The BA 30 from Sennheiser is a rechargeable battery pack for the evolution wireless SK D1 or 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.
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These are great replacements for the original batteries, and they last all day long, 12 hours as you can see on the transmitter's display). The receiver batteries (BA 20) don't last as long—around 4 hours—so I recommend getting at least two of those for every one BA 30. The BA 30s take about 4.5 hours to charge, and the BA 20s take about half that... I think the Sennheiser AVX is a great system. Yes, the batteries are expensive—we're all used to that reality—but these are totally worth it.
It works!