Just to clarify what James wrote - A car battery serves to limit temporary surges that can occur when an alternator goes from high load to low load. Without the battery in circuit, as in a disconnected battery and a jump start from a remote battery, if the cables are disconnected while the engine is running, there is no ballasting action. In this case the alternator can put out bursts of high voltage enough to fry all the vehicle electronics.
In a similar case, a completely discharged battery has a high internal impedance and may not properly ballast the alternator until 5 minutes or so of charging. As a precaution, if the dome light is not even a glimmer, I leave the jumper cables on for 5 minutes before starting the engine just to make sure that the battery impedance has normalized. A battery with any decent amount of charge has a low impedance (which is good and allows a high current to flow in and out).