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Thread: Boat DC Wiring Low Voltage Question

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    Default Boat DC Wiring Low Voltage Question

    Hey gurus,

    Question. Brought the boat home. Boat is on the trailer at the house. Doing some testing as I am going to rewire a few things and ran into this. If I test with my DVOM from the negative post on a battery to the hull I get 0volts with all the switches off. When I turn the switches on with nothing turned on in the boat I get same result. Normal and no voltage leaks. But when I start turning stuff on in the boat I start getting low voltage between the ground post of battery and hull, starts around .004 volts and climbs with the more stuff I turn on. Not sure if its normal, a lame DVOM, or a problem. It I turn on my radio and lights I will see around .010 volts from the negative of the battery and the hull. I will show .0 from a negative buss bar and the hull though. As I increase the load by turning on more stuff and turning the stereo up and increasing the draw from the batteries I will see it climb. It only reads from the battery bank supplying the power. I have got it as high as .3 volts with more stuff turned on. If isolate my house and my start banks from a positive standpoint (like they are normally) I will only see this on the bank battery supplying the voltage. If I turn on something tied to my start bank when it is isolated from the house (like the bilge or pot puller) I will start to see voltage off that bank doing the same test. If I turn off my house bank completely and bridge my start bank to my main panels I get it at the start bank and not the house, supply power from the house and I get it from the house. If I isolate the house from the boat and run something tied directly to the house with a switch (my crane winch) I see voltage when the winch motor is running. I am not sure if this is normal or if it is pointing to a problem. All my connections are real clean and all my buss bars are sprayed with battery terminal protectant and I don't see any signs of corrosion on connectors. Wondering if I am just paranoid or if thats a big deal? Like I say it is .0 with no load, when I start adding the load I see it. In fact I have 6 volt batteries in my house bank tied in parallel and series. If I go from the negative on one 12v set to the negative on the other 12v set I see the same thing as load is added. I don't see it from the other buss bars further from the bank I assume because of resistance but I am an armature trying to figure this out to prevent damage. My zincs all look like they are wearing normal. I do have some white speckles here and there on the boat but that is typical for an aluminum boat based on what I see on all the other boats. I have a few more tests to run but at this point I have bypassed 90% of the back bone wiring using jumper cables direct from the house bank to the buss bars under my dash for my main components and all the results are the same.

    Any thoughts?

    -Derrick
    ... aboard the 'Memory Maker' Making Memories one Wave at a Time!

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    I think you're probably chasing your tail. In THEORY you should see zero volts on ground, but 3/10 of a volt isn't exactly catastrophic. One thing you might check is the resistance on all of your ground/bonding straps (cable or flat braid strap from neg terminal to aluminum hull, egine block to hull, etc.) You should see less than .003 ohms. High resistance in your ground could, conceivably, cause a voltage potential difference (i.e. voltage) between battery and hull. Lastly, check and see if you have amps flowing in that circuit where you're seeing voltage. That could be a concern, if the battery is draining with the key off. The only other thing you've mentioned is you're running 6vdc batteries rigged in series to get 12dc and then in parallel. If any of those batteries is at a lower state of charge, you may see a voltage difference between negative and hull. Maybe. If it were my boat, I wouldn't spend a lot of time on it.

    Quote Originally Posted by DMan View Post
    Hey gurus,

    Question. Brought the boat home. Boat is on the trailer at the house. Doing some testing as I am going to rewire a few things and ran into this. If I test with my DVOM from the negative post on a battery to the hull I get 0volts with all the switches off. When I turn the switches on with nothing turned on in the boat I get same result. Normal and no voltage leaks. But when I start turning stuff on in the boat I start getting low voltage between the ground post of battery and hull, starts around .004 volts and climbs with the more stuff I turn on. Not sure if its normal, a lame DVOM, or a problem. It I turn on my radio and lights I will see around .010 volts from the negative of the battery and the hull. I will show .0 from a negative buss bar and the hull though. As I increase the load by turning on more stuff and turning the stereo up and increasing the draw from the batteries I will see it climb. It only reads from the battery bank supplying the power. I have got it as high as .3 volts with more stuff turned on. If isolate my house and my start banks from a positive standpoint (like they are normally) I will only see this on the bank battery supplying the voltage. If I turn on something tied to my start bank when it is isolated from the house (like the bilge or pot puller) I will start to see voltage off that bank doing the same test. If I turn off my house bank completely and bridge my start bank to my main panels I get it at the start bank and not the house, supply power from the house and I get it from the house. If I isolate the house from the boat and run something tied directly to the house with a switch (my crane winch) I see voltage when the winch motor is running. I am not sure if this is normal or if it is pointing to a problem. All my connections are real clean and all my buss bars are sprayed with battery terminal protectant and I don't see any signs of corrosion on connectors. Wondering if I am just paranoid or if thats a big deal? Like I say it is .0 with no load, when I start adding the load I see it. In fact I have 6 volt batteries in my house bank tied in parallel and series. If I go from the negative on one 12v set to the negative on the other 12v set I see the same thing as load is added. I don't see it from the other buss bars further from the bank I assume because of resistance but I am an armature trying to figure this out to prevent damage. My zincs all look like they are wearing normal. I do have some white speckles here and there on the boat but that is typical for an aluminum boat based on what I see on all the other boats. I have a few more tests to run but at this point I have bypassed 90% of the back bone wiring using jumper cables direct from the house bank to the buss bars under my dash for my main components and all the results are the same.

