up the power when the system can’t deliver enough.
Keep coin batteries away from children
The National Capital Poison Center says that if anyone swallows a battery, immediately call the 24-hour National Battery Ingestion Hotline at 1-800-498-8666 or your poison center at 1-800-222-1222. Don’t delay. Immediate treatment decreases severity.
Common questions
How often should I change the batteries in a Honeywell thermostat?
Honeywell Home's RTH2300 and RTH221 manual says to replace the batteries once a year.
From Google Nest Help, Honeywell Home’s manuals, Sensi support, ecobee’s product pages and the National Capital Poison Center, read on October 25, 2026. Checked against published data from Google, Honeywell Home, National Capital Poison Center, ecobee and Sensi.
See all 7 sources
National Capital Poison Center, Button battery ingestion (poison.org/battery)
Google Nest Help, Batteries for Nest products
Google Nest Help, How to change Nest Thermostat batteries
ecobee product pages: SmartSensor for rooms, SmartSensor for doors and windows, Smart Thermostat Essential
Honeywell Home (Resideo) owner's manuals: RTH2300/RTH221 (33-00100EF) and RTH111 (33-00137EF), PDF
Honeywell Home (Resideo), T6 Pro Z-Wave Programmable Thermostat TH6320ZW2003 and TH6320ZW2007 guide (33-00587EFS), PDF
Sensi (Copeland) Support, How do I replace the batteries on my Sensi?
Battery sizes come from battery makers' datasheets and device makers' own manuals and help pages. Check your device's manual or the old battery before you buy. If anyone may have swallowed a battery, call the National Battery Ingestion Hotline at 1-800-498-8666 or your poison center at 1-800-222-1222 straight away.