The present invention relates to a back-up battery for an electronic device with a rechargeable battery, which electronic device is provided with a contact for connecting a battery charger and which back-up battery consists of one or more battery cells. According to the invention the terminals on the back-up battery are connected to a contact adapted to the contact for the battery charger on the electronic device, with the result that when it is connected to the electronic device's battery charger contact the back-up battery will charge the electronic device's rechargeable battery, thereby providing increased service life for the electronic device.