# Common Ground For Batteries



## JPB (Feb 2, 2015)

I am in the process of converting an aluminum bass boat into an all electric boat. I have most of the work complete and I have read some literature about having a "common ground" for each of the batteries. Here is the set up I am running:

2 battery bank for transom 24 volt trolling motor
2 battery bank for bow 24 volt trolling motor 
and 1 battery for all accessories

I have a total of 5 batteries in the rear compartment of the boat connected to a 3 bank charger and a 2 bank charger from dual pro. Obviously the (2) 24 volt banks are wired in series but my question is should all 5 batteries be connected to each other for a "common ground". If so why and how? 

Thanks!


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## earl60446 (Feb 2, 2015)

I don't see a need for it from a boat operation or charging standpoint. As a matter of fact, I think it would cause you problems. Batteries don't have a ground connection, only neg and pos.
Tim


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## JPB (Feb 2, 2015)

I seem to agree and not sure why it would be recommended. 

The motorguide operation manual reads:

"Vessels with multiple batteries, must have a common ground bonding circuit. Not establishing a common ground between the vessel batteries, may cause severe corrosion, electrolysis, and electrical shock"

I would like to avoid all of that stuff if it is true?


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## DaleH (Feb 2, 2015)

On my big boats w/ multiple batteries, all round connections go to a common ground post, specifically a "power post" by Ancor Marine. It even has a ring of #8 connections around the perimeter for connecting things like stern-mounted bilge pumps.

BENEFITS - I can take out and remove ANY battery in the bank without losing the ground on any other battery or item in the circuit, even with the boat running and in use!

However - I do not use battery chargers (bigger boats are on moorings), so that could alter the configuration or purpose. I'd contact Ancor Marine for guidance.


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## Bob Landry (Feb 5, 2015)

earl60446 said:


> Batteries don't have a ground connection, only neg and pos.
> Tim



The negative pole on batteries IS the ground, not to be confused with AC Safety Ground, and all battery grounds should be connected to a common point, the preferred method being a Blue Sea Power Post or buss bar capable of handling the combined current load.
The exception would be the ground terminal used to connect two batteries in series for your 24V.
ABYC only allows 4 connections per lug so a Power Post may or may not be suitable for your application. My preference is a Buss bar with multiple screw connections as it allows you to run all of the neg wires on your devices back to a common point.If your buss bar does not have enough terminals for multiple devices, you can run a wire from it to a seperate buss bar to connect all of your DC components.
The suggestion to call Blue Sea for answers is a good one. They have an excellent tech department.


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## DaleH (Feb 5, 2015)

Bob Landry said:


> ... the preferred method being a Blue Sea Power Post or buss bar ...


Ahhh, I wrote 'Ancor' instead of 'Blue Sea', thanks! This is the one I like as it has the additional smaller connection points for items like stern bilge pumps et al.


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## huntinfool (Feb 5, 2015)

To answer your question, I'm going to say no.

With your two and three bank chargers you must NOT have any grounds common. 

In the instructions for the chargers it says you can charge batteries with out having to remove any cables, as long as you have separate grounds. Meaning you don't have to disconnect any wires on your trolling motor batteries even though the positive of one battery is connected to the negative of the other battery. Because the charger has a separate ground for each bank you can do this. However I would think if all of the grounds were connected you would not be able to get 24v or be able to use the charger. 

But I very well could be wrong.


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## Skiffing (Feb 5, 2015)

huntinfool said:


> To answer your question, I'm going to say no. ,snip>



I'm with you. 

In this instance each battery is powering discrete loads that will not be integrated and different voltages / potentials.

On boats with house battery bank and engine battery[ies] of the same voltage the loads may be intermingled via an ABC/ switches.

In that instance a common negative is warranted.


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## JPB (Feb 16, 2015)

Well...I completed the gas outboard to all electric conversion and did so with out a "common ground". I hope to get it on the water soon for a test and I will let you know if anything catches on fire. The outboard I removed from this project I put on my other boat and installed a tiller for it. Thanks for the help!


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## rabbit (Mar 1, 2015)

I read this thread so I went out and measured my battery negative to my hull and got a reading of 2 volts! I don't know why it does this so I grounded the negative to the hull. Problem solved (I think.)


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## earl60446 (Mar 2, 2015)

rabbit said:


> I read this thread so I went out and measured my battery negative to my hull and got a reading of 2 volts! I don't know why it does this so I grounded the negative to the hull. Problem solved (I think.)



What was the problem that you solved? 

Tim


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## earl60446 (Mar 2, 2015)

JPB said:


> Well...I completed the gas outboard to all electric conversion and did so with out a "common ground". I hope to get it on the water soon for a test and I will let you know if anything catches on fire. The outboard I removed from this project I put on my other boat and installed a tiller for it. Thanks for the help!



JPB

Glad you got your batteries and all electrical all hooked up. If you had hooked all the battery negatives together for a "common ground", you would have had some cabling instantly disintegrate, not a good thing. You could have got hurt too.

Tim


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## rabbit (Mar 2, 2015)

Not sure there was a problem but I must have had some inductive leakage to the hull.


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## Skiffing (Mar 3, 2015)

rabbit said:


> Not sure there was a problem but I must have had some inductive leakage to the hull.




No. It's just a difference in electrical potential that used to be isolated. Now you've set up a path for electrolysis - corrosion.


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## JPB (Mar 3, 2015)

Everything is working well but I am still not sure how to form a common grounding circuit between all the batteries like the motorguide manual makes clear if you have multiple batteries. I am waiting for a return call from motorguide to help clarify the statement in the manual.

I am mostly only thinking about this now in order to protect warranty's on the new trolling motors.


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## Skiffing (Mar 3, 2015)

^^^^

Looking at your battery wiring you have two 24 volt batteries which look to be the same make / model / capacity and age.

The 24 volt batteries will get drawn down differently because they have separate loads AND will be charged independently - not together.

Don't connect the negative terminals of the batteries together.

I have a photovoltaic system on a cabin with a 650 Amp / hour 12 volt battery. There are five batteries [130 ah each]. Each battery has ten wet cells [NiCad] I invert this power to 120 volt AC. A negative terminal on one of the chained batteries is grounded to a ground rod.

The AC system is grounded to a different ground rod more than 20 feet away.

That's a ground. Your battery system doesn't have a ground. It has a negative terminal.


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