A battery module, wherein the battery module comprises a first connecting piece and secondary batteries; the secondary battery comprises a battery cap assembly, a case and an electrode assembly, the case has an opening in a top thereof, the battery cap assembly is fixed to the case and seals the opening of the case, the electrode assembly is received in the case and comprises a first electrode plate and a second electrode plate; the battery cap assembly comprises a cap plate, a first electrode terminal and a second electrode terminal, the first electrode terminal is electrically connected with the first electrode plate, the second electrode terminal is electrically connected with the second electrode plate; the cap plate includes a first terminal hole and a second terminal hole, at least a part of the first electrode terminal protrudes above the cap plate; characterized in that the second electrode terminal includes a second protruding portion protruding above the cap plate and a second base portion connected with the second protruding portion, the second base portion extends into the second terminal hole, a cross-sectional area of the second protruding portion is larger than a cross-sectional area of the second terminal hole; a protrusion is provided on an upper surface of the first electrode terminal; the first connecting piece includes a first connecting portion and a second connecting portion; the first connecting portion is attached to the upper surface of the first electrode terminal of the battery cap assembly of one secondary battery and welded with the protrusion of the first electrode terminal of the battery cap assembly of the one secondary battery; the second connecting portion is welded with the second protruding portion of the second electrode terminal of the battery cap assembly of another secondary battery to form a welding portion, the projection of the welding portion in the up-down direction is positioned in a cross-section of the second base portion of the battery cap assembly of the another secondary battery, wherein the cross-section of the second base portion is perpendicular to the up-down direction, and the welding portion extends to the second base portion; the second base portion is connected with a bottom surface of the second protruding portion and extends downwardly; the first electrode terminal includes a first protruding portion protruding above the cap plate and a first base portion connected with a bottom surface of the first protruding portion, the first base portion extends downwardly and extends into the first terminal hole. the secondary battery comprises a battery cap assembly, a case and an electrode assembly, the case has an opening in a top thereof, the battery cap assembly is fixed to the case and seals the opening of the case, the electrode assembly is received in the case and comprises a first electrode plate and a second electrode plate; the battery cap assembly comprises a cap plate, a first electrode terminal and a second electrode terminal, the first electrode terminal is electrically connected with the first electrode plate, the second electrode terminal is electrically connected with the second electrode plate; the cap plate includes a first terminal hole and a second terminal hole, at least a part of the first electrode terminal protrudes above the cap plate; characterized in that the second electrode terminal includes a second protruding portion protruding above the cap plate and a second base portion connected with the second protruding portion, the second base portion extends into the second terminal hole, a cross-sectional area of the second protruding portion is larger than a cross-sectional area of the second terminal hole; a protrusion is provided on an upper surface of the first electrode terminal; the first connecting piece includes a first connecting portion and a second connecting portion; the first connecting portion is attached to the upper surface of the first electrode terminal of the battery cap assembly of one secondary battery and welded with the protrusion of the first electrode terminal of the battery cap assembly of the one secondary battery; the second connecting portion is welded with the second protruding portion of the second electrode terminal of the battery cap assembly of another secondary battery to form a welding portion, the projection of the welding portion in the up-down direction is positioned in a cross-section of the second base portion of the battery cap assembly of the another secondary battery, wherein the cross-section of the second base portion is perpendicular to the up-down direction, and the welding portion extends to the second base portion; the second base portion is connected with a bottom surface of the second protruding portion and extends downwardly; the first electrode terminal includes a first protruding portion protruding above the cap plate and a first base portion connected with a bottom surface of the first protruding portion, the first base portion extends downwardly and extends into the first terminal hole.