    Any thoughts?

    -Derrick

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    Quote Originally Posted by FL2AK-Old Town View Post
    I think you're probably chasing your tail. In THEORY you should see zero volts on ground, but 3/10 of a volt isn't exactly catastrophic. One thing you might check is the resistance on all of your ground/bonding straps (cable or flat braid strap from neg terminal to aluminum hull, egine block to hull, etc.)
    I dont think you want any grounding intentionally to the hull. Could be difficult to track down, maybe a eyelet in a light fixture. I dont think its enough to worry about and suspect most metal boats would show some small voltage to ground. You would have to disconnect everything and start connecting one item at a time to track it down.... I would want to know....
    “Nothing worth doing is easy”
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    Just think of what you are measuring across as a big shunt. That will explain the small readings. If you had some bad joints or to small wire size you would get big readings.
    When seconds count, the cops are just minutes away.
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    The voltage between ground and the hull needs to be ZERO volts for corrosion control and AC protection and other problems. There also need to be a single connection (cable) going from the DC main negative bus to the hull to prevent ground loops.

    Any one or a combination could cause your problem, not following ABYC wiring standard, mutable grounds causing a ground loop, using to small of wire, bad connection, etc.


    If you have white speckles that telling me there is a corrosion problem and it could be caused by a number of things, voltage difference between the hull and DC ground is one cause. Just because the zincs look good does not mean you are protected from corrosion. With the boat out of the water it will make it harder to find what is causing the corrosion problem, assuming it is your boat that has a problem and not another boat causing corrosion on your boat.

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    Hey guys, sorry for the delay getting back. Was out of town last week. Here is where this stands.

    After chasing my tail I called Blue Seas and talked to a Marine Electrical guy who then consulted with one of his Senior guys. They believe what I am seeing is voltage drop. He feels the voltage drop is a little high but still under 3%. I have always thought the backbone was too small of wire so I am going to rip the backbone out this winter and reroute for shorter runs and upgrade the wire size.

    To test the theory I did this. Made sure all batteries were charged up and pretty much equal. When I test from the negative of the house bank to the hull I get 0.000 with no load (in this case from the negative of the house bank to the hull is a long run to get back to the motors and then back up to the hull. As I increase load the DVOM shows more voltage. If I start testing moving closer to the battery in the DC negative bus the voltage slowly decreases until it reaches 0.002 or so. Another test I did was disconnect my dash from the + and - buses and used a par of jumper cables directly to the house bank, I got higher readings from the house to the dash negative bus bar as the jumper cable wire isn't as big I suspect. I added the + and - bus back in and added the negative from the jumper cables to jumper straight across to provide more ground and the voltage dropped lower. So basically, more wire between dash and house the lower the voltage goes.

    What do you guys think about the voltage drop theory??? At this point I tend to think its right since I can do the same tests off my start batteries totally isolated from house and get similar results.

    Two other things have come out of this.

    1. Your DC negative bus should only bond to the hull in one spot. On a outboard aluminum boat this is done through the motors. This prevents grounding loops which cause current. HOWEVER! I have found that many other things create this connection and I am not sure about how to get around it. For instance, my Wallas stove top and oven both have continuity from the ground wire to the hull. So how do you get around the fact that there are components in your boat that will have ground to hull through the chassis and ground to the neg bus bar?

