I'm really not sure where you got 9V from!! I posted this But that was for a driver that supplies a constant current and varies the voltage to maintain that voltage drop. A driver is designed to supply a constant current to whatever you connect to it, which is why there is a range of operation. A battery is designed to supply a constant voltage and the current will vary based on the load resistance. An LED battery tester is meant to supply a roughly 3V constant for momentary testing of an LED only. 9V will blow any LED out completely, I suppose if you knew you had about 9V of drop across a string of LEDs you could test say 4 LEDs at once with a 9V battery. But not just one.