The internal combustion engine has been around for well over a century, the first engines put to use in the late 1860s, but starting them wasn’t as easy as turning an ignition key or pressing the start-stop button. In those days, starting was done by a hand crank, which would give the engine enough compression to fire off a cylinder. The flywheel might carry it over to the next firing, or it might not, at which the operator would have to crank the engine again.

Early drivers didn’t crank their engines for long, however, with car batteries and electric starters available as early as 1911. The first airplanes were, quite dangerously, started by hand until 1930, requiring someone to turn the propeller. The introduction of the electric starter made it possible to start ever larger and more powerful engines, which would be impossible to crank by hand, but without car batteries, even electric starters would have no way to energize.

Today, all piston-driven internal combustion engines are equipped with car batteries and electric starters. The car battery is designed only to supply a short burst of high power, just enough to move the engine a couple-hundred rpm. Once the engine starts, the electric starter disengages, having drained a few percentage points off the car battery’s state of charge (SOC).

All vehicle electrical systems require power, including the ignition and fuel system, engine and transmission controls, audio and climate control, to name a few, but the car battery isn’t designed to power these for very long. In fact, it might last only a few minutes, and ruin itself at the same time. With the engine running, the generator, also called an alternator, kicks in to generate electricity for the rest of the vehicle, usually between 13.5 V and 14.5 V. This is enough power to run the vehicle and keep the battery charged.