Thermostat low battery message
A low battery message means the AA or AAA batteries in your thermostat are running down. How much warning you get varies: Honeywell says about two months on the T4 Pro, the VisionPRO TH8000 flashes "LO BATT" about 30 days ahead, and the Lux P521Uc gives about two months. Replace them with new alkaline batteries soon.
What does the low battery warning look like?
It depends on the brand and model. Some thermostats show a battery icon, some flash words like "LO BATT", and Sensi changes your setpoint as a signal. Here is what the manufacturers describe, and how much time each one says you have left:
| Thermostat | Batteries | What the warning looks like | Time left once it appears |
|---|---|---|---|
| Honeywell Home T4 Pro and T6 Pro | 2 AA, alkaline recommended | Low battery icon; then "BATT" with the alert and battery icons | About two months |
| Honeywell VisionPRO TH8000 | 3 AAA alkaline | "LO BATT" flashing | About 30 days |
| Sensi Smart | 2 AA alkaline | Battery indicator bars; setpoint jumps to 85 (cool) or 55 (heat) | Not stated |
| Sensi Lite | 2 AAA alkaline | Setpoint jumps to 85 (cool) or 55 (heat) | Not stated |
| Lux P521Uc | 2 AA alkaline (Energizer or Duracell) | Battery graphic down to 1 bar; temperature and clock alternate with "LO BATT" | About two months |
Sensi Touch and Touch 2 thermostats have no batteries, so they never show this warning. Google Nest thermostats work differently from all of the above; see Nest thermostat low battery.
What happens if you ignore it?
The thermostat gives up features in stages, and eventually it may stop running your system.
Honeywell's T4 Pro and T6 Pro guides describe three steps. First, "BATT" appears on the Home screen with the alert icon and the battery icon, which means the batteries need replacing. When power gets very low, the backlight switches off to save what is left. When it is critically low, the screen shows nothing but "BATT", the alert icon and the battery icon.
Sensi says that on its battery models, low batteries may leave the thermostat unable to turn on your heating or cooling. Lux says that once its P521Uc shows "LO BATT", you should replace the batteries as soon as practical so the heating or cooling is not interrupted.
If the screen has already gone dark, that is a later stage covered in thermostat screen is blank.
Why did the temperature setting change by itself?
On a Sensi Smart or Sensi Lite, a jump in the setpoint is the low battery warning. Sensi says that if the thermostat is in cool mode and the setpoint changes to 85, or in heat mode and it changes to 55, you should replace the batteries. You can also check battery status in the Sensi app, under About Thermostat in the settings.
That jump is easy to mistake for a schedule change or a hold someone set. Before you go looking through your schedule periods or a vacation hold, check whether the new number is exactly 85 or 55.
How often should you change thermostat batteries?
Do not wait for the warning. Honeywell says to replace the batteries in the T4 Pro and T6 Pro once a year even if the icon never appears, or before you leave home for more than a month. The VisionPRO TH8000 manual gives the same once a year advice. Lux says the P521Uc batteries should be changed at least once a year, or sooner if "LO BATT" appears.
Use alkaline batteries. Honeywell recommends them, Sensi specifies AA or AAA alkaline depending on the model, and Lux names Energizer or Duracell alkaline only.
Do the batteries run the thermostat, or are they a backup?
On several models they are only a backup. Honeywell says batteries are optional on the T4 Pro, T6 Pro and VisionPRO TH8000 if the thermostat was wired to run on system power, and required if it was not. The Sensi Smart manual says its two AA batteries are for backup and tells you to replace them when the indicator reaches 3 bars to keep the Wi-Fi connection.
If a thermostat with constant power still eats through batteries quickly, the power supply may be the cause, and that is wiring work for a licensed HVAC technician rather than something to change on the thermostat.
What should you do now?
Does the thermostat take AA or AAA batteries?
- Yes
Is the screen still lit and responding?
- Yes
Replace the batteries with new alkaline ones of the size your model takes. On a Sensi, allow 1 to 5 minutes for it to reconnect to Wi-Fi.
- No
Replace the batteries. If the screen stays dark with new ones fitted the right way round, check the signs at the bottom of this page.
- Yes
- No
It may be a Nest with a built-in battery or a model powered only by the system. See the Nest low battery page, or the blank screen page if it is dark.
The reconnect time comes from Sensi's battery page. If a Sensi or any other smart thermostat stays disconnected after that, see thermostat offline. For everything else on Sensi thermostats, see the Emerson Sensi hub.
Frequently asked questions
- Will I lose my schedule when I change the thermostat batteries?
- Not on a Honeywell Home T4 Pro or T6 Pro. Their guides say settings are kept in permanent memory that needs no battery power, and if the new batteries go in within two minutes you will not even have to reset the time and day.
- Why did my Sensi change the temperature to 85 or 55 by itself?
- On the Sensi Smart and Sensi Lite, that is the low battery signal. Sensi says a setpoint that jumps to 85 in cool mode or 55 in heat mode means you should replace the batteries.
- Does my thermostat need batteries if it has a C-wire (common wire)?
- On several models the batteries are only a backup when the thermostat is wired for constant power. Honeywell says so for the T4 Pro, T6 Pro and VisionPRO TH8000, and Sensi says the AA batteries in the Smart are for backup. Without that wiring, batteries are required.
- Can I wait until the batteries die completely?
- It is a bad idea. Sensi says low batteries in its battery models may leave the thermostat unable to turn on your system, and Lux says to replace them as soon as practical once "LO BATT" shows, so the heating or cooling is not interrupted.