The USAlert WatchDog Pager Battery Charger can charge two sets of three NiMH (Nickel Metal Hydride) batteries at the same time. One set can be installed and charged in a WatchDog Pager and another set can be charged in the spare battery compartment at the rear of the Battery Charger. The WatchDog Battery Charger is both a smart charger that detects battery levels and adjusts the charging rate accordingly, as well as a rapid charger that gets a fully discharged set of batteries back to an operational level in a minimum amount of time. The typical battery recharge time is about 2 hours for a moderately discharged set of three. For completely discharged batteries, recharge time should be about 3-4 hours. If a set of batteries is being charged while installed in a WatchDog Pager and the pager is turned on, the pager will still draw a small amount of current and the charging time will be slightly increased. The WatchDog Battery Charger is compatible with both USAlert WatchDog and WatchDog LT pagers.