Hi,
Had a similar problem where the pixhawk 2.1 power module showed voltage drops of around 7 Volts. Now this would happen if an ESC is shorted but the current draw measured with a clamp meter showed the the current draw was within limit. Tried calibrating it but the values fluctuated too much. Switched the 2.1 power module for an old APM 3DR power module I had lying around and now there is no problem at all. Current and Voltage readings are all good.
https://discuss.ardupilot.org/t/voltage-drop-during-flight/38391/3
If your vehicle is drawing more than 60Amps then I would recommend the Mauch Power modules. I have used them and the accuracy is spot on.
Regards.