An electric tester detects voltage in wires or outlets by sensing a little amount of capacitively linked current from the live circuit to tester and then back to ground.
It normally has a neon lamp that turns on when it receives enough voltage, typically around 80 V.
However, electric testers are ineffective for measuring DC current because they depend on components such as capacitors built for AC circuits with changeable polarity and direction.
DC current, with its consistent polarity, does not activate the tester.