The current from a battery flows in the same direction all the time and the amount of current does not change so long as the resistance of the circuite is constant. These are called Direct Current, or DC. A battery is a DC voltage source. The mains supply is AC or Alternating current. How does it vary with time. An easy way to compare the two is plotting the voltage as a function of time. The PVS output of expEYES is a DC source, and the SINE output is an AC source. Connect PVS to A1 and SINE to A2 and watch them using the CRO+ program. The peak voltage of our AC is 3.73 volts and the frequency is 151.5 Hz. For the mains supply voltages are in hundreds of volts and the frequency is either 50 or 60 Hz.