Is accessory circuit an isolated curcuit?

[Todd4]

This is actually a pretty simple question, if I can word it correctly. There are 2 independant electrical circuits on a boat 1) outboard motor (start, charge) 2) accessories (lights, radios, bilge pumps, horn, etc). An outboard motor will be grounded to an aluminum boat hull by it's mounting brackets - the negative battery cable from the start battery terminates on the engine block, so no choice. However, I was always under the impression (or I just assumed) that the accessory circuit was NOT grounded to the hull of an aluminum boat - all grounds were directly or indirectly WIRED back to the negative post of the battery (yes, usually through various electrical connections, etc). Obviously, this is the case with a fiberglass boat - you can't ground to a fiberglass hull.

So, if all my accessories are grounded back to the negative post of my house battery, shouldn't that isolate it to be a completely independant circuit from my start battery? My start battery only has the positive and negative battery cables connected to it which terminate at the starter or outboard motor block (no other wires are attached). I just noticed that my accessory ground wires (where they would attach to my house battery) have continuity to the aluminum boat hull (and thus, continuity with the negative post of my start battery). Is this normal? Or did a PO take a short-cut somewhere and ground an accessory directly to the aluminum hull. (Ultimately, I'm conderned about corrosion.)

So my question is: Do I have an accessory ground shorted to the aluminum hull that I need to trace down and eliminate to electrically isolate my house circuit from my start curcuit?

I hope I clearly explanied my situation. Thanks.

Todd

 

[Georgesalmon]

Re: Is accessory circuit an isolated curcuit?

You probably have an assocory that is grounded through its case or something. It won't hurt anything if the two batteries neg are electrically connected. If both the pos and neg are connected then you have a parallel battery situation and problems but theres no current to flow if just the negs.

 

[emoney]

Re: Is accessory circuit an isolated curcuit?

And doesn't your ignition switch alter that thinking? It must be wired to the main engine, AND, it's also tied into the "Accessory" for lights, etc. Not that I know the answer, but it just seems like this makes sense.

 

[Todd4]

Re: Is accessory circuit an isolated curcuit?

Thanks for the quick replies (keep 'em coming).

Yes, makes sense. Wouldn't there be a concern about corrosion if an accessory were grounded through it's case directly to the hull?

I don't need my ingnitioin switch to run my accessories. I just flip the accessory toggle switch on and everything (all accessories - radio, depth finder, lights, etc) will run without the key.

Thanks.

Todd

 

[Georgesalmon]

Re: Is accessory circuit an isolated curcuit?

other than coorosion caused by dissimiler metal, no. There is no current flow in the hull as long as there are both +12, and -12 wires to the device. What your looking at is potential, not current flow. Hook your multimeter up between the hull and the +12 your talking about. With the meter in amps, you'll see no current flow. Even though the ohm meter will show continuity. Wish I could spell.

Got a built in fuel tank? Think about it, typically you have a bonding wire from the tank fill to the tank for static electricity grounding. Then you have a ground wire to the sender. OK, somewhere thats all touching the hull, right? Then you have the motor mount touching the hull. Now all the negs are together, no problem.

 

[Silvertip]

Re: Is accessory circuit an isolated curcuit?

And doesn't your ignition switch alter that thinking? It must be wired to the main engine, AND, it's also tied into the "Accessory" for lights, etc. Not that I know the answer, but it just seems like this makes sense.

Not true. Only the instruments and possibly their internal lights are wired through the ignition (engine harness). All other circuits are wired through the "Boat Harness". That is of course, provided a prior owner hasn't done some creative wiring.

 

[Silvertip]

Re: Is accessory circuit an isolated curcuit?

Are the house and starting battery negative terminals tied together? Is there a dual battery switch, ACR or Isolator involved? Normally, two wires (one black and one red) run from the battery via a circuit breaker at the battery, to the fuse or breaker panel at the helm. From there the circuits split and go to the various loads. Ground wires are not run back to the battery but rather to a ground buss incorporated into the fuse\/breaker panel or as a separate unit. The #8 or #10 black wire carries the ground back to the negative terminal of the battery. It doesn't matter if that battery is the house or the starting battery.

 

[Todd4]

Re: Is accessory circuit an isolated curcuit?

OK, I tried to clarify my original post, but it quickly got long. I explained my concerns at the end of this post, for those that don't want to read it all. Thanks for the help.

