The BA 20 Recharging Battery Pack from Sennheiser is for the AVX EKP digital wireless system. It is compatible with the AVX-ME 2, the AVX-MK 2, the AVX-835 and the AVX-MMD 42-1 systems.
Compatibility
Sennheiser AVX 835 SET
Sennheiser AVX Combo SET
Sennheiser AVX ME2 SET
Sennheiser AVX MKE2 SET
Sennheiser EKP AVX