Sennheiser BA 370 Rechargeable Battery for PX210BT, PX360BT, MM400, MM450, MM550, PXC360BT, PXC310, PXC310BT Factory replacement rechargeable battery Replacement for the MM400 and MM 400-X, MM450 and MM 450-X, MM 500 and MM 500-X, MM 550 and MM 550-X Replacement for the PX210BT, PX 360BT, PXC360BT, PXC310 and PXC310BT