Abstract: In some examples, a high-energy-density battery pack device includes a circuit board and at least two casing structures mounted on the circuit board. In some examples, the high-energy-density battery pack device also includes at least two cells electrically connected in series or in parallel through the circuit board. In some examples, each cell of the at least two cells is positioned in a casing structure of the at least two casing structures. In some examples, the respective casing structure surrounds the respective cell with an opening on one end of the cell.