Battery Voltage is not as it should be when connected to ckt
As I know Battery is a constant voltage source.


A battery is not a constant voltage source - it has an effective series resistance (ESR) that means on high current loads the voltage on the terminals drops noticeably.

So, if your 9V battery drops to 6.7 volts and the circuit is taking 2 amps then the ESR is 1.15 ohms.

If a battery were a perfect voltage source it could provide infinite power and that is obviously incorrect.