High voltage battery cutout circuit for a motor vehicle electrical system

Protection apparatus for a vehicle electrical load normally powered by an electrical system including a storage battery and an engine driven alternator for supplying charging current to said storage battery, comprising:

• an auxiliary battery connected in parallel with said electrical load;
• a normally conductive MOSFET having a source-to-drain circuit coupling said alternator and storage battery to said electrical load and auxiliary battery;
• a voltage sensitive circuit coupled to the alternator and storage battery for detecting a voltage in excess of a threshold voltage;
• and
• a transistor circuit including a first transistor having an emitter-collector circuit coupled across a source-to-gate circuit of said MOSFET, said first transistor being activated by said voltage sensitive circuit when the detected voltage exceeds said threshold voltage to bias said normally conductive MOSFET to a non-conductive state to isolate said alternator and storage battery from said electrical load and auxiliary battery, thereby to protect said electrical load from alternator voltages in excess of said threshold voltage while maintaining power supply to said electrical load via said auxiliary battery.

Protection apparatus for a vehicle electrical load normally powered by an electrical system including a storage battery and an engine driven alternator for supplying charging current to said storage battery, comprising:

• an auxiliary battery connected in parallel with said electrical load;
• a normally on MOSFET having source-to-drain circuit coupling said alternator and storage battery to said electrical load and auxiliary battery;
• a first transistor having an emitter-collector circuit coupled across a source-to-gate circuit of said MOSFET;
• a first zener diode and resistor connected in series across said storage battery;
• and
• a second transistor coupled to said first transistor and to a junction between said first zener diode and said resistor so as to bias said first transistor to a conductive state to turn off said MOSFET when a voltage across said storage battery exceeds a breakdown voltage of said first zener diode.