Many people now enjoy the benefits of a Nest Thermostat to control their HVAC system, but depending on your installation, you may experience the Nest Thermostat low battery issue.
The Nest Thermostat is powered by your HVAC system and has a battery backup. The battery backup is only designed to work for a short time while the main power is off. If the mains power is not restored, your batteries will go flat, and an error will show on the Nest display.
Solution 1: Charge/ Replace Flat Batteries
Your batteries have gone flat. Depending on your Nest thermostat, the fix is different. The Nest Thermostat has replaceable AAA batteries, while the Nest Thermostat E and Nest Learning Thermostat have a built-in rechargeable battery.
For Nest Thermostat:
- Remove the thermostat from the base
- Replace the AAA batteries in the back of the thermostat
- Push the thermostat onto the base until it clicks
For Nest Thermostat E or Nest Learning Thermostat:
- Pull the thermostat from the base
- Connect a USB charger to the charging point
- Refit the thermostat once the battery is fully charged
Solution 2: Fix The Nest Thermostat Mains Power Disconnected Issue
Your Nest Thermostat (all versions) is powered by mains electricity, and the batteries are only there in case power is disconnected.
There can be several reasons why the power is disconnected:
- Your HVAC system power is off
- Damaged HVAC cables
- Blown fuse or circuit breaker tripped
Your HVAC system power is off: How to troubleshoot?
This is the most common cause of flat batteries in a Nest Thermostat. When your HVAC system switches off, it also disconnects power from your Nest Thermostat. Luckily, there are simple solutions to this problem.
Firstly, try to connect a C (Common) wire
The C terminal in your HVAC system can be connected to the C terminal in your Nest Thermostat. This will provide a constant power supply saving your batteries for emergency use only and keeping the rechargeable battery of the E and Learning Thermostats fully charged.
The cable for the C wire connection may have been installed by your HVAC professional but not connected. If that is the case, connecting the cable is a simple process.
In your installation, you may find a spare cable, as some systems don’t use all the control wires. In this case, use one of the extra cables to connect the common terminals.
If there isn’t a spare cable, you will need to run a wire from your HVAC system to your thermostat. Depending on the relative locations of the HVAC and the thermostat, this could be a challenging job.
How to run a new furnace cable to install a Nest Thermostat C-wire video;
Then, try to connect a USB adaptor
If you have a Nest Thermostat E or Nest Learning Thermostat, the backup battery is rechargeable via a USB connector on the back of the unit.
To ensure the battery remains permanently charged, you can connect a USB charging adapter.
You will need a mains outlet close to your thermostat. If one isn’t available, an electrician should be able to install one easily. It is then a simple job of plugging in the adaptor and connecting a USB charging cable to the Nest Thermostat USB port.
Fitting a USB adaptor for your Nest Thermostat video:
Also, try to connect a Nest Power Connector
Nest has recognized an issue in some installations where the HVAC system does not consistently power the thermostat. To overcome this simply, they have produced the Nest Power Connector.
The Nest Power Connector is ideal if you do not have a c wire installed. Instead of running a cable from the HVAC system to the thermostat, the Nest Power Connector uses the already installed thermostat cables.
Installing the Nest Power Connector video:
Damaged HVAC cables problem: What should I do to fix it?
While this is not the most likely cause, it is worth checking.
- If the C terminal on your Nest Thermostat is connected, you need to find out why there is no power.
- Use a multimeter to check the voltage on the C terminal, both at the HVAC and the thermostat.
- If power is available at the HVAC but not at the thermostat, there is a problem with your system wiring.
Depending on how confident you are, you could trace the fault or call an electrician.
Blown Fuse or Circuit Breaker Tripped: What to check and how to fix?
Fuses and circuit breakers (CB) are used to protect all electrical systems. It may simply be a blown fuse or tripped CB causing the problem. In this case, replace the fuse or reset the CB.
Fuses and CBs trip for a reason and do so to protect the electrical circuits. It is vital to note when this happened. If it happens again, there may be a more serious problem that needs investigating.
If your system fuses blow or CBs trip regularly, get a professional electrician to check the circuits. Resetting a CB regularly could damage your system and is a potential fire risk.
Do Nest Thermostats Have Batteries?
Do nest thermostats have batteries?
Yes. Although the Nest thermostats are mains powered, all versions include a battery backup for power outages.
How long does the nest thermostat battery last without power?
The battery backup should last for around two hours.
How To Replace The Batteries?
How do you replace the Nest Thermostat batteries?
- Unclip the Nest thermostat from the base
- Remove the AAA batteries from the rear of the unit and replace them with new, fully charged batteries
- Push the thermostat back onto the base until it clicks home
What type of battery does the Nest Thermostat use?
The type of battery in your Nest Thermostat will depend on your version.
The original Nest Thermostat had two AAA batteries, which are easily replaced when required.
The Nest Thermostat E and Nest Learning Thermostat have a rechargeable battery fitted. This is easily recharged via the USB connector on the back of the Nest unit. The battery is not designed to be user-replaceable.
Find purchasing options for the best AAA Alkaline Batteries online:
Duracell AAA Alkaline Battery
Energizer AAA Alkaline Battery
How often should you change batteries?
I recommend replacing the backup batteries once per year to ensure you always have a backup system. If you had a power outage for a significant time, replacing the batteries may be wise.
Where to buy Nest Thermostat battery?
AAA batteries are available in grocery stores and online.
Final Thoughts
The Nest Thermostat system is an excellent solution for saving money on your HVAC running costs, but it must be connected to a constant mains power supply. If the mains power is removed for any significant period, you could see the low battery warning.
You can eliminate the warning and enjoy your Nest Thermostat without issues by following my suggestions above.
