The Sennheiser BA 40 is a rechargeable lithium-ion battery pack made for the SpeechLine Digital Wireless range, specifically the SL Tablestand DW and SL Boundary DW transmitters used on lecterns, boardroom tables and conference desks. It is a genuine Sennheiser replacement or spare pack, useful for institutions, corporate meeting rooms, churches and lecture venues that want a charged spare on hand so a transmitter never goes flat mid-session, or to replace an original pack that has lost capacity after years of daily charging. The pack is rated at 3.6 V with a capacity of 2.1 Ah, and it slots into the base of the compatible transmitters where it is charged in the device. It measures about 75 x 60 x 33 mm and weighs roughly 70 g. The listing is for a single battery pack; transmitters and chargers are sold separately.