Grounding Batteries to Eng. Block - SailNet Community
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Old 09-26-2012 Thread Starter
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Grounding Batteries to Eng. Block

Started this thread regarding Shunts:
How do Shunts work?
A issue I solved there prompted me to research the Battery/Ground relationship. The book; The 12 Volt Bible For Boats, by Miner Brotherton states:

"Ground is considered to be either an infinite "sink" or an infinite "source" for charge. That means if we have an excess of charge, ground will take all we can give it. If we have a deficiency of charge, ground will supply us with all we need."

I think I understand the "sink", meaning ground gives excessive charge some where to dissipate. However, I do not understand how ground supplies us with all the charge we need. If my alternator fails (for example), ground ain't going to help me, right?
Can someone explain what Botherton means?

Last edited by L124C; 09-26-2012 at 04:27 PM.
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Old 09-26-2012
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Re: Grounding Batteries to Eng. Block

Well just based on that snippet alone I can't know exactly where they are going with that but there are a few things.

First, a simple example of ground sourcing is when you have negative voltage supplies. Negative voltages are common in all sorts of electronics for various reasons (especially perhaps in measurement devices like your shunt monitor). There is nothing special about them except that if you hooked up a multimeter you'd see a minus sign in front. If you wanted, and I've never heard of it done, you could wire your entire boat upside down with + connected to the engine etc, and - fused and switched to all your loads. In this case "ground" would be "sourcing".

Second you could have short term negative voltages due to electrical noise, static, or inductive devices like motors that can generate voltages especially when they turn on or off. In this case ground can be seen to sourcing current.

However, I think the bottom line that they're getting at is that ground is your reference - it doesn't matter what you do to it, it stays the same. In usual boat applications, 99% of the time this means that ground is sinking current.

Last edited by asdf38; 09-26-2012 at 11:10 PM.
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Old 09-26-2012
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Re: Grounding Batteries to Eng. Block

In what context was he using this statement as it relates to the DC system on a boat???

Theoretecally and ideally, earth ground is an infinite source or sink of charge which means it can give electrons to a positively charged object until it becomes neutral and vice-versa.

On a boat, you might have a floating ground, a ground connected to earth ground via a seawater connection or you may not even have a ground at all. Either way, I would not consider your boat ground as an infinite source/sink of charge and certainly not something that is going to magically provide a source of power

Eric
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Re: Grounding Batteries to Eng. Block

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Re: Grounding Batteries to Eng. Block

Quote:
Originally Posted by asdf38 View Post
In this case ground can be seen to sourcing current....99% of the time this means that ground is sinking current.
While the DC negative (DC grounded conductor) may be connected to ground, they are not the same. DC grounding conductors are normally non-current carrying.

Eric
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Old 09-27-2012 Thread Starter
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Re: Grounding Batteries to Eng. Block

Quote:
Originally Posted by Capt. Gary Randall View Post
If you have a specific question you need answered, feel free to email it to us at service@marinesurveysplus.com and we will be happy to reply with an answer.
Thanks, but I thought the question in my OP was specific. I wouldn't know how to pose it any other way.
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Re: Grounding Batteries to Eng. Block

Quote:
Originally Posted by fairbank56 View Post
While the DC negative (DC grounded conductor) may be connected to ground, they are not the same. DC grounding conductors are normally non-current carrying.

Eric
Fair. But I'm still not sure which one he was asking about. I consider it safe to assume that when someone says ground, especially in a DC system they mean return, or in this case battery (-). I also suspect this text was trying to lay out what ground is in a general sense which blurs the lines between earth, return, (-) etc.

On and I missed the most obvious example of where ground can sink or source current - AC power. In your house the neutral line, which is supposed to be connected to earth ground, both sinks and sources current. Also if the hot wire accidentally hits something that's grounded then ground will both sink and source current.
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Re: Grounding Batteries to Eng. Block

Here is a link to the best article I have come across about grounding - by Stan Honey.

Grounding

Well worth reading.

Brian
Living aboard in Victoria Harbour
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Old 09-27-2012
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Re: Grounding Batteries to Eng. Block

Quote:
Originally Posted by L124C View Post
Started this thread regarding Shunts:
How do Shunts work?
A issue I solved there prompted me to research the Battery/Ground relationship. The book; The 12 Volt Bible For Boats, by Miner Brotherton states:

"Ground is considered to be either an infinite "sink" or an infinite "source" for charge. That means if we have an excess of charge, ground will take all we can give it. If we have a deficiency of charge, ground will supply us with all we need."

I think I understand the "sink", meaning ground gives excessive charge some where to dissipate. However, I do not understand how ground supplies us with all the charge we need. If my alternator fails (for example), ground ain't going to help me, right?
Can someone explain what Botherton means?
There is a lot at play here:

Wiring batteries to the engine block is required with case grounded starters and alternators or there is an incomplete circuit. On some boats, such as metal hulls, the starter can be only momentarily connected via solenoid then disconnected and the alternator is isolated ground.

There are four main things at play:

Lightning grounding (earth potential non current carrying)
Dissimilar metals bonding (earth potential non current carrying)
Negative DC current carrying conductors
AC grounding

There is a difference between "grounded" and "grounding".

DC Negative Conductor (grounded conductor) = Black OR Yellow (Yellow is ABYC preferred but black still acceptable)

DC Grounding Conductor = Green or Green W/Yellow stripe. This is for AC and DC GROUNDING or EARTHING and is not for negative return conductors. DC grounding is optional on the DC side unless you are wiring an inverter or charger and then the case needs to be earthed with green or green/yellow tracer. Bonding wires and lightning grounding also use green for earthing...

Your boat is attached to "Earth" for safety reasons or to help minimize corrosion. Your inboard engine is connected to the batteries to complete the starter, fuel pump or alternator circuits unless other means of connecting the neg conductors to the battery exists. With a DC only system you can have a system totally isolated from Earth, as many small OB powered sailboats do, but Earthing the boat somewhere is normally preferred and this spot is most often the engine on inboard sailboats..

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Last edited by Maine Sail; 09-27-2012 at 01:21 PM.
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Old 09-27-2012 Thread Starter
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Re: Grounding Batteries to Eng. Block

Quote:
Originally Posted by asdf38 View Post
Fair. But I'm still not sure which one he was asking about. I consider it safe to assume that when someone says ground, especially in a DC system they mean return, or in this case battery (-). I also suspect this text was trying to lay out what ground is in a general sense which blurs the lines between earth, return, (-) etc.
I (and I think, Botherton) was referring to the battery-motor Neg connection. He begins the paragraph with: "GROUND refers to a connection made to the earth itself"....(paraphrasing)"On land, through a pipe driven into the ground, on our boats, ultimately to Seawater". He goes on to say that ideally (and by ABYC recommendation): "Each item in the circuit is connected by its own pair of wires and the common ground carries no load current." (except on my boat on which I now realize, it completes the circuit for the Alt. and Starter!).
Anyway, I was specifically asking how GROUND "will supply all (charge) we need". (My boat notwithstanding!)

Last edited by L124C; 09-27-2012 at 02:37 PM.
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