A Current Interrupt Device (CID) is a protective device in car batteries that interrupts the current supply when the charging current is too high. The Current Interrupt Device is based on a PTC thermistor whose resistance value increases with increasing charging current.
In battery protection devices, there are those that are reversible, others are irreversible. Both methods are often combined. A reversible method uses a PTC thermistor whose resistance is low during normal operation and which increases as the charging current increases, reducing the current flow. Once the charging current returns to normal, the temperature decreases and so does the resistance value of the PTC thermistor.
An irreversible method relies on the increasing pressure within the battery during charging. The pressure increase in the battery cell is triggered by higher temperatures, which are related to the level of the charging current. A thin metal foil over which the charging current flows and which is equipped with predetermined breaking points serves as a protective device. If the internal pressure exceeds a certain level, the metal foil tears and the charging current is permanently interrupted.