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Old 09-22-2021, 12:04 PM   #1
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Negative 12V bus

I'm helping out a fellow boater in my marina with some electrical issues on his 36 foot open sportfish style boat. The boat has 4 battery banks:


Starboard engine/house
Port engine
Genset starting
Bow thruster


The boat had horrible galvanic corrosion in the past. The owner installed a galvanic isolator and is hoping that fixed the problem.


Other people have found and fixed starting and dim light problems with the starboard engine/house +12V wiring. AFAIK, they did nothing to the -12V connections


I checked continuity of the bonding bus and -12V bus to the battery banks. Only the starbard engine/house bank -12V has good continuity to the bonding a -12V buses. The other three banks have 55 to 75 ohms of resistance from the -12V battery posts to the bonding and -12V buses.



I don't see any deliberate connection made between the -12V battery posts to the bonding and -12V buses on the other three banks. It seems like any "continuity" (55 to 75 ohms) is purely by chance.



I plan to recommend adding a bus bar and 5 wires, one to each battery bank -12V post, and one to the existing -12V bus (it's too small to connect properly-sized battery cables to). This will tie everything together at the batteries. Any reason not to do this?
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Old 09-22-2021, 01:32 PM   #2
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I think your approach to creating a common negative bus is a good one.


BTW, galvanic corrosion comes from stray DC current, not stray AC. So your focus on the DC is the right place to be looking. A positive DC wire exposed to bilge water is a classic source of corrosion, so check wiring to bilge pumps, bilge pump switches, etc.
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Old 09-22-2021, 04:06 PM   #3
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There should be one central ground bus where all - connections can be traced to. All batteries, all engine blocks, etc. A sub-bus that traces back to the central neg bus with one cable is good. for the purposes of bonding 55-75 ohms is practically open.

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Old 09-22-2021, 06:26 PM   #4
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For bonding it should be 1 ohm or less. I would also pull every connection off and make sure they are both clean and tight.
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Old 09-23-2021, 12:10 AM   #5
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Quote:
Originally Posted by twistedtree View Post
I think your approach to creating a common negative bus is a good one.


BTW, galvanic corrosion comes from stray DC current, not stray AC. So your focus on the DC is the right place to be looking. A positive DC wire exposed to bilge water is a classic source of corrosion, so check wiring to bilge pumps, bilge pump switches, etc.
this LINK
Quote:
GALVANIC CORROSION on the other hand is an electrochemical reaction that causes electrons to flow from one metal to another metal.
STRAY CURRENT CORROSION is simply corrosion caused by stray (leaking) current from a bilge pump or a battery charger among other things.
Stray current might be either AC or DC through the bonded ground wire > shore power > another boat > water. IMO (This is a tough subject)
Quote:
Recent tests have shown that AC current from shorepower in the water can also cause corrosion to underwater parts although at a much slower rate than DC
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Old 09-23-2021, 06:15 AM   #6
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So the boat owner in question does not pussyfoot around (he's a retired merchant marine captain). After sharing my advice, he offered to pay me really well to do the work right away, went to Hamilton Marine and got all the stuff necessary to make a good ground bus, and a couple of hours later I had the work complete. I have decided to start accepting work like this when I have free time, it's easy money for me and there's a huge demand for this kind of work in my marina. I don't "need" the money, but it's nice.
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Old 10-11-2021, 08:09 PM   #7
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Originally Posted by Mischief Managed View Post
So the boat owner in question does not pussyfoot around (he's a retired merchant marine captain). After sharing my advice, he offered to pay me really well to do the work right away, went to Hamilton Marine and got all the stuff necessary to make a good ground bus, and a couple of hours later I had the work complete. I have decided to start accepting work like this when I have free time, it's easy money for me and there's a huge demand for this kind of work in my marina. I don't "need" the money, but it's nice.
I highly recommend you research using a half bridge such as a silver-silver chloride and actually MEASURE the voltages between it and EVERY bonded ground on the boat.

I use a single divers plate style aluminum anode mounted onto my boat's transom. All of my exposed metals are bonded to the boat's grounding system. If metals are deteriorating at a slow pace, most likely the problems are galvanic; if at a rapid pace the problems are usually the result of unwanted currents in the water.
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Old 10-13-2021, 07:48 AM   #8
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Originally Posted by foggysail View Post
I highly recommend you research using a half bridge such as a silver-silver chloride and actually MEASURE the voltages between it and EVERY bonded ground on the boat.

I use a single divers plate style aluminum anode mounted onto my boat's transom. All of my exposed metals are bonded to the boat's grounding system. If metals are deteriorating at a slow pace, most likely the problems are galvanic; if at a rapid pace the problems are usually the result of unwanted currents in the water.

I may eventually do that, but only if I need to. At this point everything is working (read below), so I won't waste his money looking for problems he's not having.


The owner had the boat hauled last week and the anodes were about 20% to 30% depleted after a full season in the water, so it seems the galvanic isolator solved his original corrosion issues. Ironically, I learned that I was the person, via a middle-man, that suggested that he put the galvanic isolator in. One of the professional boat repair people at the marina had asked my advice about a boat that had bad corrosion issues last year and I recommended firstly making sure there was a galvanic isolator installed. Turns out this is the same boat he was working on...



I did some electrical installation work for him yesterday and the owner reported to me that all of his electrical stuff is working better than ever, so I am guessing the ground bus I installed solved some other issues too. Sadly, I found quite a lot of random non-boat-wire installed by the factory in his boat and lots of poorly made crimp connections. I fixed what I could, but I suspect he will eventually discover more electrical problems. The boat is headed to the Bahamas for the Winter soon, so he will have ample opportunity to uncover new electrical problems. At least his battery, starting, charger, ground, nav lights, horn, and helm electronics wiring is all checked out and good.
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Old 10-13-2021, 07:59 AM   #9
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Excellent thread about checking grounding, which is often the problem in DC circuits.
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