A battery module includes a top cover and a bottom cover affixed together to accommodate a battery set, which has conducting terminals positioned in respective bottom side holes of the bottom cover for the contact of respective metal contacts of a battery charger or power hand tool. A movable member is sleeved onto the battery set inside the bottom cover and held in a first position by spring members to block side holes of the bottom cover and movable from the first position to a second position to open the side holes of the bottom cover for ventilation to dissipate heat upon insertion of the battery module in a battery charger for charging.