Insufficient ground supply on my circuit? Ground capacitive foil
This CapSense 
library distinguishes between touch and no touch through the increase/decrease of the time delay caused by the series resistance and capacitance to the earth. This obviously requires the Arduino to be connected to the earth. 

If you use a power supply the Arduino is either directly connected to the earth via the mains plug or via the stray capacitances through the PS, which are considerably higher than the sensor's capacitance to the earth. The stray capacitance between the battery and earth is far to low for this to work. 

Solution:


Connect the Arduino board to a heating pipe or earth pin of a socket.
Make use of capacitive sensing though an oscillator and PLL like proximity sensors do. Is more reliable, but also more complex, requires additional electronics and is not always possible depending on your application.