Arduino works bad when powered by 9V battery At first I am sorry for my bad English.

I am making robot which can do a lot of things... 

If I connect arduino on my robot with PC via USB, the robot works properly. If I use external power supply (9V battery) the robot is doing almost nothing.. The diodes are little bit blinking and sometimes dc motors make motion... And thats all. Batteries are charged. Where's the problem? 

Everything I want is make into motion dc motors.. With 5V USB it works. With external power supply not (9V battery).
I got a fresh 9 V out of a packet and measured it.

 9.58 volts


Now with a load of 500 mA (which you quite likely would get with a robot) it dropped to:

 7.4 volts


With a load of 750 mA the voltage dropped to 6.6 V which is below what you are supposed to supply via the power jack.

And that was with a fresh battery, testing for a few seconds.

Those 9 V batteries are just not designed to drive motors.

I suggest a somewhat heavier-duty battery, like 5 x AA batteries, at least for the robot part.