A battery pack, comprising: a battery housing; a battery cell assembly within the battery housing; and at least two connection terminals associated with the battery housing and capable of transmitting electrical energy or signals; wherein said connection terminals are designed to be connectable to sheet-shaped charging terminals of a charger and/or to sheet-shaped tool terminals of an electric tool; wherein the battery housing comprises an insert housing wall, the battery cell assembly comprises more than one battery cell unit, the insert housing wall comprises an insertion structure and terminal ports for each of the at least two connection terminals, and the insertion structure at least has an insertion direction which forms an angle greater than or equal to 0 degree and less than or equal to 45 degrees with respect to a maximum projection surface or a maximum size direction of the battery cell assembly; characterized in that said battery cell assembly comprises a left unit body and a right unit body which are mirror-symmetrical, and a connecting unit body provided there between, wherein said left unit body and said right unit body form mirror symmetry, and a symmetrical mirror surface is perpendicular to a maximum projection surface of said battery cell assembly, wherein, if said maximum projection surface of said parallel battery cell assembly is regarded as being in the horizontal direction, said left unit body and right unit body go beyond the connecting unit body on both upper and lower sides and form two grooves and having different sizes on both sides of said connecting unit body; wherein a distance of each of the connection terminals from an outer surface of the insert housing wall is greater than or equal to 7 mm. a battery housing; a battery cell assembly within the battery housing; and at least two connection terminals associated with the battery housing and capable of transmitting electrical energy or signals; wherein said connection terminals are designed to be connectable to sheet-shaped charging terminals of a charger and/or to sheet-shaped tool terminals of an electric tool; wherein the battery housing comprises an insert housing wall, the battery cell assembly comprises more than one battery cell unit, the insert housing wall comprises an insertion structure and terminal ports for each of the at least two connection terminals, and the insertion structure at least has an insertion direction which forms an angle greater than or equal to 0 degree and less than or equal to 45 degrees with respect to a maximum projection surface or a maximum size direction of the battery cell assembly; characterized in that said battery cell assembly comprises a left unit body and a right unit body which are mirror-symmetrical, and a connecting unit body provided there between, wherein said left unit body and said right unit body form mirror symmetry, and a symmetrical mirror surface is perpendicular to a maximum projection surface of said battery cell assembly, wherein, if said maximum projection surface of said parallel battery cell assembly is regarded as being in the horizontal direction, said left unit body and right unit body go beyond the connecting unit body on both upper and lower sides and form two grooves and having different sizes on both sides of said connecting unit body; wherein a distance of each of the connection terminals from an outer surface of the insert housing wall is greater than or equal to 7 mm.