Study of the electric power balance in a vehicle for the choice of the battery

The number of electrical components in vehicles is day by day increasing, requiring a careful dimensioning of the electric system and a correct choice of the components that will have to provide electrical power to the whole system. The amount of electric energy that may be required during the usage of a car is very variable, depending on how many and which loads are active at the moment; the choice of the power generating elements must then be done in a way that guarantees a certain performance, usually relative to the most critical working conditions that may be encountered by the vehicle in its lifetime. In the present paper a study of the fundamental characteristics of the electric system of a vehicle is proposed with particular attention to the two main components, the alternator and the battery; the fundamental equations for the electric power balance in the vehicle are reported and discussed as a preamble for the implementation of a Matlab model able to simulate the behavior of the charge and discharge of a car battery just considering the overall energy balance. The graphical visualization of the trend of the battery charge over many operating cycles allows the user of the program to choose the battery with the most suitable capacity avoiding oversizing or downsizing of this fundamental component. Particular emphasis has been put on the choice of the requirements that the battery charge has to satisfy throughout its lifecycle.