A battery module comprises a case, an extension/retraction positioning module, a press element, a battery body and a cap. The extension/retraction positioning module is disposed in the case to enable an extension/retraction operation along a first axis; the press element is connected with the extension/retraction positioning module to drive the extension/retraction positioning module to enable the extension/retraction operation when the press element is pushed; the battery body is disposed in the case to move relative to the case to an initial position or a power supply position by the extension/retraction operation, the battery body comprises a power connecting portion; the cap combines with the case and the portable electronic device in a rotational manner. The battery body moves to the initial position or the power supply position by the extension/retraction operation to determine whether the power connecting portion is electrically connected with the portable electronic device.