No matter which model you are using, be it the 1st, 2nd, 3rd generation, or the latest Nest Learning or E Thermostats, they all come with a built-in battery. In this guide, we explain its purpose, life-span, and what to do if it won’t charge or is showing as low.
Purpose of the Nest Thermostat battery
The Nest thermostat battery is charged directly from your HVAC system via the Rh and/or Rc wires. A C-wire or common wire is also used as a neutral to complete the circuit. The wires provide a continuous flow of power. This helps maintain power to the thermostat, even if your HVAC system is off for a prolonged period of time.
The purpose of the battery is to power the Nest interface even without direct power flowing from the HVAC unit. This also helps in cases where there has been a power cut.
Nest thermostats come with backlit displays, Wi-Fi activity sensors, and many other advanced features. All of these have to be powered by the battery.
How do I charge my Nest Thermostat Battery?
In most cases, the Nest thermostat will maintain its charge from the Rh and/or Rc wiring from the HVAC unit. That means it will not have to be manually charged.
When your HVAC system is running, the thermostat charges itself. It relies on battery power when the system is off. If the HVAC unit is off for a prolonged period of time, the thermostat can cycle the unit on and off for short periods to give the battery a boost in charge.
If you are experiencing a low battery warning, the battery is failing to charge itself or your Nest thermostat has died, there are a few ways you can fix the issue.
Option 1: Ensure the Nest thermostat is correctly connected to the base
Sometimes, if the thermostat is not correctly mounted onto the base, the connectors are not lined up correctly and the power cannot flow through.
Option 2: Manually charge via USB cable (Temporary Fix)
Take the Nest device off the wall and, depending on the model, use a Mini-USB or Micro-USB to charge the device for 1-2 hours. This is by its nature a temporary fix which will give you time to further troubleshoot the issue.
Option 3: Utilize a C-Wire to provide a constant flow of power
If a C-Wire is already installed, check the wiring to ensure it is correctly in the terminal housing.
If a C-Wire is not installed then your options are:
1. Hardwire a C-Wire Directly or Indirectly
This requires running cabling directly from your HVAC unit to your thermostat. This can possibly be a complex task as the wiring is already embedded in the walls.
Alternatively, Fast-Stat has a product which allows you to wirelessly connect a C-wire while providing a constant source of power for the thermostat. Definitely worth a look.
Includes a sender embedded in the wall behind the thermostat and a receiver installed at your furnace. Works for distances up to 300ft and no batteries are required.
2. Install a C-Wire Transformer
Possibly a cheap fix that will work for some people. Essentially, you utilize a C-wire transformer which is plugged into an AC outlet close to the thermostat and you run a cable up into the device. You can cover over the wiring with a concealer.
3. Hijack the G-wire (Fan)
This method involves utilizing the existing G-wire for your fan control as a C-wire. You swap over the G-Wire at the thermostat and then also swap over the G-Wire within your HVAC unit.
You will lose fan control, but you’ll solve your low power issue!
Option 4: If the battery can no longer hold the charge
If the unit is still under warranty, you should contact the retailer to replace the unit. If the unit is outside warranty, you can either replace it or replace the internal battery within it. At your own cost, of course.
The battery is not a serviceable part so it requires a little DIY know-how to replace it on your own. You can find detailed instructions below.
What happens if the battery is showing as low?
“Battery Low” is an indication that the battery on your Nest thermostat is running out of charge. You will also see a blinking red light on the Nest.
This can result in unusual behavior with the thermostat. For example:
- It may not be able to call the HVAC system for heat or to cool.
- The connection between your Nest thermostat and your Wi-Fi could be disrupted.
- The display on your Nest thermostat might not work or become non-responsive.
When you get this message, you should immediately charge your Nest via USB to temporarily fix the issue.
Once resolved, you will need to troubleshoot why the wiring of the Thermostat is not providing enough charge.
This will typically be down to the lack of a C-wire or an issue with the voltage coming from the Rh or Rc wire on your HVAC unit.
Check to see if your HVAC unit has power and if the breaker has tripped or a fuse has blown.
What is the Battery Life of a Nest Thermostat?
The Nest thermostat internal battery should last from a number of hours to a couple of days without direct charge from the HVAC system.
The lifespan of the internal battery depends on the type of continuous charge it gets from the HVAC system. With a constant stream of power, the battery can last 5 – 10 years without issue. However, if there is a strain on the battery, it has been known to fail after 2 years.
How do I check the Nest Thermostat Battery levels?
If your Nest Thermostat is running on a low battery, your first port of call is to determine the exact battery level. Follow the steps below to check the battery level.
- Press and hold the thermostat ring until you see the Quick View menu.
- From there navigate to Settings >> Technical Info >> Power.
- If you see your battery is 3.7V or higher, your thermostat should not be disconnected due to a low battery. The lin current should be over 20mA
- However, if the battery level is below 3.7V or your lin is or under 20mA, your thermostat can show a low battery indicator.
Is it possible to replace a Nest Thermostat battery?
If you have been using your Nest thermostat for a long time, the battery might need to be replaced. With time, the old battery starts taking more time to finish charging. It also loses its capacity. Replacing the old one with a new one will ensure the same level of performance as when you first bought it.
The Nest Thermostat battery is not a user-serviceable part as it is built into the device. If you are facing low battery or power issues, we recommend you follow the troubleshooting advice above before considering battery replacement.
For scenarios where the internal battery is completely dead and cannot hold a charge, and the device is still under warranty, we recommend you contact your provider and seek a replacement.
If the device is out of warranty, then you can either attempt to replace the internal battery or bring it to a local repair shop. Doing so will void any remaining warranty you have on the device, though.
At SmartHomePerfected we always advise readers not to undertake any work they are not competent in performing and if in any doubt, it is best to seek the assistance of a professional.
Battery Replacement Instructions
To replace the battery on your Nest Thermostat, hold on to the display’s metal outer ring, and gently pull it off of the base. Turn over the display and remove the 6mm screws from the back using a #0 screwdriver.
Hold on to the grey 20-pin connector and gently pull it off the metal plate. Now you need to disconnect the ribbon cable that powers the base. After that, you have to lift the battery from the base and disconnect it.
Check out the following tutorial on how to accomplish this – https://www.ifixit.com/
In this blog, you have seen the purpose of the Nest Thermostat Battery, how you can charge it, how to troubleshoot issues with low battery, what the expected battery life is, and how to replace it.
We hope you found this article helpful. Leave a comment below if none of the above worked in resolving your Nest battery problems.
Do you know about how much time i would need to schedule the fan to run per day to keep the battery charged if I don’t have the C wire and don’t do the Green wire changeover?
Thanks for this. VERY helpful as I bought a used Nest thermostat, installed and verified correctly hot wired to the furnace, am getting the right charge and Lin my numbers are good. However, when The furnace runs for a while (catching up more than say 5 degrees in warming (heat so far, haven’t tested cool) it seems like it *drains* the battery rather than charges it. Hmmm.
Anyways your post gives me solutions. Thanks!
Also note that your link to IFixIt for the battery replacement has changed. It’s this now:
Thanks! I’ve updated the link 🙂