Magicshine MJ-60024400mA - 7.4V Li-ion Battery pack.Uses a 5.5mm x 2.1mm Co-Axial connectorBuilt of 4 - 18650 cells in a 2S2P configuration Includes an integrated protection circuit to prevent over-charging or over-discharging and to balance the charge between cells.These packs are built by BAK Battery Co., One of the largest manufacturers of Li-ion batteries in the world. A Tier One supplier for the portable electronic, auto, and computer industries.