NOVELTY - The system (10) has an electricity supply system (10A) provided with a generator (11A) which is driven with a drive shaft (2a) of an internal combustion engine (2). Another electricity supply system (10B) is provided with another generator (11B) that is driven with a transmission shaft (4) between the internal combustion engine and a driving wheel (3). Charging processes of two chargers (13A, 13B) are performed when boost pressure is greater than acceptance boost pressure, and electric power generation with the latter generator is performed through upslope path start time control process.    USE - Power source system for motor vehicles i.e. lorry. Can also be used for an automatic transmission vehicle i.e. automatic transmission car.    ADVANTAGE - The charging processes of the chargers are performed when the boost pressure is greater than the acceptance boost pressure, and electric power generation with the latter generator is performed through the upslope path start time control process so as to improve fuel injection quantity of the internal combustion engine while absorbing engine motive power with the generators at time of increasing charging capacity of the chargers, so that battery over discharge can be prevented by performing electric power generation and charge efficiently, and operation of the internal combustion engine in an inadequate charge condition of the chargers can be enhanced, thus suppressing deterioration of fuel consumption in an effective manner.    DETAILED DESCRIPTION - The chargers are battery. An INDEPENDENT CLAIM is also included for an electric supply method for a power source system for motor vehicles.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of a power source system.    Internal combustion engine (2)    Drive shaft of an internal combustion engine (2a)    Driving wheel (3)    Transmission shaft (4)    Power source system (10)    Electricity supply systems (10A, 10B)    Generators (11A, 11B)    Chargers (13A, 13B)