The major responsibility of the battery is to supply current to start the engine. The current to start the engine. The current required to crank the engine varies from car to car. It depends on the engine stroke and bore of the number of cylinders engine/starter cranking ratio, circuit resistance, temperature engine oil viscosity and the accessory load. A four-cylinder engine may require as much cranking current as an eight-cylinder engine with greater displacement. All of these factors are considered when a battery is specified for your car.
The extra boost
Every car has its own charging system designed to meet your vehicle’s general needs in terms of carrying the electrical current under normal driving conditions. When additional current demands are made on your car, your battery must be able to provide that additional electrical load to keep you going if the charging system fails. Those demands occur for example when starting or idling. The following chart gives you an idea of the electrical current demands made on the battery of typical small and full size cars with 12 volt electrical system