    2. Your AC ground should only be connected to the DC ground in one spot. However several things like inverters, battery chargers, AC panel face, etc. are connected to both AC ground and DC ground (DC ground is typically a chassis ground) and they provide continuity from AC Green wire to DC Neg bus, I have tested this thoroughly. So, what do you do in the case where your AC is gournded to DC in multiple spots? This is all marine equipment!

    Lastly, I think I have found the source of my corrosion spots, I have some inside the hull as well. Some of this may be just normal salt water environment but I suspect it may have been accelerated by a reverse polarity issue I have had. I use a Honda eu2000i generator to charge my house bank when I am on the water. When running my generator I get a reserve polarity light. My boat is wired with the shore power direct to my AC main breaker as I don't have an inverter to run through first. I have tested my Generator on other boats and they don't get it although I suspect this was due to the fact they went through an inverter/charger first. I was concerned about the reverse polarity light when I first got the boat so I did some research and found lots of articles that talked about reverse polarity when using a Honda portable generator and they all pretty much came to the conclusion that it was a false positive becuase the portable generator has a floating ground. Everyone seemed to just play it off. I read a few places where people had made a bonding plug for the ground and neutral on the generator to 'turn the light off'. I didn't want to just turn the light off to potentially mask an issue.

    After talking to Blue Seas engineers and doing a bunch of testing here is the deal for you guys that run portable generators. Ground and Neutral should be bonded at the source of power. When I did the testing without the bonding, even though Ground to Neutral was 0.00v, as soon as you plugged it into the panel suddenly the DVOM confirmed voltage accross the Neutral and Ground panel bars, sometimes up to 60vac. I suspect this is what caused my corrosion issue (glad I caught it when I did, I don't appear to have any pitting just color spots). As soon as I bond the neutral and ground at the generator this voltage goes away and I get normal voltage readings at the panel. Hot to Ground = 120vac, Hot to Neutral = 120vac, Neutral to Ground = 0.00vac. Without the bonding I would get Hot to Ground = 60vac, Hot to Neutral = 120vac, Neutral to Ground = 60vac, I still don't understand why I would get this since without being plugged in the generator sitting by itself would get Hot to Ground = 0.00vac, Hot to Neutral = 120vac, Neutral to Ground = 0.00vac.

    So, for any of you guys that have read about reserve polarity using a portable generator and heard it to be no big deal, it is a big deal and can be fixed by making a bonding plug. Marine generators come with the neutral and ground bonded from what I understand. Portables are made for power tools and such that do not require a ground.
    ... aboard the 'Memory Maker' Making Memories one Wave at a Time!

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    Voltage drop theory makes sense, if you are looking for more info and have a desire to know electrical systems (and it sounds like you do), this book is a great resource.

    I bought one from Amazon and am part way through it, I will need to go back and reread some sections twice, maybe more. But Nigel lays it out very well and is quite thorough in his explanations.

    You may want to get a copy of the ABYC standards as well, it has a ton of info but not necessarily the why and how.

    I think its worthwhile getting it right, I suspect there are many boats that have electrical issues that are never caught until something fails or damage is done.
    “Nothing worth doing is easy”
    TR

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    What kind of voltage and resistance are you measuring from Batt+ to hull? From Batt- to hull? I am seeing what I think are large size discoloration pitting marks on mine. Going to do some checking tonight. I checked last Thursday night on the batt+ to hull and I basically see a dead short at .4 ohms and am reading the same batt- to batt+ or batt+ to hull - - - same voltage reading.

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    Measuring the battery voltage to batt- and to the hull is not the proper way to determine what is causing the corrosion. Also .4 ohm from the battery ground to the hull is two high of a reading, it should read 0.001 ohms.

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    Quote Originally Posted by Bullelkklr View Post
    What kind of voltage and resistance are you measuring from Batt+ to hull? From Batt- to hull? I am seeing what I think are large size discoloration pitting marks on mine. Going to do some checking tonight. I checked last Thursday night on the batt+ to hull and I basically see a dead short at .4 ohms and am reading the same batt- to batt+ or batt+ to hull - - - same voltage reading.
    Whenever I do a continuity test I am getting 0.00 from the battery - to the hull. Which is should be. If you are getting .4 you have resistance or corroded connectors somewhere. If you do a continuity test from the battery + to the hull you should get no continuity because you shouldn't have connectivity from your + to the hull otherwise you have a positive wire touching or corroded to the hull somewhere I would think. Also if you do a continuity test from battery - to battery + you shouldn't get any continuity either.
    ... aboard the 'Memory Maker' Making Memories one Wave at a Time!

