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12v wiring bamboozlement


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I have an electricity bamboozlement that I can't get my head round. I think it's obvious but I'm not seeing it, please help me understand what's going on with this. Here's my problem:
Last night I was wiring up my two MPPT controllers via busbars to the 12v batteries. The controllers currently are NOT connected to each of the solar arrays so there's no solar input yet.
Sod's law, I managed to fumble the only live cables (1 and 2 on the diagram) so that they touched and sparked. Apart from making me jump I didn't notice anything else untoward happen at the time, so I completed the wiring as in the attached diagram. (The solar panels are still not connected to the controllers). Had all been well I would have expected to measure 12-13v at (point C) which was the battery bank voltage. However my multimeter read no voltage at all. Indoor lights and tele were on so the batteries were fine. I checked the multimeter on a AA battery and it read that voltage fine. I assumed that my earlier spark must have blown the fuse shown in the diagram. The fuse isn't accessible from indoors so I disconnected cables 1 and 2, and sealed off the ends till I could try again the next day.

Today is the next day.
When I started tinkering today, the engine was running. Before I unwrapped the ends of cables 1 and 2, (not yet attached to busbars), out of curiosity I checked the voltage with my multimeter: it showed 13-point-something volts at point A, suggesting that the fuse had not blown after all. Hoorah. The spark had been a red herring, I think. (I've not actually checked the fuse.)
I then worked my way through each cable and crimped terminal, replacing a few wires so everything is the same diameter (4 or 6sqmm, I think - yellow connectors) and making sure everything seemed secure and tidy. I turned the engine off and once again checked the voltage across point A with my multimeter. It showed nearly 13 volts so that's good.
Finally I connected cables 1 and 2 to the busbars themselves (this involves cables 1 and 2 bending round in an S shaped curve but is otherwise unremarkable, as some of the other cables are S shaped  too, it's all in quite a small space).


Then when I checked the voltage across point C again, it was only 3v! I checked the voltage across the two busbars and again only about 3v. :(
I detached cables 1 and 2 from the busbars and again measured the voltage between points A, and it had miraculously  returned to nearly 13v. :D

Where are my volts going!? How can busbars/cables just eat 10v and then return them once disconnected?


It seems there is some major resistance somewhere but how do I find it and solve it, please?  I don't know if the problem is one or both busbars, or one or more cables, the curving of the cables. Or something else.

 

Deep in the distant recesses of my bwainz I suspect the answer is probably a simple one, but I'm gonna need it spelled out to me please, coz I can't think how to eliminate the possible causes.

For now I have once again disconnected cables 1 and 2 and taped up the ends till next time.


Thanks in advance. :)

20200519_194241.jpg

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This sounds like a bad connection (a high resistance). Multimeters in isolation are not the best way to track this down because they have a high resistance (impedance) themselves so can sometimes measure the correct voltage despite the high circuit resistance. Getting a bit of current flowing down your suspect wires will bring out the fault which can then be localised with the multimeter. Have you got an old "proper" light bulb or similar load that you can connect between your two C's ???

 

...............Dave

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As an aside, to avoid accidental surprises ilke your wire-fumbling, the first thing I always do is disconnect the negative at the battery itself before doing anything other than minor fiddling. The very last thing is to reconnect it. Voila - no more accidentally shorting positive cables against anything bonded to the negative (ie the hull), and no need to think about which wires you need to connect first to avoid any sparking.

Edited by tehmarks
Bad typing, even worse proof-reading
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8 minutes ago, BlueStringPudding said:

Thanks. I don't have a lightbulb. What else might "get the current flowing"? And why at point C? (Just so I can learn) What am I looking to measure in this situation?

Because point C is where you have measured the suspect voltage, and its also the end of the new bit of your wiring. The bad connection must be somewhere between the busbars and one of the C's. So, if we get current flowing from the red busbar to C, through our "load" to the other C and back to the black busbar that current must go through the bad connection, it hopefully won't be able to so logical use of the multimeter will isolate the fault. You can sometimes get 12v test screwdrivers with a small internal lightbulb, a useful tool to have, but must be the 12volt type with a real lightbulb, not a mains neon. An old lightbulb with some wired attached is a good alternative.

 

Any 12volt device will probably do, but most will likely have a cigar type connector on them and getting them off and back on can be a pain.

 

Maybe using the mutimeter and doing some cable wiggling (wiggle test) will show the fault, but you might need three hands to do that.

 

................Dave

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Thanks, Dave. I won't be able to buy new equipment like sonic screwdrivers for this. ;) So I'll have to piece something together.  With the 12v contraption connected between C, how should that change the voltages I'm measuring at various points round the circuit? I still don't understand what it changes in the measurements I'll be taking and what they will indicate.

