Emergency system for motor vehicles

The system has an on-board power supply system triggering a radio emergency call during emergency based on a state of the on-board power supply system of a motor car. A radio communication device triggers the emergency call. A back-up battery comprises multiple electrochemical cells (101a-101c) that supply electric energy to a radio communication device. An electric heating device i.e. heating foil (102), is supplied with power by the on-board power supply system with which the back-up battery is heated when temperature is below or drops below a lower limit value. The back-up battery is designed as a nickel metal hydride or lithium ion rechargeable battery. An independent claim is also included for a method for operating an emergency system for a motor car.