Synergy Digital Wireless Headset Battery, Works with Sennheiser MM 400-X Wireless Headset, (Li-Pol, 3.7, 270mAh), Compatible with Sennheiser 0121147748, BA 370 PX, BA370, BA-370PX Battery - Walmart.com
Sennheiser MM 450-X Wireless Review - RTINGS.com
Sennheiser Black MM 400-X On-Ear Foldable Bluetooth Phone Headset - Newegg.com
Amazon.com: HQRP 2.5mm to 3.5mm Audio Cable Compatible with Sennheiser MM400-x MM450-x MM550-x PXC-550 Urbanite-XL Bluetooth Headphones Replacement : Electronics