Thanks. 

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A good 12V load would be a car headlight bulb. Something like this. Big advantage of this style is that you can connect to it with standard 1/4" crimp connectors. The only issue with them is they get very hot when on, so be careful not to touch it. Handle by the wires and don't leave it on too long. It will take around 5A, so plenty enough to show up high resistance joints that a multimeter measuring volts can miss. @dmr is probably right. It does sound like a high resistance joint somewhere. They can cause a lot of head scratching. There is a good chance you'll find a branch of Halfords open as they sell push bikes and so were considered essential.

 

Jen

Edited by Jen-in-Wellies
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4 minutes ago, BlueStringPudding said:

Thanks, Dave. I won't be able to buy new equipment like sonic screwdrivers for this. ;) So I'll have to piece something together.  With the 12v contraption connected between C, how should that change the voltages I'm measuring at various points round the circuit? I still don't understand what it changes in the measurements I'll be taking and what they will indicate.

Thanks. 

OK, lets say the bad connection is in the red wire (it could just as well be the black). The current won't get through the bad connection so the voltage will change at that point. Multimeter black lead directly onto the black bus bar, red lead on the red busbar, should read 12volts (ish) now start moving the red lead towards point c in steps, first the connection on the busbar etc etc, at some point the voltage will drop from 12 to very little?

 

Another thought....are you measuring the volts correctly? sometimes you have to press the multimeter probes in really hard and even do a bit of scraping, even shiny wires and multimeter probes can develop an insulatiing tarnish sometimes.

 

Even with a lightbulb you might still have to do some serious wiggling to bring out the fault.

 

...................Dave

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I really need to be able to do this with bits and bobs I've got round the boat. I'll need to think of an appliance that can be sacrificed. I need at least a 5A draw, do I? Or will less do?

And I still don't understand: how does this "show up high resistance joints" when I'll still be measuring with the same multimeter? What exactly will I be measuring if not volts  between various points, as I was measuring today? How will those measurements differ from what I've measured today?

 

Edited to add: the above was in response to Jen's post - Dave, I think we were typing at the same time

Edited by BlueStringPudding
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4 minutes ago, dmr said:

OK, lets say the bad connection is in the red wire (it could just as well be the black). The current won't get through the bad connection so the voltage will change at that point. Multimeter black lead directly onto the black bus bar, red lead on the red busbar, should read 12volts (ish) now start moving the red lead towards point c in steps, first the connection on the busbar etc etc, at some point the voltage will drop from 12 to very little?

 

Another thought....are you measuring the volts correctly? sometimes you have to press the multimeter probes in really hard and even do a bit of scraping, even shiny wires and multimeter probes can develop an insulatiing tarnish sometimes.

 

Even with a lightbulb you might still have to do some serious wiggling to bring out the fault.

 

...................Dave

That's exactly where I did place the multimeter. It read 3v across point C. It also read 3v between the two (brand new) busbars when cables 1 and 2 were attached to the busbars. It's read 3v on the posts that cables 1 and 2 were connected to. The voltage only dropped when cables 1 and 2 were attached to the busbars. This is why I'm confused. And I still don't understand what the current-drawing gizmo will do to these readings? 

The multimeter read just under 13v across points A when cables 1 and 2 were NOT connected to the busbars. It's the act of connecting them that is reducing the voltage even at the posts where cables 1 and 2 are attached. 

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As an example, when cables 1 and 2 are disconnected there is no current flow, assume the fuse has a high resistance for some reason, as there is no current flowing through it it has no voltage drop, so you read approximately 13V at point A.

 

Now you connect up something with a current draw, the solar controllers, a lamp for example, now there is some current flow through the fuse, therefore, the fuse will have a voltage drop due to the high resistance and you will only read 3V say at point A, B or C because 10V is being lost over the fuse.

 

With your readings, with everything connected, you are measuring 3V on the busbars, therefore your  high resistance is somewhere between the batteries and the busbars.

 

You add a load and start with the multimeter on the batteries, then move one step at a time from battery terminal -ve to the +ve crimp, then fuse input, then fuse output, then crimp at the +ve busbar. If at some point the voltage drops then you have found the length with the high resistance in. If you do not find it then put the +ve multimeter back to the battery +ve terminal and work along the negative route.

Edited by PeterF
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14 minutes ago, BlueStringPudding said:

I really need to be able to do this with bits and bobs I've got round the boat. I'll need to think of an appliance that can be sacrificed. I need at least a 5A draw, do I? Or will less do?

And I still don't understand: how does this "show up high resistance joints" when I'll still be measuring with the same multimeter? What exactly will I be measuring if not volts  between various points, as I was measuring today? How will those measurements differ from what I've measured today?

