 In this video, we're going to learn how real batteries differ from the ideal voltage sources that we've been discussing so far in this course. A perfect voltage source has two terminals that always maintain a fixed voltage drop, regardless of the load resistance. This means it will always apply a current of I equals V over R. However, a real voltage source can only supply a finite maximum current, and they generally behave as an ideal voltage source in series with a small resistor. The smaller this resistance, often called the internal resistance, the better. So, for example, a normal 9-volt alkaline battery, as marked in red here, behaves like a perfect 9-volt voltage source in series with a 3-ohm resistor. This means it can provide a maximum of 3 amps when one terminal is connected directly to the other. However, if you did this, it would drain the battery completely in a few minutes and the battery would heat significantly. The battery will behave more like an ideal voltage source when the load resistance R here is increased. So, when R equals 0, our battery behaves very differently to the ideal battery source that we discussed over here. However, when R is very large, but when R is large compared to the internal resistance, then these two circuits pretty much behave in the same way.