One of the main reasons people buy Nest thermostats for their HVAC systems is because it comes with a rechargeable built-in battery or replaceable AAA alkaline batteries. So, theoretically, your thermostat will always have power. But as you are frequently receiving error messages that the battery is low there must be a reason why it is draining all the time. You wonder now whether you can troubleshoot and fix this problem by yourself? 

The most common reason for the Nest thermostat’s low battery warning is the improper placement of the thermostat on its base. This will prevent the Nest thermostat from charging, which will drain its backup batteries. Removing the thermostat from its base and putting it back correctly will fix this issue.

There are quite a few things you can check to try and find out why the battery is not charging. Let’s look at the most common things you can do to locate and fix the problem.

1. Make sure the thermostat is connected to the base

Sometimes the thermostat is not properly placed on the base. If it is not placed correctly, the thermostat is unable to connect to the power. If no power is flowing through the thermostat, the battery can’t be charged and will at a stage fail. 

The solution is easy. Just reposition the thermostat correctly so that everything aligns. If the problem persists, you’ll have to troubleshoot further. 

2. Replace the AAA batteries

If your thermostat is equipped with replaceable AAA alkaline batteries, replace them. The charge level should be at the maximum again. Leave the thermostat for a day or two to check whether the charge level is what can be expected after a few days. If it seems as if the batteries are draining very quickly again, you’ll have to troubleshoot to find the reason.

Replacing AAA batteries on a Nest thermostat

3. Charge the Nest thermostat’s built-in battery manually

Ideally, you should never have to charge it yourself. Because of the Rh and Rc wiring connected to the HVAC system the Nest Thermostat should always be fully charged. If the HVAC is offline, then the thermostat will rely on the battery for power.

But if the HVAC is offline for a long time, the thermostat tries to get the unit to cycle on and off, in an attempt to keep the battery charged. If you find that your Nest thermostat has a low battery warning, it means that the battery is unable to charge itself.  

Manually charge the thermostat with a USB port

You can manually charge the thermostat’s built-in battery as a temporary solution. The thermostat will soon be low on power again, but the temporary solution might give you enough time to look for the battery’s real problem and time to solve it. You can manually charge the thermostat through the use of a USB port. Charge the thermostat for about 2 hours. For more detail consult your owner’s manual.

Recharging nest thermostat battery

Manually charge the thermostat by using a C wire

If you already have a C wire installed, check whether it is properly connected, and placed in the proper terminal. If you don’t already have a C wire installed, install one. There are excellent YouTube videos available showing you how to install a C wire. 

Check the wiring between the Nest thermostat and the HAVC system

If the wiring between the HVAC and the thermostat is damaged or disconnected power cannot flow and the thermostat’s battery can’t be charged. 

Find the loose connections with the “Tech Info Diagram”

With a Nest thermostat, it is relatively easy to detect loose connections. By using the “Tech Info Diagram” feature of the Nest thermostats you can immediately see where loose connections are. To get to the diagram, go to the main error page where you’ll find the option to view the “Tech Info Diagram.” Tap on it and the connection diagram on the thermostat will open.

The diagram highlights all loose connections in red. Note down which wires are red and should be checked and reconnected.

Example of finding loose wires using Nest tech info diagram

Reconnect loose connections

Turn off your HVAC system’s electricity completely. Although the thermostat’s wire only carries a 24-volt current, it is always good to turn off the power to the whole system before you start checking or working on your thermostat and other HVAC components. 

Then remove the thermostat display from the base on the wall and inspect all connections and wires.

Ensure that they are all connected, and if some wires are loose, re-insert them into the correct thermostat terminals. Remember, to re-insert a wire properly, you have to press down the connector button and push the wire as far as it will go. The wire has been properly installed if the connector button remains in its position when you let it go. 

Always remember that if there are loose wires to be re-connected, some wire ends may need to be stripped again. Approximately 1/3 to 1/2 inches of straight, corrosion-free copper must be exposed.

