Three things: I am glad thar someone mentioned that there is plastic wrap around the battery that must be removed. Second, it is a challenge to get the battery connectors to line up when reinserting the battery since the unit connectors are not stationery.
I had to work on it for a bit but got them lined up & the battery seated into the unit to allow the cover to slip back into place.
I tested the light & pushed the "On" button once. Three of my dogs came running so we shall see how things go once my broken foot will allow me to walk again!
Product or Service Mentioned: Petgentle Ultrasonic Pet Trainer.