The BA 30 from Sennheiser is a rechargeable battery pack for the evolution wireless AVX Bodypack Transmitter (sold separately). It can be used to replace the two AA size batteries that power the transmitter-unit. The BA 30 uses a lithium-ion 3.7V rechargeable battery.
Compatibility
Sennheiser AVX Combo SET
Sennheiser AVX ME2 SET
Sennheiser AVX...