Which is perfect DC supply?

Answer 1

A perfect DC power supply has zero impedance, and infinite capacity, so that fluctuations in load do not affect its output. A perfect DC voltage source generates constant voltage, while a perfect DC current source generates constant current.

In the imperfect world, power supplies have low, but non zero impedance, usually represented by some very small series resistance. This causes a voltage drop when current increases. Even the wires used as conductors are imperfect, possessing impedance of one sort or another, so they are a factor as well.

A car battery is a good example of a near perfect DC supply, as its internal impedance is usually too low to measure with normal equipment. It does, however, have limited capacity.