NOVELTY - The electronics (400) has a battery cell internal resistance input port (318) for inputting battery cell internal resistance value (316), and a battery cell open circuit voltage input port (322) for inputting battery cell open circuit voltage value (320). A comparator (402) determines the prognosticated battery cell voltage from battery cell open circuit voltage value, battery power instantaneous value (411) and internal resistance value, and produces the alarm signal (312) in dependence of comparison of prognosticated battery cell voltage with battery cell momentary voltage.    USE - Security electronics for battery cell of battery cell monitoring unit used in battery system mounted in motor car (all claimed) such as hybrid car and electric car. Can also be used in laptop and mobile phone.    ADVANTAGE - The security electronics can ensure safe operation of the battery cell. The security electronics compares the battery cell momentary voltage with battery cell voltage threshold values, and also compares the battery cell momentary voltage with prognosticated battery cell voltage. The security electronics can strongly produce the alarm signal, if the battery cell momentary voltage threatens to fall below the prognosticated battery cell voltage. The security electronics forms a redundant, independent and/or supplementary measuring instrument to the well-known battery cell monitoring units, so that a high security rating for battery systems permitted with security electronics is enabled. The high input resistor can be provided easily and inexpensively, and the working reliability of security electronics can be increased. The uncritically small voltage fluctuations at the battery cell can be obviated, during the inadvertent production of alarm signal. The battery management unit can disconnect the entire battery with receipt of alarm signal by the battery separation unit from a load so as to cause safe battery state.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) battery cell monitoring unit; and    (2) battery system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the security electronics.    Alarm signal (312)    Battery cell internal resistance value (316)    Battery cell internal resistance input port (318)    Battery cell open circuit voltage value (320)    Battery cell open circuit voltage input port (322)    Security electronics (400)    Comparator (402)    Battery power instantaneous value (411)