Why no common ground for two 12v deep cycle marine batteries connected in series?

Question

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.

Answer 1

If you tried connecting a common ground to two batteries that are in series, you would be effectively shorting out one of the batteries. You need a common ground for two batteries in parallel by definition. In order for them to be in parallel, their grounds must be connected to each other (and to the - side of the motor) and their positives must be connected to each other (and to the + side of the motor). You would connect two batteries in parallel to supply more current, or to reduce wear on a single battery (or make it discharge more slowly). You would put batteries in series to supply a higher voltage.

In layman's terms, don't try tying together the grounds of two batteries connected in series, because you'll be shorting out the first battery.

Answer 2

My hunch is that it's a bad use of the term "common ground." In the 12 V example the trolling motor voltage is assumed to be the same as the rest of the boat, so it is safe to make a direct connection between the boat ground and the trolling motor.

In the 24 V scenario, with two batteries in series, I think the concern is that something could go wrong and 24 V could get connected to the 12 V system. This could damage components that are rated for 12 V. To avoid this they want you to isolate the two systems and maintain a separate ground for the trolling motor.

Answer 3

The batteries with shared current in parallel at 12V have common V- (gnd) and common V+ , by definition of parallel.

The two 12 V batteries in series to a 24V trolling motor might have current surges with some voltage drop in the cables and act like antenna for RF noise and disturb main battery by conducted noise, such as an AM radio or ultrasonic sensor.

ISolation on either side reduces induction current noise from motor commutation noise,again just useful for radios and maybe Ultrasonic transducers.

A common ground might not hurt anything if cable if battery routing is far apart, but to avoid potential aggregation, this is the preferred solution. There is also no need to share a common ground for a trolling motor.