I have a Pixracer and a 3DR Powermodule but it shows only 0.06V when i plug in a 4S Lipo. When i try to click other and calibrate it, i have severe Fluctuations in the measurement (13V-17V is shown and it changes all the time).
Assuming you have the voltage and current sense lines going to the right pins…
The method for current measurement with the 3DR power module is quite crude and is not reliable. It uses the voltage drop across a resistor to calculate the current flow. But most cheap resistors are not thermally stable and are not linear. So the measurement will vary with temperature and current flow.
It you want accurate voltage and current measurement use a power module that features a Hall effect sensor. This device is a magnetic sensor that is not actually in the circuit. So it does not affect the circuit in any way and can not damage the power module if it fails.
One such module that is well regarded is the Mauch power module.
But its fluctuating in a range of 4V within seconds…
Like @mike says, a reliable power module is the key. Also not sure if all the 3DR power modules were specced for 4S…
Get a Mauch http://www.mauch-electronic.com 
If I value the vehicle, I put a Mauch PM
3DR modules are fine with 4S batteries.
I prefer Mrobotics power modules rather then Mauch modules (had problems in the past with one of them), also because they are specially designed by the same guy of the Pixracer.
But the best power module isfrom Airbot Services
Ardupilot simply reads a voltage provided on the correct pins and calculates the values it presents in the HUD etc. There is nothing in the setup of Ardupilot that would cause the voltage to fluctuate. You could have the wrong sense pins connected which would cause the voltage to fluctuate with the current and vice versa but other than that the power module is providing a fluctuating measurement voltage to the Pixhawk and it is simply displaying what it “sees”.
Mike is right.
I guess you have a wrong pin , check the last updated data sheet for the Pixracer.
Yep, Mike is right. I’ve seen this happen before. The voltage and current sense lines are reversed and the numbers are whack-o.
I’ve also seen the numbers go whack-o because a sense line was open and the input pin is “floating”. Check the wires. They can break right at the point where the connector is crimped over the insulation. Try pulling each one gently. If the insulation stretches you just found the break…