i'm using 3 feetech 25kg/cm 7.4 v servo per leg for quadruped robot
no load current is 120mA and stall current is 3.5 A
i cant make a choise for a battery, should i buy a battery for arduino for better performance
and is one battery enough for 3 servos in one leg. As for the connection connecting the servos parallel better than series.
You can use a 2S lipo or 6V NiMH RC battery for those servos. It must be rated for at least 15 Amperes peak discharge current. Servos briefly draw the stall current every time they start moving.
Use a separate power supply for the Arduino, or a switching 5V step down converter.
2s lipo voltage is nominally 7.4 volts, but a good deal of the energy you will get out of it is between 7.4 and 8.4 volts.
The specifications for the "feetech 25kg/cm" servo do not state a maximum supply voltage, nor is any figure greater than 7.4 volts mentioned.
So @ahmad4419 may want to be careful before supplying more than 7.4 volts.
a7
It is my understanding that when servo suppliers rate servos for 7.4V, they have LiPo batteries in mind, with a NOMINAL voltage of 7.4V. Nothing else makes sense.
Can You make a little drawing, using pen and paper to explain?
So that's 12 servos....??
Yes
Cool project! You'll need a battery that can handle the maximum combined current of your servos (10.5A), like a 2-cell LiPo for the needed 7.4V. Connect servos in parallel, not series, so they each get full voltage. Consider separate power for Arduino and servos to avoid electrical noise issues. Good luck! 
Think you need to recalculate that outcome........ 
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