A car's charging system is composed of an alternator (or DC generator), a voltage regulator, battery and indicator light or gauge. While the engine is running, it's purpose is to provide power for the car's electrical load, e.g., ignition, lighting, accessories, etc., and to recharge the car's battery.
Open Circuit Approximate Average Cell
Battery Voltage State-of-charge Specific Gravity
12.65+ 100% 1.265+
12.45 75% 1.225
12.24 50% 1.190
12.06 25% 1.155
11.89 0% 1.120