Ok,dumb question(s): Isn't Impedance=Resistance in DC?
Not so dumb.
Resistance is a single dimension property where as impedance is a two dimensional property. Both are measured in Ohms but only to a first approximation can they be considered the same.
When KeithRB says:-
But output resistance has nothing at all to do with how much current it can supply.[/quote
He is talking about an active regulated power supply where the output resistance encapsulates the regulation of the circuit. That is by how much the voltage drops as the current increases. These system normally have over current protection which kicks in way before any output impedance limitation of the supply.
In a battery however it is another matter where the output impedance does define the maximum current output. The trouble here is that it is not a constant and will change with current draw and with temperature.