Sennheiser 450 TRAVEL 550 Travel MM 400 MM 400-X M Replacement Battery: BatteryClerk.com Headphone
Sennheiser MM 400-X Review
Sennheiser MM400 Bluetooth Headphones with Invisible Microphon - QVC UK
Amazon.com: LIFH Replacement Battery for Sennheiser MM 400-X, MM 450, MM 450-X fits Sennheiser 0121147748, BA 370 PX, BA370, BA-370PX(Li-Polymer 270mAh / 1.00Wh) : Electronics