A battery device includes a battery set, and a casing; the casing includes a right covering part and a left covering part, which are joined together to provide a holding room for the battery set; the battery set include at least two batteries, and are held in the casing with positive and negative poles thereof being exposed through front and tail ends of the casing respectively; a socket is held in the casing, and electrically connected with the positive and the negative poles of the battery set; thus, after a plug of a charger is inserted in the socket, the single-part batteries together will be charged through the charger without the battery device being located on the charger.