If you are like me, you are thinking what. I had no idea our thermostat worked on batteries until I saw the blinking light on the thermostat itself. It read change batteries. I looked around the thermostat until I saw the side lifted right out of the thermostat. I grabbed that side, pulled out the two batteries and replaced them. The part of the thermostat goes in just like it goes out. There is even markers to show you where to put the positive and negatives of the new batteries. Now that I know my thermostat takes batteries, I will keep my eyes open for that blinking light.