My trolling motor instructions advise that a common ground MUST be established for connecting the trolling motor either to one 12 volt marine battery or to two 12 volt deep cycle marine batteries connected in parallel. This is to prevent corrosion, electrolysis, battery deterioration, etc.
However, if two 12 volt deep cycle marine batteries are connected in series, to produce 24 volts of power, the instructions specifically say NOT to try and establish a common ground. Instead, they advise to just keep the trolling motor wiring and other component wiring separate on different sides of the boat to prevent electrical interference.
I'm not an electrician or an engineer. I have only limited understanding of electrical wiring. I don't understand why a common ground is essential in the first instance and not advised at all in the second instance. Please explain in terms a layman can understand. Thank you.