vinceherman:
Can those servos run on 18V?hmmm, I could be wrong, but it looks to me like you are applying the full battery voltage to the VIN pin. Is that what you intend?
Hi,
thank you for your answer!
No, I am planning to put a DC-DC-Stepdown Converter between the battery and the Arduino, as well as between the servos and the battery. So the battery will connect to the DCDC-Converter and from that I gonna connect the servos and the VIN of the Arduino Nano. The DCDC-Converter can be adjusted and I would set it to 6V and 5A as the servos run on 6V and 2500mA each.
I hope I understood DCDC-Converters correct 
Best,
Jules