One of mine had a battery and the other didn’t. These are truly “open-box”. They take the weirdest battery size I have ever seen. CR2477. It’s like a button cell battery that you would find in your cars’ remote, just way thicker. Had I known this, I would have gone with the Honeywell. At least that one is compatible with the Ring app and uses batteries that you can find just about anywhere.
You bought this online and you are complaining about going to a store to buy batteries? Why not just buy some on Woot’s parent company… you know… Amazon (https://www.amazon.com/Panasonic-CR2477-Litium-Coin-Battery/dp/B002LHE678)? You still don’t have to leave the house. Even if you do have to buy a new battery, it’s still cheaper than hopping in the car and getting these at Home Depot (https://www.homedepot.com/p/ecobee-SmartSensor-2-Pack-EB-RSHM2PK-01/309221992).
There’s a bigger issue with the batteries other than ecobee deciding to go away from a more standard button battery size like 2032… They apparently drain within weeks. Hopefully, they can remedy the situation with a firmware update instead of a component swap in new models.
These arrived (with batteries) and are setup and working with my eco bee, confirming that my office is warmer than the main thermostat temperature… but that’s it. Now both me and the ecobee know my office is warmer than desired, but it doesn’t do anything in response to make it cooler.
Easy to install etc, maybe I was expecting too much. My main relief is that these were a woot bargain and I didn’t get gouged further by bestbuy. Woot!
Hi! Make sure you add the sensor to be participating in the comfort setting/s.
That’s what tells it to average out the temps.
I’ve had mine (have two ecobee thermostats and two sensors) for about a year and the batteries are still good in them, where is that info from?
Interesting! You are using the newer Smart Sensors, correct?
I read a bunch of reviews on the Smart Sensors that stated the battery life was a big issue for high occupancy areas that are constantly picking up movement/heat (whatever sensor they use to detect occupancy). I set up my parents’ home with ecobees both upstairs and downstairs with a few of the older Room Sensors… Thought about upgrading them to the Smart Sensors to add some coverage, but was spooked off by the reviews that I read. If the battery died and I wasn’t around to check on em, they’d never get replaced! 
I have the smart sensors and the battery life has been good, I’ve had them for a year so far. If you have an apple HomeKit you can use them as motion detectors as well.