Time Pilot,
Batteries are electro chemical devices with different characteristics. About the same analogy can be drawn between bipolar transistors and FET devices. With one it's easier to supply current with the other voltage. You are not going to get a simple explanation as to why beyond that without getting text books and having a chemical background. For some idea take a look at.
http://www.sciam.com/askexpert_quest...B7809EC588F2D7

U-Dud,
Yah it's along for the ride as long as its higher than what you are attempting to charge.

John