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    That is what I thought. So how should the measurements be? Batt+ to hull I am getting 12.56 volts...same as the batt+ to batt-. Not good I think. I am going to take a closer look tonight. I basically am seeing a dead short between batt- and the hull. Which I think is bad. Does the outboard motor ground the engine block to the hull and then the negative (-) wire from the engine to the battery- connection make a looped circuit. It's a yammi 90 4 stroke outboard. Aluminum hull. No shore power. I plan to measure resistance as I disconnect each individual ground (-) wire in attempt to isolate. I must have a short somewhere or an accessory tied solidly to the hull at some point, unless it is the motor.

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    Quote Originally Posted by Bullelkklr View Post
    That is what I thought. So how should the measurements be? Batt+ to hull I am getting 12.56 volts...same as the batt+ to batt-. Not good I think. I am going to take a closer look tonight. I basically am seeing a dead short between batt- and the hull. Which I think is bad. Does the outboard motor ground the engine block to the hull and then the negative (-) wire from the engine to the battery- connection make a looped circuit. It's a yammi 90 4 stroke outboard. Aluminum hull. No shore power. I plan to measure resistance as I disconnect each individual ground (-) wire in attempt to isolate. I must have a short somewhere or an accessory tied solidly to the hull at some point, unless it is the motor.
    The motor is your connection from the DC Ground Bus to the Hull. Your hull is grounded to the DC Bus this way. So if you test DC from the Batt+ to Hull you should read same voltage as going from Batt+ and Batt-. If you get less voltage then you have resistance I think...

    If you turn on your switches and you see voltage by going from the batt- to the hull then you have a voltage leak for sure. In this case the batt- is your ground and the hull is your positive, you don't want to see current there. Thats where I am seeing the .005 - .3ish depending on how much I turn on and am being told it is voltage drop and normal. The concern would be if it was 1v or higher, still trying to decide if I buy that story or not.
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    Quote Originally Posted by MacGyver View Post
    There also need to be a single connection (cable) going from the DC main negative bus to the hull to prevent ground loops.
    This is one I am trying to figure out how to do. There are a couple things in the boat that have continuity from the ground wire to the hull through the device, like my stove and oven, and it appears to be due to their through hull exhaust connections..... Blue Seas is telling me that while it is not preferred a lot of equipment does that for safety, so you take safety over corrosion prevention. Same thing on the AC side, there are several things like chargers and inverters that are grounded to both AC and DC making the connection in several spots. No wonder so many boats are speckled, the manufactures of equipment are making the connections happen in multiple locations.
    ... aboard the 'Memory Maker' Making Memories one Wave at a Time!

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    Quote Originally Posted by DMan View Post
    This is one I am trying to figure out how to do. There are a couple things in the boat that have continuity from the ground wire to the hull through the device, like my stove and oven, and it appears to be due to their through hull exhaust connections..... Blue Seas is telling me that while it is not preferred a lot of equipment does that for safety, so you take safety over corrosion prevention. Same thing on the AC side, there are several things like chargers and inverters that are grounded to both AC and DC making the connection in several spots. No wonder so many boats are speckled, the manufactures of equipment are making the connections happen in multiple locations.
    A lot of confusion your having is not understanding you have several type of grounds; grounded item, grounding conductors, AC neutral, and AC ground, and lets not for get out beloved Hull ground. Each type of ground is needed to protect a boat from corrosion, electrical shock, and keeping battery system working properly. In order to understand each system and how they are all wire together. You need to get out the books and learn everything about each system. Once you done that, it will start to make sense it really not that bad.


    I did not hear the conversation, between you and the Blue Sea guy but he may not have known you have an Aluminum boat, unless he saying you need to add a ground wire like they do in a fiberglass boat for bonding.

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    There are some forum threads on-line that advocate a direct tie between the battery negative and the hull. I can't see that as being a good thing, but IF everything is functioning properly and you have complete solid return to battery grounds from every device, then it should be okay - theoretically.....

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    Quote Originally Posted by MacGyver View Post
    A lot of confusion your having is not understanding you have several type of grounds; grounded item, grounding conductors, AC neutral, and AC ground, and lets not for get out beloved Hull ground. Each type of ground is needed to protect a boat from corrosion, electrical shock, and keeping battery system working properly. In order to understand each system and how they are all wire together. You need to get out the books and learn everything about each system. Once you done that, it will start to make sense it really not that bad.


