DC Engine Bond point

Tom Metzger

Sustaining Partner
Probably so. But the highest drawing circuit on my boat is the 20 A glow plug circuit. Does a lower resistance in the battery grounds really make any difference then (1/3 of zero is still zero, right?). Or is there another reason to have each battery bank (and perhaps the DC panel itself) independently grounded to the block? Charging perhaps? I have the original 50A alternator.
Ah yes, it's a sailboat. For those of us who use their engines the starter draws more than the glow plugs. it's 1000 watts (~83 amps) if fully loaded That's why I put the battery leads on the engine block. Everything else goes to the bus.

Certainly not worth changing. Now, that grey wire hanging... :cool:
 

Tom Metzger

Sustaining Partner
With power circuits the multiple grounds is actually the better practice and having multiple ground points will not have any negative effect on your radios.
How we wire our own boats is fine as long as we don't electrocute our neighbors and the fire stays aboard, but when working on other's boats it is generally best to stick to ABYC code.
 

wynkoop

Member II
Tom-

I find nothing in ABYC that says multiple ground leads to different points on the engine, or in the case of power boats with multiple engines, ground leads to each engine back to the battery is a problem, and from the physics of it you have a lower resistance, less heating, and higher voltage due to lower IR loss.

I am open to anyone who can show me either the danger of multiple ground points, with real science backing it up, or the physics as to why it is a bad idea.
 

debonAir

Member III
The only problem I've seen with multiple ground wires is when more than one goes to the same bolt. The rings, especially the thicker ones, can get slowly flattened and perhaps loosen up and the little slots between them soak up moisture and corrode. As long as you check the nuts occasionally should be no issues.

On another topic, how the heck do you keep your engines and stringers so darn clean? (like I need another time consuming project!)
 

wynkoop

Member II
debonAir-

My suggestion is multiple ground wires to different points. If you look back at my posts I am suggesting to a bolt at the starter and a bolt at the alternator.
 

debonAir

Member III
Oh I got that..I am doing the same as you. I was just commenting in general, I've seen problems with stacks of ring terminals on the same bolt, so try not to do that myself.
 

Tin Kicker

Member III
Oh I got that..I am doing the same as you. I was just commenting in general, I've seen problems with stacks of ring terminals on the same bolt, so try not to do that myself.
I see nothing in ABYC that prohibits a second ground.

As far as stacking ring terminals, the max is 3 on a post in aviation and while I haven't looked to see if ABYC is the same, it is good guidance to prevent excessive resistance and heating.

Speaking of heat, it's been raining a lot lately outside so I've started to re-wire and came across this spade connector behind the alternator. The shrink tube on the end of the purple wire should be translucent but in addition to the gold overspray the widest edges are blackened from resistive heating. The metal in it is blackened even though the corrosion on the spade doesn't look bad. This mess will be removed and routed to a terminal on the new connector strip on the engine stringer that takes the place of the trailer connector.
 
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Tom Metzger

Sustaining Partner
Christian can be very quiet and is, therefore, often ignored so I will repeat what he said. "I WOULD HATE TO GIVE UP MY BATTERY MONITOR".

I will also repeat what ABYC says: "The negative terminal of the battery, AND the negative side of the DC system, shall be connected to THE engine negative terminal or its bus."

And as far as reducing resistance goes, Ericson battery circuits were wired with 1/0 cu wire so we are talking about 0.01V per foot at 100 amps. I'm curious where on your boat there is 100 amps flowing for any length of time where a few hundredths of a volt will matter.

I'm not saying anyone is wrong if their wiring works safely. Just don't suggest it's THE "correct" way. I suspect the insurance surveyors and the "glass kickers" prefer things done by code.

OK, those are my last words on the matter.
 

wynkoop

Member II
I can not fathom how multiple parallel connections to the negative of the battery would force anyone to give up their battery monitor.

You will note Tom the ABYC says it shall be connected. It does not say it shall be connected once. Just it shall be connected. If it is connected once it is connected. If it is connected twice it is connected if it is connected 3 times it is connected.

My E-27 was not wired with 1/0. It was wired with 4 gauge from the battery to the old Palmer, and that 4 gauge was connected in multiple places on the Palmer that I pulled out last fall. If you have 1/0 great.

So far as when does it matter to have lower resistance, well when batteries are low every bit matters if you want to start, but more importantly if you have a SSB radio installed in your boat you want the best ground system with the least resistance to both have best radio operation and to keep stray RF currents out of places where you do not want them.

As I said above most folks probably will not notice the difference, but I will stick with multiple ground leads from the battery bank to the engine tied to the starter and the alternator.
 

trickdhat

Member III
Blogs Author
Ah! I finally understand the connection. If you literally take multiple ground leads from the battery, one of the ground leads would be bypassing the shunt making it useless. I don't think that was the intent, at least not mine.

