Open the battery cover on the back of the detector. This cover slides out of place. To open, place both thumbs on the back of the battery cover where the battery cover hooks snap into place. Push down where the battery cover snaps into place with your thumbs and slide the cover out. Some force is required. Connect the included lithium battery by inserting the battery harness connector into the product. Do NOT remove the plastic wrap surrounding the battery pack. The battery connector has a large notch on it that faces up when installed correctly. There is only one way in which the connector can be installed. A small portion of the connector will be exposed when connected properly. You should hear it click into place. After connecting the battery, place the battery with the labels facing down and the battery wires wrapping around the top side of the battery pack. For the best fit, it is recommended to have the negative (-) terminals face toward the outer edge of the detector.