A battery pack, comprising: a housing having a socket located thereon, the socket having a plurality of terminals received therein, the terminals including a first , a second , a third and a fourth conductive terminal; and two battery cell groups are received in the housing, each having a positive electrode and a negative electrode, respectively, wherein the first conductive terminal is electrically connected to the positive electrode of the first battery cell group , the second conductive terminal is electrically connected to the negative electrode of the first battery cell group , the third conductive terminal is electrically connected to the positive electrode of the second battery cell group and the fourth conductive terminal is electrically connected to the negative electrode of the second battery cell group ; wherein the two battery cell groups are switchable between a first state when connected to a first male plug of a high rated-voltage power tool and a second state when connected to a second male plug of a low rated-voltage power tool, in which first state the first conductive terminal and the fourth conductive terminal are electrically connected with the first male plug and the second conductive terminal and the third conductive terminal are electrically connected to each other by a connecting terminal of the first male plug and in which second state the first conductive terminal , the second conductive terminal , the third conductive terminal and the fourth conductive terminal are electrically connected to the second male plug and wherein the two battery cell groups in the first state are connected in series and in the second state are connected in parallel. a housing having a socket located thereon, the socket having a plurality of terminals received therein, the terminals including a first , a second , a third and a fourth conductive terminal; and two battery cell groups are received in the housing, each having a positive electrode and a negative electrode, respectively, wherein the first conductive terminal is electrically connected to the positive electrode of the first battery cell group , the second conductive terminal is electrically connected to the negative electrode of the first battery cell group , the third conductive terminal is electrically connected to the positive electrode of the second battery cell group and the fourth conductive terminal is electrically connected to the negative electrode of the second battery cell group ; wherein the two battery cell groups are switchable between a first state when connected to a first male plug of a high rated-voltage power tool and a second state when connected to a second male plug of a low rated-voltage power tool, in which first state the first conductive terminal and the fourth conductive terminal are electrically connected with the first male plug and the second conductive terminal and the third conductive terminal are electrically connected to each other by a connecting terminal of the first male plug and in which second state the first conductive terminal , the second conductive terminal , the third conductive terminal and the fourth conductive terminal are electrically connected to the second male plug and wherein the two battery cell groups in the first state are connected in series and in the second state are connected in parallel. <EOS>