A device for a battery charger, includes connection elements 14, 16 connected to the output lines of the charger, connection elements 16, 24 for connection to the terminals of a battery to be charged. The device includes elements 80 for detecting a voltage over the output lines of the charger, elements 80 for detecting a positive voltage over the terminals of a connected battery, switch elements 60 for connecting at least one of the output lines to the connection elements to the terminals of the battery, and a switch activating element 70 arranged to the voltage detection elements and designed and arranged such that it is capable of activating the switch when a voltage is detected over the output lines of the charger and a positive voltage is detected over the terminals of the connected battery, thereby connecting the charger to the battery.