The battery in the HT1 sensor device generally has a 1 to 2 year lifespan, often closer to two (more in warm conditions and less in extreme cold), but there are a few details to consider when changing it.
Thankfully, the CR2477 model battery is a replaceable battery. You will mostly likely need to purchase the replacement online from a specialized battery retailer. One possible vendor is DigiKey (https://www.digikey.com/product-detail/en/panasonic-bsg/CR2477/P120-ND/107123). They're a very reliable electronic components retailer. Grainger is another good source, as is Mouser Electronics.
When it's time to change it, the process is quick and easy. Just follow these simple instructions in the video below.
(Note: the sensor in this video is from an early production run. Later model sensors do not have the additional wires and disc on the inside of the device).
The easiest way to open the case is with an old credit card or similar plastic card. Hold the sensor with the back facing you (the side with the seam) and with the text upright. Using a corner of the credit card, slide back and forth along the top seam pushing inward until the card penetrates slightly (squeezing the left and right sides of the sensor can help).
Then slide the card along the top edge to the left side and pry outward and the back cover will come off. The circuit board will then come out on its own. It's positioned by two plastic pegs but is only held in by the back cover. The battery is on the back of the board. It's a CR2477 lithium coin cell. The + side of the battery should face away from the circuit board.
Reassembly is the reverse, with one trick: it's easier to replace the back cover if you put the top side in first, as the bottom has two smaller pegs that are designed to snap in more easily.
After you change the battery, if your sensor does not pick up in the app after a few moments, please walk through the steps listed at this link: I changed the battery in my sensor, so why is my Gateway not picking it up?
Going Forward - Here's how to check the battery voltage:
1.) From the home screen with the word "Devices" at the top, please tap the sensor.
2.) Scroll to the bottom of the following screen (this is the screen with the time graph). You should see "Battery Voltage."
Anything LESS than 2.3V is when it's too low and you should replace the battery. Also, "Battery Low" appears next to the sensor name once it's low.