Li-Polymer Wireless Headset Battery for Sennheiser, 3.7V, 270mAh, Compatible Models: 450 TRAVEL, 550 Travel, MM 400, MM 400-X

Li-Polymer Wireless Headset Battery for Sennheiser, 3.7V, 270mAh, Compatible Models: 450 TRAVEL, 550 Travel, MM 400, MM 400-X

AliExpress