Parallel systems, which are most commonly produced at present, connect both the electrical and internal combustion systems to the mechanical transmission. They can be subcategorized depending upon how balanced the different portions are at providing motive power. In some cases, the internal combustion engine is the dominant portion and is used for primary power, with the motor turning on only when a boost is needed. Others can run with just the electric system operating alone. Most designs combine a large electrical generator and a motor into one unit, often situated between the internal combustion engine and the transmission, in the location of the flywheel, replacing both the conventional starter motor and the generator or alternator. A large battery pack is required, providing a higher voltage than the normal automotive 12 volts. Accessories such as power steering and air conditioning are powered by electric motors, so that they continue to function when the internal combustion engine is stopped; this offers the possibility of further efficiency gains, by modulating the electrical power delivered to these systems, rather than having them run directly from the engine at a speed which depends on engine speed.
