The USAlert Rechargeable Batteries for the WatchDog Pager are included with each WatchDog Pager as the primary power source, but as with any battery type, they eventually do wear out and have reduced performance over time. Battery life may be affected after 12 - 18 months of use, so it is recommended that the batteries be replaced at regular intervals to maintain optimum pager performance. The WatchDog Pager uses three “AAA” sized NiMH (Nickel Metal Hydride) rechargeable batteries, each rated at 1100 mA/hr. The NiMH rechargeable battery type is the preferred power source for small electronic devices that are used frequently and require high power. Replacement batteries are typically shipped in packs of three. Some alternate rechargeable batteries or batteries with a lower capacity rating have been found to create pager performance issues, so replacing your batteries with 1100 mA/hr capacity batteries is recommended.