When you’re satisfied that all the connections are securely in place and correctly aligned, fit the display back to the base. 

Put the power on again and wait for about two to three hours. Then check whether the “battery low” error code is still coming on. 

Check whether the wires and thermostat are compatible

For your Nest thermostat to work effectively the existing wiring in your home must be compatible with your Nest thermostat. If it is not compatible the power might not be flowing and the battery not being charged. Wires which are not compatible can even cause the draining of the battery.  However, this is usually not the problem if the system has worked for quite a time already and is only now encountering problems. 

But if you‘ve bought your Nest thermostat recently, you can use a compatibility checker tool to check the compatibility. If the old wiring and the new thermostat are not compatible, you’ll either have to get another thermostat or get an electrician to modify your existing wiring. 

Check for a blown fuse

A blown fuse could prevent the power to reach your Nest thermostat and then the battery can’t be charged. 

The fuse is connected to the control unit within your HVAC. You can quickly check to determine whether the fuse is working. 

Turn off the power to your HVAC system and remove the fuse. If the connector center and fuse housing are full of dust, clean them. 

The fuse has a transparent “window” with a u-shaped wire visible inside. The fuse is blown if the u-shaped wire is broken. Then you have to replace it. If the u-shaped wire is still intact, ensure that the contact points on the fuse are clean and put the fuse back into the cleaned fuse housing.

HVAC fuse replacement

When you replace the fuse, ensure that you replace it with the same model as the one that has blown. The color of a fuse is meant to indicate the current rating. So, for example, if the old fuse is purple, ensure you replace it with a purple fuse.

Also remember that if you’ve got a blown fuse, there is a great possibility that your HVAC has other underlying issues that caused it. When you’ve replaced the blown fuse and it blows again it is recommended that you contact a licensed technician to rule out and fix any issues. 

Replace the battery

If you’ve checked and solved all the wiring issues, and the thermostat is in position and correctly wired and connected, but the battery stays on low, then the problem is the in-built battery itself. 

If you are still within the warranty period, Nest will replace the battery. If the warranty has expired you can still get the battery unit replaced but at your own personal cost. 

More about the Nest thermostat’s battery/batteries

Built-in battery

The main reason why there is a rechargeable built-in battery in a Nest thermostat is that the battery can power the Nest interface, even when the HVAC unit is powered off.

The Nest thermostat’s built-in battery is designed in such a way that it is charged directly from your HVAC system, through the Rh and Rc wires. Commonly, a C wire is used to complete the circuit if needed. This system provides a continuous flow of power so that the thermostat’s battery is always fully charged. 

When the power is interrupted, the system normally still has some power running through it, and the battery should be able to power the thermostat. 

When the built-in battery charge is frequently on a low level, there might be an issue in the system that is either draining the battery continuously or preventing the charging of the battery. Your troubleshooting will have to look at both possibilities.  

Replaceable batteries

Some Nest thermostats use AAA alkaline replaceable batteries. They fulfill the same function as the built-in battery but have to be replaced when their power is low. If they frequently lose power because they are continuously needed for backup, you have to troubleshoot the reason why backup power is needed so often.    

Apart from error messages, how can you detect a faulty battery?

Usually, your Nest thermostat will display a warning code for a low battery. Sometimes it can show you that the battery is running low by displaying a blinking red light. 

There are also other indicators that can alert you that your thermostat’s battery is low on charge. You can watch out for any of the following signs:

  • The thermostat will start to act differently than usual.
  • The connection between the Nest thermostat and your Wi-FI is frequently failing.
  • The display of your Nest thermostat might be blank from time to time or not be responding. 
  • The temperature is not always properly regulated. 


There are several reasons why a Nest thermostat’s battery can be low. It is often caused by a lack of power flowing to the thermostat. If the thermostat is not receiving power, the battery cannot be charged and the thermostat and HAVC system are using the battery to keep on working. This can drain the battery over time. 


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