After all this conversation, I'm leaning towards a negative bus in the battery compartment acting as the main DC ground bus where the batteries, dc distribution panel, & 24 hr circuit negatives are connected. This will also be where each negative post of the battery banks will terminate (eventually through a BM shunt on the house side). The main negative bus will be connected to another bus in the engine compartment acting as the main ground bus wich will be connected to the starter ear (to provide minimal voltage drop to the starter) and to the B- part of the alternator (to provide more efficient charging) . The ground bus is where the ac ground and all other ground wires will land.

I understand the ground bus will technically add resistance and an additional connection, but the benefit of not having a ton of cables connected to the engine as well as the starter and/or alternator not having to rely on the block for their return path outweighs the negative. I'll even get to add a battery monitor like the Victron on Luffalee (although the Balmar looks intriguing).

This configuration should get all the connections off the exhaust elbow, clean up the negative terminals of the batteries, provide the single return where I can insert a BM shunt, and reduce voltage loss on the starting and charging circuits. Now I just have to dust off the cable sizing calculator and get to work.
 
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wynkoop

Member II
trickedhat-

Why is your shunt in the negative lead? Mine is in the positive lead.

So I take it your diagram looks like:


Battery ________ shunt _________ Engine

That just seems odd to me?

In general ground never has any intentional resistance introduced in it's path.

I am curious about what Battery Monitor you have. I want to look up the manual and see why they are doing what they are doing.

My shunt is on the common output of my battery switch.
 

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
FWIW, our previous Link 10 required a shunt for the negative side, near the battery terminal.
(When it started malfunctioning after about a decade, I replaced it with a BalMar Smartgauge, which at the time did not use a shunt).

Note that when you use a high quality terminal block to consolidate your ground cabling, you are not adding much/any resistance to the circuit. I went with a Blue Seas bus bar.
 

wynkoop

Member II
Loren-

Very interesting. I can not fathom why they made that engineering choice, but now I understand why I have never had the negative lead shunt experience.
 

Tom Metzger

Sustaining Partner
Because of the multiple wires usually going to the battery positive terminal the shunts normally go in the negative line. Victron, Xantrex, Blue Seas, et al.

Connecting to the common terminal of the 1-2-both switch tells you the boats use of power, but not the state of charge because you are connected to multiple batteries and the charger current is probably not included. Most people can afford .0001 ohm of the shunt, equal to about 3/64"of #4 wire.
 

trickdhat

Member III
Blogs Author
I don't currently have a shunt on the new boat I'm asking about, but I have a Victron BM-702 on my 25+. I don't have the manual in front of me, but if I remember correctly, they recommend placing it on the lower potential side for safety reasons. It's rare, but why not remove the potential of arcing when given the chance?

For the DC plants I work on professionally, the shunt is placed on the positive side, but that's because it is a -48 vdc system and the positive side is the lower potential to ground. We all say we're careful and know where the other end of the wrench is at all times, but accidents happen.
 

Tom Metzger

Sustaining Partner
Typically in an industrial situation the shunt is strictly intended to read current and is not tied into an integrated unit as a battery monitor such as the Victron BMV series with both the voltage and current tied into the shunt circuit. It can go wherever the current to read exists.

On a boat frequently the only place the current to be monitored exists is in the negative lead. Charger wires go to the positive terminal.
 
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debonAir

Member III
All the manuals I looked at for battery monitors (Victron, Renogy, Blamar) say to put the shunt on the ground side. I bought the Renogy because it shows volts, amps, and SOC all at the same time, handles 500A, and cost 1/3 the others. Once I got the shunt it was pretty obvious why you want it on ground: a big hunk of metal hanging off the positive terminal would be sparks and shorting hazard. Even tied to the ground side I plan on putting the thing under some kind of cover.
 

wynkoop

Member II
Interesting, well my boat came with the shunt at the Common of the battery switch when I got it in 1984, and I do not have my charger wired directly to my batteries, but rather to the common of the battery switch. Yes that means I do not charge with shore power the off battery, but it has not been an issue for me.

The only thing connected directly to the battery, besides the cables to the engine and the switch is a pair of automatic bilge pumps. I have one on each battery direct via fuses so they will operate no matter the state of the battery switch, and if one battery has died the other pump will still run.

I suspect if I had a shunt in the negative I would set it up like this:

battery ---------shunt========engine at 2 points======negative bus

But I doubt that will ever happen. My instrumentation for batteries consists of the Ammeter in the panel and a pair of digital voltmeters tied to the battery connections on the battery switch. With this I know the state of charge of both my batteries and I know current drain or charge.

I suspect if I had a bigger boat with a larger bank and a dedicated starting battery I would do things differently, but for my needs dual voltmeters and a single ammeter do the trick.

I am glad to have learned that so many systems have the shunt in the ground.
 

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
To avoid a crowd of wires to the battery terminals, you should add a BlueSeas fused terminal to the battery plus.
Look at one of the pix in this thread to see it in use. If you must (!) wire a bilge pump directly, this lets you safely fuse it as well.
 
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