hardik311095:
My HC-05 is 5v version with onboard regulator not the 3.6v-6v
The chips on that board run at 3.3V. The regulator reduces 3.6V to 6V (e.g. 5V) to 3.3V. 8.4V is too much for the regulator and probably already destroyed it, which is why your HC-05 doesn't power up.
A single lipo cell might have worked.
Steve