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The battery readings on your Nest thermostat help show the state of electric current flowing through your thermostat. You will want the voltage to be at ideal levels for the thermostat to keep functioning properly. Below these levels, the thermostat may not be able to regulate your temperature correctly.

Checking the battery voltage helps you know where to start in diagnosing your system. 

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You might also need to install the C-wire on your Nest thermostat as it can likely help address problems with your HVAC system. Let’s get you started.

What is the ideal Nest Thermostat battery voltage?

You can check the current voltage of your internal battery through the display. You’ll find this in Settings, under Device Information, in the Power section. The Battery label shows you the voltage.

For some specific actions, a certain voltage is required to activate them. When the voltage falls some features like Wi-Fi, software updates, or motion sensing cannot function. In lower voltages, the Nest thermostat will also preserve the battery and disable the screen.

You’ll ideally want to keep your Nest thermostat battery voltage above 3.6V. This is a threshold level for the thermostat. A dip below this is a signal for the thermostat to preserve battery and disable features.

Below 3.6V the display won’t automatically turn on when sensing you come close. A 3.7V level is required for software updates.

What is the ideal Voc, Vin & lin voltage on a Nest Thermostat?

In the Device Information > Power section of Settings, you’ll find the Battery section. Here you’ll find Voc, Vin, and lin readings of your Nest thermostat. These indicate the flow of electricity through your wiring. Check that these readings don’t go below a certain level.

For Voc, Google advises it stays from 29V to 42V.

For Vin, 29V to 42V is ideal when you have a C-wire connected. If not, that is also the ideal range when the system is not operating, and 6V to 7V when it is.

For lin, 100 mA to 200 mA is best when a C-wire is connected. Otherwise, 20 mA to 40 mA is ideal.

How to check voltage with a voltmeter/multimeter

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.

HVAC systems in the US are typically 24V. If you're having issues with battery voltage, you will need to confirm the correct voltage is through the R wire and out the W, Y, or C wire. There might be a problem with the fuses or wiring, possibly a short circuit.

To check voltage, you’ll have to use a voltmeter or multimeter. Only do this if you are competent and comfortable in handling electrics. 

You’ll have to set your voltmeter to ṽ mode. Select the next voltage range higher than 24V rounded up. You’ll be holding the red probe on the red wire or Rh-terminal most of the time. While the red probe is held on the red wire, alternate measuring the black probe on the other non-red wires/terminals. Check if you’re getting voltages from 24V to 28V.

We recommend calling a professional to get a broader scope of the problem and solutions. They can inspect inconsistent voltage, or faulty, improper, or loose wiring, and other electrical issues that might be contributing to equipment problems.

What if my Nest Thermostat is continually showing a low voltage?

For most low voltage warnings you probably need a C-wire to provide the needed power.

Hardwire a C-Wire

To install a C-wire, you have to connect both ends (usually in blue) to your HVAC system and Nest thermostat. This is commonly the furnace of your system. Before doing so, it’s best to ensure that the power is off. Check the switches for your HVAC on the breaker. Don’t forget to also use recommended safety equipment.

The task of installing a C-Wire is complicated by the fact that the walls are typically covering over the wiring. You will need to try and fish the wire through or create some holes to work in and patch up after. 

Install a C-Wire Adapter

This would be your next option if you don’t have a C-wire, or installing one is impractical.

There are a couple of approaches available. The first is to use a system by Fast-STAT while allows you to wireless provide a c-wire.

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 required

The second is a little more unsightly and requires a local power outlet. Most adapters are easy to install like the Fyve adapter and also work well with the Nest thermostat. 

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.

These adapters can look unsightly with wires hanging from your thermostat. You could use trunking to hide the wiring or fish it down the wall to come out by the power outlet.

Reuse your G Wire as a C Wire at both the thermostat and the furnace

Check your Nest thermostat wiring and you’ll likely find a G-wire, conveniently in green. You have the option to use this for the C-terminal if you don’t have a C-wire in place. This is a convenient way to get the needed power to your thermostat. But it does have its caveats.

Since the G-wire is the fan wire, you’ll have to sacrifice fan functionality when using this. You can use your fan independently or in “fan-only” mode. Same as installing an adapter, this would also be challenging if you have a zoning panel. Check also your fan for any other connected equipment.

You might also need to install a jumper cable between the empty G-terminal and Y-terminal.

Conclusion

Maintaining the correct amount of power to your Nest thermostat is important for it to function properly. If you find your HVAC system delaying heating and cooling for longer than usual, try to check the battery levels on your Nest thermostat. You’ll want them at the ideal levels specified by the manufacturer.

If the voltages are off, your system likely won’t be able to function correctly. In most cases, attaching the C-wire will get your system back up and running.

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2 Comments

  • Gordo
    Posted January 31, 2021 at 12:07 am

    Thanks for providing the ideal ranges for Voc, Vin, and lin. It’s helpful to know these, as I am trying to resolve a low battery issue with multiple Nest thermostats. They all worked fine for 2-3 years, but now they are all going into low battery mode every few days. It seems that when the units are calling for heat, they start to run low on power.

    Knowing the ideal ranges for these values is helpful, but I would like to know what Voc, Vin, and lin actually mean… any insight there?

    Also, on the value for lin, I see something like 40 mA (i) or 20 mA (a) — any idea what the i and a mean?

    Thanks so much for your help!!

    • Esai
      Posted September 19, 2021 at 8:11 pm

      Probably, you are missing a C-wire. I had seen similar issue with my friend. Following the above mentioned solutions would fix the problem.

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