    I did not hear the conversation, between you and the Blue Sea guy but he may not have known you have an Aluminum boat, unless he saying you need to add a ground wire like they do in a fiberglass boat for bonding.
    I just ordered a couple of books. I thought I understand this stuff pretty well but my head is starting to spin. HA!
    ... aboard the 'Memory Maker' Making Memories one Wave at a Time!

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    Quote Originally Posted by DMan View Post
    I just ordered a couple of books. I thought I understand this stuff pretty well but my head is starting to spin. HA!
    Yeah thats what I thought too......! I
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    Quote Originally Posted by Bullelkklr View Post
    There are some forum threads on-line that advocate a direct tie between the battery negative and the hull. I can't see that as being a good thing, but IF everything is functioning properly and you have complete solid return to battery grounds from every device, then it should be okay - theoretically.....
    In the real world all theories apply.

    What happens when you put a aluminum boat in salt water does that count?

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    1. Your DC negative bus should only bond to the hull in one spot. On a outboard aluminum boat this is done through the motors.

    You are correct and the reason they do is to save themselves a couple of $ and it not the correct per ABYC standards. This prevents grounding loops which cause current.

    Ground loops do not cause current. If you place the connection close to the engine and it has no resistance there will not be a problem.

    HOWEVER! I have found that many other things create this connection and I am not sure about how to get around it. For instance, my Wallas stove top and oven both have continuity from the ground wire to the hull. So how do you get around the fact that there are components in your boat that will have ground to hull through the chassis and ground to the neg bus bar?

    When you learn about bonding this will make sense.

    2. Your AC ground should only be connected to the DC ground in one spot. However several things like inverters, battery chargers, AC panel face, etc. are connected to both AC ground and DC ground (DC ground is typically a chassis ground) no it not---- it's a floating ground and they provide continuity from AC Green wire to DC Neg bus, I have tested this thoroughly. So, what do you do in the case where your AC is gournded to DC in multiple spots?
    You fix the problem.

    After talking to Blue Seas engineers and doing a bunch of testing here is the deal for you guys that run portable generators. Ground and Neutral should be bonded at the source of power. When I did the testing without the bonding, even though Ground to Neutral was 0.00v, as soon as you plugged it into the panel suddenly the DVOM confirmed voltage accross the Neutral and Ground panel bars, sometimes up to 60vac. I suspect this is what caused my corrosion issue (glad I caught it when I did, I don't appear to have any pitting just color spots). As soon as I bond the neutral and ground at the generator this voltage goes away and I get normal voltage readings at the panel. Hot to Ground = 120vac, Hot to Neutral = 120vac, Neutral to Ground = 0.00vac. Without the bonding I would get Hot to Ground = 60vac, Hot to Neutral = 120vac, Neutral to Ground = 60vac, I still don't understand why I would get this since without being plugged in the generator sitting by itself would get Hot to Ground = 0.00vac, Hot to Neutral = 120vac, Neutral to Ground = 0.00vac.

    The short answer is removing the ground and neutral is giving you false reading, don't do it.

    So, for any of you guys that have read about reserve polarity using a portable generator and heard it to be no big deal, it is a big deal and can be fixed by making a bonding plug. Marine generators come with the neutral and ground bonded from what I understand.
    Portables are made for power tools and such that do not require a ground.

    It may not be require, When used on a boat you better have it grounded to keep from killing someone.

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    [QUOTE=MacGyver;
    2. Your AC ground should only be connected to the DC ground in one spot. However several things like inverters, battery chargers, AC panel face, etc. are connected to both AC ground and DC ground (DC ground is typically a chassis ground) no it not---- it's a floating ground and they provide continuity from AC Green wire to DC Neg bus, I have tested this thoroughly. So, what do you do in the case where your AC is gournded to DC in multiple spots?
    [B]You fix the problem. [/B]

    Should have read
    QUOTE=MacGyver;
    2. Your AC ground should only be connected to the DC ground in one spot. However several things like inverters, battery chargers, AC panel face, etc. are connected to both AC ground and DC ground (DC ground is typically a chassis ground)

    no it not---- it's a floating ground

    and they provide continuity from AC Green wire to DC Neg bus, I have tested this thoroughly. So, what do you do in the case where your AC is gournded to DC in multiple spots?

    All the AC neutrals and grounds need to go to a single connection on the main DC buss. per ABYC

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