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If you use Nest thermostat in your home, sometimes, it could happen that you experience the Nest low battery issues. There could be various reasons for the low battery and you will need to find the root cause. There are a couple of methods to fix the Nest thermostat low battery problem.

Generally speaking, there are few common ways to fix Nest low battery issue:

  1. Charge the built-in battery. This method helps in cases when the battery is drained way below the minimum operating level of 3.8V.
  2. Check and troubleshoot the wiring between your Nest and your HVAC system. If Nest is improperly wired to your HVAC system, or if there are third party issues with the HVAC system, then, the Nest won’t get the required power supply, therefore the battery will start draining.
  3. Power the Nest thermostat using a compatible C Wire Adapter. If your HVAC doesn’t have a C wire, or it has a C wire that doesn’t work properly or the wires used to power the thermostat do not provide enough power, then this may also lead to battery drainage. In this case, connecting the Nest with a stand-alone C wire adapter (24V transformer) will solve the low battery issues.

The Nest thermostat uses the voltage from your HVAC wires to be powered on and also to keep its internal battery charged. If there is a blackout, or your HVAC system is powered off for any other reason, the battery has a capacity to continue working for a while. The thermostat will also turn off some of the features such as WIFI in order to preserve the battery life. However, if the battery continues to drain, the thermostat will shut down completely.

Let me explain each of these methods individually.

1. Fix Nest Thermostat Low Battery By Charging the Battery

A typical situation to use this method is when your Nest thermostat has been kept in a box for too long. This is normal because even if it is not in use, the battery drains slowly over time. If the battery level is below the minimum operating level, it will become unusable.

Another situation is if your HVAC system has been turned off for a longer period of time. In a typical installation, the Nest thermostat gets power from the HVAC system which also keeps the Nest battery charged. If the HVAC system is powered off, then your Nest thermostat will also power off in a few hours due to battery drainage.

Each Nest thermostat has a built-in USB charging port on the backside that can be used to manually charge the battery.

How to charge Nest thermostat battery:

  1. Pull off the Nest display and use the USB port on the back to charge it using USB cable and wall charger (typical Android wall charger is fine). The USB cable should be either micro USB or mini USB, depending on the model you have.
  2. Charge the battery for 2 to 3 hours.
  3. Connect the display to the thermostat base and using the Menu settings, check whether the power is 3.8V.

2. Troubleshoot the Wiring Between the HVAC and the Nest Thermostat

Troubleshooting in general, and specifically troubleshooting the wiring between the HVAC and the Nest thermostat is something that depends on many factors, thus it doesn’t make sense to be covered in this article.

However, if I could sum up the main check points to troubleshoot the wiring between Nest thermostat and your system , here they are:

  • It all starts with checking the compatibility between your system and the Nest thermostat. There is also a compatibility checker that you can run to see whether your existing system is compatible with the Nest thermostat.
  • In most cases, the Nest thermostat is powered from wires coming from the HVAC system. In some cases, it is powered with the system’s wires for heating and cooling, and in others, C wire is also required in order to deliver enough power, and in a consistent way.
  • The type of wires also plays a role in troubleshooting. Check this resource to read about which wires are supported and which are not. Maybe, you will need a separate stand-alone power supply for your Nest thermostat.
  • Consult a professional electrician to check how much power is being supplied through the wires – RC wire (red) and C wire (blue). If you are experienced, you can do this using a voltmeter.
  • Check the fuses on your system’s control board. A blown fuse may prevent the power to reach the Nest thermostat.
  • Some HVAC systems are equipped with many sensors which make them pretty sensitive to various fluctuations in terms of power, current, etc. You may contact a local Nest Professional who can check the wiring, and potentially service your HVAC system.

3. Power the Nest Thermostat Using a C Wire Adapter

This approach is used when you can’t solve the problem with powering the Nest thermostat through the HVAC system. More specifically, this method is applied if using C wire doesn’t work or the HVAC system does not provide enough power to the thermostat, or simply they are not compatible with each other.

In this case, using a Nest compatible C Wire Adapter will do the job.

Nest Thermostat C Wire Adapter

Here are the steps how to use the C Wire Adapter:

  1. Shut off power at the breaker
  2. Install any wire from your 24 volt adapter to the “C” terminal and the other wire to the “RC” terminal. If the HVAC system has a cooling, you may need to use the included jumper and connect from “RH” to “RC” terminals
  3. Plug the power adapter into the outlet
  4. Turn the power on at the breaker
  5. Attach faceplate to your thermostat

How to check Nest thermostat battery level?

Press the thermostat ring to open Quick View Menu, then go to Settings > Technical Info > Power. Find the number labeled Battery (it should be the first one).

If the battery level is 3.8V or higher, then the battery is in good condition.

What are the Nest Thermostat battery levels and what they mean?

Press the thermostat ring to open Quick View menu. Then choose Settings > Technical Info > Power. You will find the values for Battery Level, VOC, VIN, and IIN.

Battery Level represents the current voltage of the thermostat’s internal rechargeable battery. It varies from 3.6V to 3.8V.

  • 3.7V is required to install a software update.
  • 3.6V is required for the display to turn on when you approach.
  • below 3.6V, your thermostat will turn off Wi-Fi, delay software updates, and disable the screen to preserve the battery charge and delay shutting down completely.

VOC, VIN, and IIN measure the electrical current sent over your system’s wires to the Nest thermostat.

For more info check this link.

How Long Does Nest Thermostat Battery Last Without Power?

If your HVAC is turned off, or if there is a power outage and there isn’t a continuous power supply to the Nest thermostat, the Nest thermostat battery can last one to two hours.

Nest thermostat battery won’t charge

There might be many reasons why your Nest thermostat battery won’t charge. In order to isolate the problem, the best approach would be to use a wall charger with an appropriate USB cable and charge the battery using the designated USB port located on the thermostat’s back.

Check the battery after 3 hours. If it is charged, then most likely you have a problem with the wiring between your HVAC system and the Nest thermostat.

In case the battery is not charged after 3 hours, or it couldn’t be charged at all, then possibly the battery is drained way below the minimum operating level and has entered in “sleep mode”.

If the battery is still within the warranty period, you may ask your supplier for replacement, otherwise, you can try to jumpstart the Nest thermostat battery. If you want to see how you can revive Li-Ion batteries yourself, check this article which explains how to do it.

Why is my Nest thermostat blinking red?

A blinking red light on the top of your thermostat’s display means that the battery charge is very low but it’s being charged. There are different troubleshooting steps depending on when you see the red light. For more information visit this resource.

What type of battery does a Nest thermostat use?

Nest thermostat uses 3.7 VDC (380 mAh) Lithium-Polymer battery pack.

If you need to replace the Nest battery, you can check this battery pack on Amazon which is compatible with Nest learning thermostat (2nd and 3rd generation models), as well as other models. The full list is provided in the product description section when you open the link.

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