Question

A low voltage supply from which one needs high currents must have a very low internal resistance . Why?

Answer 1

we know that 
I∞V
I=VR
V=I/R
V∞I
V∞1/R
.:  low resistance = high voltage supply 
                            = hence , high current supply 

Answer 2

Answer:

in case of using formula Imax =E÷r for r to be low I maximum