Internal combustion engine
The cars internal computer, also know as the Electric Control Unit (ECU) decides many things some of which include the fuel injection, exhaust system requirements and throttle response and for petrol engines it also can control the spark plug timing to ignite the mixture of air and fuel in the cylinder. It is primarily powered by the alternator but before the car is started, it must be powered by the battery as it decides the fuel injection and spark timing which are one of the key factors in getting a car started. The internal battery of the car also works as a surge protector (protecting the ECU from AC voltage spikes that typically last from 1 to 30 microseconds and can reach over 1,000 volts) for the ECU.