Most use cell batteries that are widely available in stores. Some use AAA batteries or 9V batteries. To see what batteries you need, you can remove the battery compartment cover and see what your thermometer has. You may also see the battery type in the product description or on the packing. If you are using your thermometer at home, the battery will last 3-5 years. In a hospital, due to more frequent use, the battery will last only 1-1.5 years.