Less than 5A is fine.

It is tricky to explain. A high resistance joint will drop a lot of voltage across it when a current is flowing, like when it is connected to a light bulb, or to a solar controller. If there is no load appliance on the end of the circuit, then a multimeter reading volts will draw an infinitesimally small current, so the voltage drop across the high resistance drop will be tiny and almost all of the 12 to 13V from the battery will reach the multimeter. Put the appliance across the end of the circuit and the current increases, so more voltage is lost across the dodgy high resistance joint and less voltage will be read at a multimeter placed across the appliance.

 

Jen

A                     B                  C                     D

+ -------------------**bad joint**--------------------

Batt                                                             Appliance drawing current.

- -------------------**-----------***-------------------

A                    B                   C                      D

So measuring across the battery terminals A-A will give say 13V. With the load on, measuring across B-B will give 13V, measuring across either C-C, or D-D will give perhaps 3V. Fix the bad joint between B and C in the +ve line and you will measure 13V at A-A, B-B, C-C and D-D. The bad joint is as likely to be in the negative wire as the positive, or there could even be more than one!

 

 

Jen

Edited by Jen-in-Wellies
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Geez guys, you’re making this complicated. BSP - first check is the connections to the fuseholder. Then swap out the fuse for a replacement. Now see how it performs. 

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Thanks. I think I get it now. So the act of actually connecting cables 1 and 2 to the controllers was creating the current draw that's revealed the resistance problem which could go anywhere back towards the battery?

As there is only one join in the battery-busbars circuit (excluding the controllers) - and that's the solar fuse holder - am I right in thinking it's the most likely culprit? 

Measuring voltage between the battery and the fuse either end will be "fine" because they're close to each other in the engine area under the cruiser stern deck. However all of the rest of the connections (bus bars, controllers etc) are in a cupboard high up inside the boat and so cannot have a reading taken directly between them and the battery or them and the fuse. 

So do I start the investigation under the back deck with the batteries and the solar fuse holder? (Physically difficult for me but if it's the most likely culprit I would rather use my energy wisely) or do I start inside the boat with all the various busbars and the like?

6 minutes ago, WotEver said:

Geez guys, you’re making this complicated. BSP - first check is the connections to the fuseholder. Then swap out the fuse for a replacement. Now see how it performs. 

Thank you :D I think you just answered my question before I even finished typing it. 

Edited by BlueStringPudding
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Hold on, I assume the fuse is between the batteries and the busbar (shown by a resistor symbol in the drawing). If this has blown then the whole boat would be dead and this has not been mentioned????

 

If all electrics have indeed failed then.......you've blown the big fuse ? ?

 

sometimes a blown fuse does go to a high resistance state rather than a total fail, probably due to molten metal getting sprayed about.

 

..................Dave

Edited by dmr
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Sorry for the wrong symbol for fuse - I think it should have been a wiggle rather than a zigzag? It's been 30 years since I did this stuff at school ?

The diagram is of my solar wiring only. The whole boat wiring is WAY more complex than that. :D

The main boat wiring has a fuse box of multiple fuses for different lighting circuits, fridge etc. It's got nowt to do with my issue above. I would have noticed and mentioned in my first post, had the lights etc have all stopped working :D

The only fuse relevant to this thread is the solar set up's own dedicated heavy duty inline fuse. Interesting you say that it could be giving these strange multimeter readings if it's blown. I've spare fuses so I'll check the connections are okay at the same time as changing the fuse. It certainly sounds like the place my detective work should start. Thanks 

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In place of a test bulb you could buy an electric train set, most are 12vdc, like Hornby ''00'' gauge, you can vary the voltage with the controller and the electrical resistance by adding or removing trucks or coaches. :closedeyes:

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5 hours ago, bizzard said:

In place of a test bulb you could buy an electric train set, most are 12vdc, like Hornby ''00'' gauge, you can vary the voltage with the controller and the electrical resistance by adding or removing trucks or coaches. :closedeyes:

ah so this is why boats have gunwales?  :)

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Voltage drop across a resistance is current times resistance. Tiny current means tiny volt drop. Think of it as compressed air trying to get through a tiny pinhole. Pressure will equalise at both sides of the obstruction until you try to use it to drive an air tool then the air cannot get through the obstruction fast enough to replace the flow through the tool and pressure plummets. Electrical pressure is voltage, flow is current.

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12 hours ago, BlueStringPudding said:

Sorry for the wrong symbol for fuse - I think it should have been a wiggle rather than a zigzag?

