I don`t know what you call it in english but after you have installed the battery into console you by a calibration plate that you put under the compass, it will take away all the magnetic field around the compass.
Offcourse you have to calibrate the compass by driving a fixed course.
We use it in our boats here in Swe even if battery is sitting somewhere else.
There is always magnetic fileds from electronics etc.
The plate does`nt cost very much just a couple of €uros.
Go to Silva in Finland they have a great one that we are using.