Are the house and starting battery negative terminals tied together? No, there are no wires installed between the house and start negative battery terminals. However, I discovered there is continuity between the two, so something (the hull) has to be tying them together. Since the negative battery cable is grounded to the motor block and the motor is bolted through the aluminum transom, the negative start battery terminal is grounded to the hull. Consequently, there must be a path from the negative house battery terminal to the hull, somewhere on the boat. I was under the impression there shouldn't be one - on purpose, anyway. So, either someone purposely used the hull as a ground (connected the ground wire of an accessory to the hull instead of the 12V negative house circuit (buss bar, whatever), or someone inadvertently connected the metal case of an accessory to the hull (bolted into an aluminum cross member for example), as suggested above. Is there a dual battery switch, ACR or Isolator involved? No, not yet. There will be once I finish my wiring (incidently, that's how I discovered my house ground-to-hull condition). There will soon be onboard AC and DC chargers (Minn Kota). They are 100% isolated so one battery will never see another battery. But as it stands today, this cannot be an issue, since I haven't connected any wires, yet. However, I hope to resolve my little issue (if there even is one) before I hook up another 16 wires to the batteries. Normally, two wires (one black and one red) run from the battery via a circuit breaker at the battery, to the fuse or breaker panel at the helm. From there the circuits split and go to the various loads. Ground wires are not run back to the battery but rather to a ground buss incorporated into the fuse\/breaker panel or as a separate unit. The #8 or #10 black wire carries the ground back to the negative terminal of the battery. Yeah, that's what I was trying to say in short hand by, "all grounds were directly or indirectly WIRED back to the negative post of the battery (yes, usually through various electrical connections, etc)" It doesn't matter if that battery is the house or the starting battery. Yes, that's basically my system too, except I have two 10 ga. red wires and three 10 ga. ground wires from/to the house battery. That's not totally unexpected, since I have two livewells and a bilge pump, besides the usual console accessories. All wires (except the battery cables) connect to the house battery terminals. I know terminating wires back to the start battery would also work, but the start battery will only used to start the motor, nothing else. (BTW - There is also an isolated 24V trolling motor w/ 2 batteries at the other end of the boat, but there are no connections between the trolling batteries, the hull, or the other two batteries - I'm positive [or should I say "sure"] - I installed it.)

I'm not sure where each wire from my house battery goes to /returns from. The negatives are ganged together with one ring terminal at the battery. I was hoping I didn't have to cut wires and trace. I know the red wires can only be going to the console (I have power to everything) or the bilge pump (it works) - I suspect one red wire powers the accessories from under the console and the other red wire keeps power to the automatic bilge when everything is turned off, but I haven't verified this. What the odd 3rd black wire is for, remains unknown.

Sorry about the way I quoted above - hope it's still readable and understandable. Thanks for asking for clarificatioins.

So I guess my questions up for discussion are:

Do you think having 12V negative continuity in my hull to my house battery is an indication of a potential problem (from a corrosion stand point) and I need to investigate further, as indicated below? (If I bought the boat new, I wouldn't be as concerned, but I have no idea who has wired what over the years.)

Or, as GeorgeSalmon implies, should I verify that each and every accessory has a dedicated ground wire (via buss bars, or however) back to the negative house battery post (i.e. ensure someone didn't connect the ground wire of a live well aerator pump directly to the hull with no negative wire connection back to the house battery post (through buss bars, etc)?

Or, should I go further and isolate every accessory case to ensure it's negative connection cannot be inadvertently grounded to the hull (even though it may have a negative wire connecting back to the battery, via buss bars, etc). This step should totally isolate the house battery negative connection from the hull (like I initially thought it would be). This may seem like overkill (overdesign), but it would positively guarantee that there is no way the house battery could possibly contribute to any hull corrosion issues, regardless of how the accessories are hooked up.


My concern: It's probably apparent by now that I suspect I have an electrolysis corrosion issue (with a relatively low hour boat).

I've read on this forum where a boater has had no corrosion problems for years until a technician wires up a radio, or bilge pump, or aerator and suddenly their boat/outdrive begins corroding at an alarming rate. The response has typically been, "take it back and make the technician correct his mistake and wire the new device up correctly". I have some experience with wiring circuits - and every electrical device on my boat appears to work properly - that's not my issue at hand. However, I've never had an aluminum boat before, so I am unfamiliar with what wiring designs are correct; and which designs are not correct for an aluminum boat and, consequently, may have the potential to cause a corrosion issue. So I'm trying to learn what indications to look for, what to do, and what NOT to do.

So far, I'll hook my ammeter up through the house ground and see if I get any current flow, but I should put it on the water to run every circuit, so that may be a while.

Thanks for discussing this issue with me.

Todd

 

[Todd4]

Re: Is accessory circuit an isolated curcuit?

So if I'm correctly understanding what evryone's trying to tell me; I should not experience any type of hull corrosion (due to wiring) because of a negative battery terminal grounded to the hull. Ok Thanks - that's good to know. So, how is something wired incorrectly to cause the extreme corrosion reported by others on this forum. Were they all interacting with stray shore power? I may not have caught that while reading other posts on the subject.