Fuse symbols:

20388085-548C-4AB8-9971-DF89299CE05F.png.86f64eca655043cc760009446feb9fb2.png



Resistor symbol:

624C2424-0547-44F1-A4ED-13305AA7EC40.gif.2cbd10c0a1eb16913faa0d1301cd1330.gif

 

 

Old and discontinued resistor symbol:

BEA67D54-D414-4F83-822A-D9AE3A30D813.gif.d6cff2295cf1a1a944e907cd4eec0a84.gif

Edited by WotEver
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1 hour ago, Sir Nibble said:

Voltage drop across a resistance is current times resistance. Tiny current means tiny volt drop. Think of it as compressed air trying to get through a tiny pinhole. Pressure will equalise at both sides of the obstruction until you try to use it to drive an air tool then the air cannot get through the obstruction fast enough to replace the flow through the tool and pressure plummets. Electrical pressure is voltage, flow is current.

Kinetic energy. A bit like steam boiler,''steam locos'' . The injector which tops up the boilers water by using the actual boilers steam pressure to inject water into the boiler against that pressure.

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Today I dismantled the solar fuse holder and put it back together again with a new fuse. While I was down there I checked other connections were tight and that voltages across each battery and across the fuse holder were the same. The only current draw from the solar circuit while doing this would have been from the two controllers (with no PV input) because I couldn't think what 12v appliance I could sacrifice to make a 5A draw. 

 

Anyway, good news in terms of the resistance problem being improved:

Before running the engine today, the voltage across the domestic batteries was 12.58v. The voltage across the solar fuse holder was 12.56v. And the voltage across the SunStar controller (which is about 10ft from the batteries, aka point C on the diagram) was 12.52v. This was the same voltage as at point A and B across the busbars, and same across the Victron controller. So there's no measurable resistance among the group of controllers and busbars anymore.

But there's 0.6v drop on the cable run from the batteries to the controllers (including any room for error from my multimeter). But I can live with that. 

 

Half an hour after running the engine there was 13.02v at each of the same controller and busbar points. So all good still.

 

Both controllers have happy little lights glowing on them now, which they didn't last night. 

And another bonus is that ever since having new domestic batteries installed a few months ago, my solar controller stopped being able to detect when the engine was charging the batteries (indicated by flashing light) but it did know when the solar panels were charging the batteries. That flashing light when the engine runs has now returned. So obviously there has been one or more unhappy connections around the solar fuse holder for a while. And they are now fixed. ?

 

I noticed that the large fuse that goes in the solar fuse holder is rated at 30A, same as the two solar controllers. Is that likely to cause problems? 

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13 minutes ago, BlueStringPudding said:

I noticed that the large fuse that goes in the solar fuse holder is rated at 30A, same as the two solar controllers. Is that likely to cause problems? 

Possibly, in the long term. Are the controllers 30A each? What size panels do you have to each?

14 minutes ago, BlueStringPudding said:

So obviously there has been one or more unhappy connections around the solar fuse holder for a while.

Told ya! :)

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1 hour ago, BlueStringPudding said:

Today I dismantled the solar fuse holder and put it back together again with a new fuse. While I was down there I checked other connections were tight and that voltages across each battery and across the fuse holder were the same. The only current draw from the solar circuit while doing this would have been from the two controllers (with no PV input) because I couldn't think what 12v appliance I could sacrifice to make a 5A draw. 

 

Anyway, good news in terms of the resistance problem being improved:

Before running the engine today, the voltage across the domestic batteries was 12.58v. The voltage across the solar fuse holder was 12.56v. And the voltage across the SunStar controller (which is about 10ft from the batteries, aka point C on the diagram) was 12.52v. This was the same voltage as at point A and B across the busbars, and same across the Victron controller. So there's no measurable resistance among the group of controllers and busbars anymore.

But there's 0.6v drop on the cable run from the batteries to the controllers (including any room for error from my multimeter). But I can live with that. 

 

Half an hour after running the engine there was 13.02v at each of the same controller and busbar points. So all good still.

 

Both controllers have happy little lights glowing on them now, which they didn't last night. 

And another bonus is that ever since having new domestic batteries installed a few months ago, my solar controller stopped being able to detect when the engine was charging the batteries (indicated by flashing light) but it did know when the solar panels were charging the batteries. That flashing light when the engine runs has now returned. So obviously there has been one or more unhappy connections around the solar fuse holder for a while. And they are now fixed. ?

 

I noticed that the large fuse that goes in the solar fuse holder is rated at 30A, same as the two solar controllers. Is that likely to cause problems? 

Yes. Both controllers are rated at 30A. 

One solar array is 390W and the other is 400W (latter has very long cables and are second hand so probably not putting owt like 400W into the controller). 

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