1 of 1 people found the following review helpful
No complaints except price.,
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This review is from: Sennheiser BA 370 PX Rechargeable Lithium Polymer Battery for Sennheiser Headphones (discontinued by manufacturer) (Electronics)
What's to say, it fits and powers both the BA-370PX and the MM-550X Bluetooth Headphones. I can relate to Sennheiser going for a proprietary battery because of size constraints but the price is a bit steep. My old chorded headphones used stock rechargeable AA batteries.