The Automotive Battery
The automotive battery has several important functions. It provides current in excess of 100 amps to operate the starter motor and to provide power for engine systems during cranking and starting.
Once the engine starts the charging system takes over and provides electrical power to all systems and accessories. It also charges the battery, keeping its vital surface charge for quick or emergency starting.
Batteries have six 2.1 volt cells connected in series, providing 12.6 to 12.7 volts total. Due to the nature of their design, they act as capacitors for the charging and electrical system. The battery smoothes ripple or A/C current caused by the alternator. An older worn-out battery provides not only less voltage but also less capacitance.