This is the primary 3.6V Lithium battery Optex recommends to use for the wireless AX and SL infrared beams series to get the best performance. It is a primary 3.6V lithium battery providing high voltage which is packed and sold individually. Four batteries (two for the transmitter and two for the receiver) are required for the wireless AX- and SL- models and the SL series can accomodate a maximum of eight batteries (four in each beam) to extend the operation life.