Thermostat Battery: Which Kind, and How to Change It

By Ray Bennett · Updated 2 October 2026 · 6 min read · Checked against the sources at the end

Thermostat Battery: Which Kind, and How to Change It

Short answer

Many Honeywell and Sensi thermostats take two AA or two AAA alkaline batteries, and Honeywell says to change them once a year. A Nest Thermostat takes two AAA as a backup. A Nest Learning or Thermostat E has a built in battery you cannot replace, and an ecobee runs on the C wire. On a Honeywell RTH2300, set the system to Off before the batteries come out.

A thermostat battery is a five minute job, and the right one depends on the thermostat. Some run entirely on batteries, some only use them as a backup, and some have none you can change at all.

This page brings together what Honeywell, Google Nest, Sensi and ecobee say in their own manuals and help pages. It also covers the one warning that is not about the batteries at all: a low battery message that keeps coming back.

Before you start

What to carry: fresh alkaline batteries of the right size, and a torch. Five minutes.

Gas or electric? Either. The thermostat is the same whether it runs a gas furnace, a heat pump or an air conditioner.

Check the size first. Pull the thermostat off its wall plate and read the battery holder, or use the table below. AA and AAA look alike from across the room.

What the makers are firm about. Switch the system off first. Honeywell’s RTH2300 manual says to set the System switch to Off before removing the batteries, or the heating or cooling could stay on with the batteries out.

Alkaline batteries. Google says to use only high quality 1.5 volt AAA alkaline batteries in a Nest Thermostat. Honeywell recommends alkaline too.

No battery to buy for some. Google says the built in battery in a Nest Learning Thermostat or Nest Thermostat E is not user replaceable. It charges from the system wires.

Which battery yours takes

On some thermostats the batteries do all the work. On others they only keep it going when the power drops, and on some there are none to change.

Thermostat	Battery
Honeywell RTH2300	2 AAA alkaline
Honeywell RTH6360	2 AA alkaline, always needed, even with a C wire
Honeywell FocusPRO 6000, T6 Pro	AA alkaline; only a backup when wired to the system’s power
Nest Thermostat	2 AAA alkaline, as a backup
Nest Learning Thermostat, Nest Thermostat E	Built in and not replaceable; charge it by USB
Sensi smart thermostat	2 AA alkaline
Sensi Lite	2 AAA alkaline
Sensi Touch, Touch 2	None
ecobee	Runs on the C wire, or ecobee’s Power Extender Kit; its SmartSensor takes a 3 volt CR2477

Changing them

Check 1

Swap them in under two minutes

Honeywell. Set the system to Off, pull the thermostat away from its mount and fit fresh batteries the way the holder shows. On the RTH6360 and FocusPRO 6000, Honeywell says that if the new ones go in within two minutes, the time and day do not need resetting. The RTH2300 keeps everything but the clock and the day.

Nest Thermostat. Google says to pull the display off the base, take out the old batteries from the back, fit two new AAA alkaline batteries and push the display back until it clicks.

Sensi. Sensi says to grip the face plate with both hands and pull it straight towards you, take out the batteries and let the display go blank. Fit fresh ones and push the face plate back until it snaps, then allow 1 to 5 minutes for it to reconnect to Wi-Fi.

ecobee. ecobee says its smart thermostats are powered through the C wire, or its Power Extender Kit where there is none. Its SmartSensor takes a 3 volt CR2477 with up to five years of life, and ecobee says CR2477N batteries are not compatible.

When to change them

A Honeywell warning is advance notice. A Nest Learning or E running low is telling you about its power supply, not its battery.

Check 2Free to check

Change them on the warning, or once a year

The warning. Honeywell’s RTH2300 flashes Lo Batt for 60 days before its batteries run out. The RTH6360 shows a low battery icon about two months ahead, and the FocusPRO 6000 flashes REPLACE BATTERY about two months ahead.

The alert codes. On the RTH6360, alert 405 means the batteries are getting low and should be replaced within two months. Alert 407 means they are almost flat.

Once a year anyway. Honeywell says to replace them once a year even with no warning, and before leaving home for more than a month.

Nest. Google says a Nest Thermostat shows the alert on its display and in the Google Home app, where you can also check the battery level.

A low battery warning that keeps coming back

Check 3Free, or a C wire adapter

Look at the power, not the batteries

Nest Thermostat. Google says repeated low battery messages can mean the C wire needs checking, and that without one you may need to add one so the backup batteries stop draining. Heat only, cool only, zone and heat pump systems need a C wire or the Nest Power Connector.

Nest Learning or Thermostat E. Google says that if the battery will not stay charged, it is usually because the thermostat is not getting enough power from the wiring. To keep the heating running, it may turn off the screen and drop Wi-Fi.

A USB charge. Google says a charge takes about half an hour, or up to 2 hours if the battery is flat. It gets the thermostat going again but does not fix the power: a Nest that is not working.

The C wire itself. What it does, and the ways to add one: thermostat wiring.

A blank screen after new batteries

Check 4

Check the fit, then the power

The batteries. For a blank display, Honeywell’s battery models list one check: fresh alkaline batteries, fitted correctly. Google says a low battery message at setup can mean loose batteries or a piece of the battery tab still stuck to the contacts.

The power. On a Honeywell wired to the system, such as the T6 Pro, the list adds the circuit breaker, the power switch at the furnace and the furnace door closed securely.

Still blank. The full checks, in order: a thermostat that is not working, or by brand: Honeywell and Sensi.

Questions people ask

What battery does my thermostat take?

Many Honeywell and Sensi thermostats take two AA or two AAA alkaline batteries. A Nest Thermostat takes two AAA. Check the holder on the back, or the table above.

How often should I change thermostat batteries?

Honeywell says once a year, even with no warning, and before leaving home for more than a month. Its warnings appear about two months before the batteries run out.

Will I lose my settings when I change the batteries?

Not on a Honeywell. It keeps its settings in permanent memory. On the RTH6360 and FocusPRO 6000 the time and day are kept too if the new batteries go in within two minutes; the RTH2300 needs its clock and day set again.

Does a Nest thermostat have a battery?

The Nest Thermostat takes two AAA alkaline batteries as a backup. The Nest Learning Thermostat and Nest Thermostat E have a built in battery that charges from the wires and cannot be replaced.

Does an ecobee have a battery?

ecobee says its smart thermostats are powered through the C wire, or its Power Extender Kit where there is none. The batteries are in its SmartSensors: a 3 volt CR2477.

Why does my thermostat still say low battery with new batteries?

Check they are seated firmly and the right way round, with no battery tab left on the contacts. On a Nest, Google says a warning that keeps coming back points at the C wire.

Sources: Honeywell Home (Resideo) RTH2300 operating manual 69-2326EF, RTH6360 quick installation guide 33-00201ES, FocusPRO 6000 user guide 69-1921EFS, FocusPRO N100 battery replacement and the T6 Pro installation instructions; Google Nest Help "How to change Nest Thermostat batteries", "Batteries for Nest products" and "Nest thermostat USB port"; Sensi (Copeland) "How do I replace the batteries on my Sensi?"; ecobee's Power Extender Kit page and SmartSensor article