12 volt query...

[casper ghost]

I've been having trouble getting laptop to work via my invertor, though the tv worked fine. Laptop always used to work. I've now found a black/negative 12 volt cable has come apart at a connector in my engine room. I fixed them back together and now laptop will work again. I guessed there must be something touching the steel to get power with the cable apart but I can't find anything. I'm guessing the lights sockets etc shouldn't be working unless a circuit is made so i'm stuck. Do I need to rewire incase there's a wire toughing the steel behind the lining or forget it now it's working. I do have a fuse box and none of the fuses have gone....

[Keeping Up]

I've been having trouble getting laptop to work via my invertor, though the tv worked fine. Laptop always used to work. I've now found a black/negative 12 volt cable has come apart at a connector in my engine room. I fixed them back together and now laptop will work again. I guessed there must be something touching the steel to get power with the cable apart but I can't find anything. I'm guessing the lights sockets etc shouldn't be working unless a circuit is made so i'm stuck. Do I need to rewire incase there's a wire toughing the steel behind the lining or forget it now it's working. I do have a fuse box and none of the fuses have gone....

It sounds, as you suspect, as if there must be an unintentional connection between the negative and the steelwork somewhere, which allows most things to work (after a fashion) even when your wire is disconnected.

 

This spurious connection needs to be located, but it is not an emergency - you can spend weeks or even months tracking it down without it being a problem, but ultimately it will lead to corrosion problems or possibly, in conjunction with a second fault, it just could become a fire hazard. There are many things which can cause this: a radio aerial (connected to the case of the radio at one end and grounded to the steelwork at the other) or a bulkhead light are two of the favourites.

 

The most useful thing you could do towards diagnosis, if possible, is to find out what is the intended purpose of the wire which you have now reconnected. Is there any way you can find out what is on the other end of it (if it is twin flex, you may be able to find out by disconnecting its corresponding positive supply for example).

[casper ghost]

After the black connection, the one that was apart, the cable splits, one running along the top for 2 cabin lights. The second runs along the gunwale to 2 sockets and a car radio. The radio does have an aerial which is fixed through the steel roof..

[Keeping Up]

After the black connection, the one that was apart, the cable splits, one running along the top for 2 cabin lights. The second runs along the gunwale to 2 sockets and a car radio. The radio does have an aerial which is fixed through the steel roof..

Is the inverter is getting its negative supply that way too, I wonder? If so, that could well explain the symptoms you describe with your laptop. If not, it could be a bit more complicated - possibly two faults at the same time. It is worth tracking it down, but as I said it's not an emergency thing.

 

If you disconnect that negative wire again, do the cabin lights go out and the radio stop working? They should. If they don't, try pulling the radio aerial out from the back of the radio. Do the lights and radio (and possibly theinverter) all stop working? If so, that means you are using the outer sheath of the aerial cable as a spurious earth and you should correct that by inserting a capacitor into the braiding. If not, there must be another spurious connection.

 

It took me 3 years to track down the last of my spurious connections, which was due to a problem in the plug for the spotlamp, but I got there in the end.

[casper ghost]

The invertor is only 150 watts and plugs into one of the 12 volt sockets. Everything workks when the cable is disconnected. I'll try disconnecting the aerial lead and see what happens, if not i've got more investigating to do. Just thought I have an outdoor light on front deck, switch is inside off this same cable, maybe that is earthing, it uses one of those 12 volt deck fittings where you can remove the lamp.

[GUMPY]

The radio does have an aerial which is fixed through the steel roof..

 

That's possibly the place where the link is if its a car type aerial..........

Edited February 21, 2010 by idleness

[casper ghost]

That's possibly the place where the link is if its a car type aerial..........

 

It is a car aerial, just bolted through the steel roof. I fitted that 10 years ago before lining out the boat...

[Keeping Up]

Yes you've definitely got at least one, and possibly two, connections that shouldn't be there.

 

The radio aerial is a definite favourite; for some reason you can't buy one where the outer braiding is suitably isolated, yet if you buy an extension cable for them the centre connection is usually isolated for you (which makes checking them for continuity impossible!) and most boats that use a car radio with a car aerial will have an unwanted earth connection. Not only can the negative return through the hull gradually eat away the steel, but also the battery isolator (if it is in the negative lead, as was popular some years ago) may be by-passed so that you cannot actually switch off the batteries - worse still, high currents can pass through the aerial braid and set fire the cable or the plug.

 

Luckily it is usually quite easy to check, by unplugging it. Curing it involves cutting the braid at one end or the other, and soldering a small capacitor (typically 0.1 microfarad) across the new break. I have two radios; for one of them this was most easily accomplished at the plug, for the other it was easier at the roof end.

 

I also found that my two bulkhead lights were causing an unwanted connection. Although they are wired with twin-flex, the metalwork of the backing plate is connected to the negative supply so the mounting screws provide a spurious connection to the hull. For these the cure was to use a couple of plastic sleeves (such as are sold in Maplins for use when mounting power transistors to a heatsink) which insulate the screws from the backplate.

 

Those were my first 4 out of the 9 inadvertent connections which I found. It sounds as if you may have two of them; you'll perhaps know by seeing whether or nor the lights go out when you unplug the radio aerial.

[casper ghost]

I didn't think a negative current through the boat did any damage to the steel at all, thought only if the positive touched it would be a problem? I know that my starter motor and alternator are not isolated so they form an unavoidable negative bond with the hull.

[Keeping Up]

I didn't think a negative current through the boat did any damage to the steel at all, thought only if the positive touched it would be a problem? I know that my starter motor and alternator are not isolated so they form an unavoidable negative bond with the hull.

I don't have any references to a definitive article on the subject (maybe someone else has) and I'm not an expert on this area of the subject. However the point is that current through the hull is current through the hull; it flows from one point on the hull to another point which is slightly more negative (or slightly less positive). The hull has no way of knowing which end is which, or whether the difference in potential is derived from a positively connected battery at one end or a negatively connected battery at the other. Corrosion will simply occur at the appropriate point; if you were to reverse the battery connections, the same corrosion would simply occur at the other end.

 

The current through the hull will usually be relatively small when all the cables are still connected, but of course will be much greater if a wire falls off (as you discovered), and sometimes the path through the hull will be a lower resisitance than a long wire so the return current will favour that route anyway

 

Yes the starter and alternator are connected to the engine, which will (probably) have some a variety of connection paths to the hull - via various moving parts, operating cables, etc. It would probably be ideal to bond the negative to the hull and to bond the engine to the hull at the same point. However this is rarely done, on the basis that the negative is usually bonded to the hull at a point which is quite close to the engine and also to the batteries, so that the hull will not then offer a "prefferred" path except under fault conditions which were so significant that you'd be aware of them anyway (eg when nothing works, or when the starter won't turn).

[WotEver]

I didn't think a negative current through the boat did any damage to the steel at all, thought only if the positive touched it would be a problem? I know that my starter motor and alternator are not isolated so they form an unavoidable negative bond with the hull.

In addition to the possibility of hull corrosion the biggest potential issue ('scuse the pun) is if two or more faults occur. Take an extreme example where the hull's neg bonding from the battery has become poor (say through corrosion or a loose nut) and you have the neg being bonded to earth via the radio aerial. Now if a 3kW inverter negative feed falls off the battery and connects to the hull, the inverter will complete its circuit via the hull and the aerial braided screen. The hull will be able to take 250+ Amps, but your poor little radio aerial lead won't.

 

Sure, the above scenario is unlikely but it's still possible. As Keeping Up pointed out in his post, it can take a while to track all the little suckers down but it's worth doing.

 

Tony