Trolling motor common ground?

[Floridaman79]

Question for you guys. So I have 3 lifepo4 batteries on order for a trolling motor installation. I naturally assumed that I would run a ground wire from the negative of the 36v bank to the negative of the house and start batteries, therefore holding the ground at the same electrical potential throughout the boat. When reading the installation manual of the motorguide xi5 manual, it says to only connect a common ground to a 12v trolling motor system and not on a 24 or 36 volt bank. Does anyone know the reason behind this? The only thing that I could think of is if the grounds are not connected, the relative potential could be reduced during a galvanic event like a pull down resistor whereas if the grounds were connected the voltage potential would be fixed at 36v. Does anyone have any thoughts on the common ground between different voltage banks?

 

[HangOutdoors]

On my 24v System I connected the ground to the 24v circuit only. 2 LiFePo4's . MinnKota Terrova.

 

[FSH 210 Sport]

I asked my friend who has been a marine mechanic for some 30 years your question and this is his answer.

“I believe the reasoning behind this is almost exclusively due to the way the split bank charging works on most of these systems. Even though the bank when viewed in series is 36V, the chargers most often used on these systems charge them as 3 individual 12V banks by basically floating the negative terminal of each battery so the charger only has a reference to each isolated battery for charging voltage. If you were to give a true ground or zero volt potential to the negative terminal of battery 1 of the 36V bank and the internal workings of the charger were able to reference that then you would have the potential to radically alter the reference voltage of batteries 2 & 3 of the 36V bank. In a what I will call a “big boat” system one would find on a yacht that lives in the water where galvanic corrosion is a genuine concern and the chargers are referencing true DC system voltage as a total bank instead of 3 individual batteries then yes, it is the industry norm to use a common ground or zero potential point of reference for the entire vessel. On your typical trailerable small fishing boat that problem is largely negated. Long story short, it is because of how the chargers work more than anything”.

 

[Floridaman79]

Interesting theory about the charger but it doesn't really make sense to me. I am not one to argue with motorguide about trolling motors so I'll leave the two systems isolated, it just strikes me as odd.

 

[212s]

Question for you guys. So I have 3 lifepo4 batteries on order for a trolling motor installation. I naturally assumed that I would run a ground wire from the negative of the 36v bank to the negative of the house and start batteries, therefore holding the ground at the same electrical potential throughout

You answered your own question - the potential will not be equal as the 36v system will have a -36v ground potential. You must keep them isolated on their own 12v and 36v circuits.

 

[FSH 210 Sport]

I see I forgot to post this…

It doesn’t make sense to you as you probably don’t have a good understanding of electrical theory. Another possible reason for leaving batteries of different voltages, and or same voltage, as stand alone systems is noise. The Cannon electric down riggers suggest running those DRs on a completely isolated stand alone battery is to keep any noise, aka interference, from mucking up the works.

Your trolling motor has sensitive microprocessors in it that could easily be affected by electrical transients within the boats systems, aka noise. And keep in mind, this noise may not always be present, but it’s intermittent appearance could leave you totally scratching your head as to why there would occasional issues. Following the manufacturers recommendations is always best when installing their equipment.

 

[Floridaman79]

I was able to find out in some more Motorguide literature that the common ground between 12v and 36v systems could result in electrolysis which would cause rapid corrosion of the trolling motor.