Thanks.

Todd

 

[emoney]

Re: Is accessory circuit an isolated curcuit?

It might help clear things up, Todd if you post a link to the discussion you're referencing.

 

[glenn property of pam]

Re: Is accessory circuit an isolated curcuit?

run a jumper wire between the negatives of the batteries to start with. the wire should be sized for the largest load so 10g is fine if you add a dual switch then you will have to upgrade to a wire the same size as used for the starter.

make sure at least one point of the hull is grounded this is for safety as it means the hull can never be anything other than ground (helps to blow fuses and breakers when things go wrong)

ok now im going to try to keep this simple .......... there are 3 types of wire 12dc+, 12dc-, 12dc- non current carrying ground.
the first 2 are the ones things use on the boat these are standard plus and minus the other is a wire thats not designed to carry any current unless theres a fault then its to carry the current required to activate the safety system (fuses)

so bow lights 12dc- must not go to the hull but has to come back via a ground wire to the battery if you was to ground the dc- at the light then you are making the "non current carrying ground" which is the hull of the boat carry the current which will lead to corrosion.

if a red wire was to get damadged and hit the side of the boat then the current will be able to flow to the hull and back to the battery which will cause a direct overload and blow the fuse which is a good thing.

 

[Fed]

Re: Is accessory circuit an isolated curcuit?

So my question is: Do I have an accessory ground shorted to the aluminum hull that I need to trace down and eliminate to electrically isolate my house circuit from my start curcuit?

Yes.
Probably only repeating what others have said above.

I also think it's best to only have the hull grounded in one location, that being via the outboard motor.

 

[glenn property of pam]

Re: Is accessory circuit an isolated curcuit?

Shore power, a battery charger, etc.

another good way to get a badly corroded boat is to dock under high voltage cables or park trailers under them.

 

[Todd4]

Re: Is accessory circuit an isolated curcuit?

It might help clear things up, Todd if you post a link to the discussion you're referencing.

Well, I guess I read too much on the internet. The discussion was on another boat forum (I paraphrased below so as not to break any rules) If I made a mistake please feel free to correct (without banning me Thanks):

"....I just got my boat back after a big electronics job....."

Start there, recheck all the electrical work done, and find out what they didn't do right that's now causing the corrosion.

It may have been your neighbor and you need to get the boat moved. If the problem continues, then the wiring job done recently is the next obvious place to look...

Could be as simple as a loose ground wire on one of your batteries. This happened to me and it was a loose nut on a negative battery terminal.

BTW - I checked for ampearge from the ground wire bundle to the hull = none (nothing was running though). I separated the 3 ground wires where they connect to the house battery (as part of my wiring job) and will see if all three have continuity to ground (was raining all weeekend). Maybe that'll tell me something. My horn, bilge, and livewell pump quit working (while everything else worked), then magically everything worked again - so somethings going on. I need to sort this out before adding 'shore power' to my battery charger. Thanks.

Todd

 

[Todd4]

Re: Is accessory circuit an isolated curcuit?

Well, it may be time for a little update. I ran all my wires and hooked them up to all my batteries. Then I tried to turn on the radio and.... NOTHING. I was only getting 2.5 volts to the console. This made absolutely no sense to me. I eventually traced it back to a corroded breaker in one of Minn Kota's battery boxes. I knew I left them on the deck when it rained (hard) once, but I just assumed they could get wet. Apparently, not. Who would make a battery box for a boat that cannot get wet? Anyway, I put a temporary breaker inside the lid (I didn't have the same style to replace it with) and eveything has been working fine. I'll fix the boxes soon (and then keep them dry). So far, I've confirmed all batteries charge correctly from the AC charger. Hopefully, I can take it out on the lake this weekend and check the DC charger operation.

I did separate the 3 ground wires (at the battery end) and confirmed they all have continuity to the hull, but showed zero amperage (current flow) between the wires and the hull (a test suggested above). I also learned that there are two schools of thought to protect a fiberglass boat from stray current corrosion 1) isolate everything, or 2) bond everything together. Great! I know some of my cuicuits are isolated (trolling motor, anchor winch). The battery chargers have no direct connection to ground (other than the negative wires in question). So I'm stumped, but I'm leaning towards isolation.

I think I'll put everything back together (12 access panels/pieces) and take it out on the water and trace the wires, one circuit at a time, later. I've worked on this little project almost all summer long and I want to take her out and enjoy it. Besides, it'll be a lot more fun tracing down wires while floating on a lake than sitting my my driveway (it's supposed to be 84 this weekend).

Thanks all for sharing your knowledge.

Todd