Hi I'm new in this arduino world but i'm very fascinated of electronics.
I want to build a robot with a few sensors(like temp. sensor , PIR sensor, gas sensor etc) that will run autonomous in my house.The robot will have the basic obstacle avoidance sistem(a micro servo motor and the HC-SR04 ultrasonic sensor) and will move using 2 DC motors 6V 142rpm.First i wanted to use a Arduino Uno but i realized that i need more pins so i've got an Arduino Mega.I am gonna power the arduino using 6 NiMH rechargeable batteries(6*1.2V=7.2V) at 2200mAh but i dont know what should i use to power the motors that are connected to a DRV8833 driver.(Also i would like to know if the DRV8835 driver is better in my situation)
Also I want the robot to run autonomous for 2 or 3 hours.
Any advices are welcome, thanks.
If the capacity of the DRV8833 is sufficient for your motors it should be fine. I use them to control small low-voltage motors. The max voltage for a DRV8833 is about 10v IIRC.
How much time out of the 3 hours will the motors actually be working? How much current (amps) do the motors draw when they are working?
...R
I looked over the specification and i see it consumes 200mA at no load and 400mA loaded.One more thing i notoiced is that it says the stall current is 2A.From what i red on the drivers page it supports a max of 1.5A so i think it will over-heat and shutdown... 
so should i change the motors (i see that the 12V have 1.2 A stall) or look for other driver?
And the motors will be running a lot since the robot will patrol around the house and i calculated more and its enough to run for 1 or 2 hours.
Unless the motor is likely to stall regularly the DRV8833 will be fine. It won't be damaged by being overloaded.
If you have two motors each drawing 0.4 amps then the two together will draw 0.8 amp hrs every hour or 1.6 amp hrs over a two hour period. I suggest getting a battery with at least twice that capacity - 3 times would be better.
...R
Can i use another set of 6 AA rechargeable batteries they have 2200mAh ?(btw what voltage should i put in ?is it enough 7.2v? or should i get a set of 8 AA rechargeable batteries which will be 9.6V)
I can only suggest that you experiment. I would start with the 6xAA cells.
...R