A safety device (19) for a battery (12) being designed so that an electrical connector (18) can be connected to the positive (14) and negative (16) electrical terminals of the battery (12) in a first (A+, A−) or a second (B+, B−) connection position, the safety device (19) being adapted to be selectively positioned in a first or a second safety position so that only the desired connection positions are accessible to an electrical connector (18). A battery including the safety device or a set of such batteries and a method for making such a set of batteries safe.