Quote Originally Posted by jawjatek View Post
You don't "have to". In electrical engineering we deal with a phenomena called a "ground loop" that can introduce common-mode noise between two devices that are at different ground potentials, or that share grounds with other signals. However, since the connection between your head and the GLS10 black box is ethernet, it may very well be transformer-coupled, which would attenuate the common-mode noise. Also, the ethernet signals are differential, which provides even more attenuation of common-mode noise.

Using two separate batteries on a boat means one battery's ground will be "floating" with respect to another battery, unless the grounds are bonded otherwise. As soon as you connect the 2 devices, the grounds will be connected by the shield or ground of the connecting cable. Any difference in ground potential will cause noise current to flow in the shield conductor. This tends to increase the common-mode noise. A common trick in audio engineering is to float one end of the shield, this is called a "ground lift".

In sonar/chartplotter installs, it is always best to avoid sharing a battery with a trolling motor, in case you were thinking of this, because the arcing inside the brushed DC motor, as well as the pulse-width modulation of the drive voltage, generates lots of wide-band noise which can couple into sensitive signals in the sonar system.

I'm not sure why you need to use different power sources for the head and the GLS 10 box. It may very well work fine. Try it.

Thanks for the feed back jawjatek, I have my 93sv running off the engine battery and was planning on adding a Duracell Ultra 12V 35AH Deep Cycle AGM SLA Battery - SLADC12-35J at Batteries Plus Bulbs up front to run the black box. I was concerned that a voltage drop difference between the two units may cause a problem, but that was the only thing I could see.

John