Voltage Drop

[Rusty123]

Ketch, are you still around? What's the verdict?

[sailinglegend]

A good way to find high resistance on a wire is to measure the voltage drop along the wire. You will need a 27 foot extra wire connected to one of the meter leads, but as the meter takes so little current that will not cause a problem. The wire may have many switches, breakers, connections, take off points, etc.. I found a 0.24v drop across my main battery isolator switch and just cleaned the internal contacts. If it is a straight piece of wire with no other connections then there will be no voltage drop.

Now do the same on the negative wire and there will be a voltage drop of 0.24v. Somewhere on that wire another load is sharing the negative wire and causing a current flow and a high resistance point is causing the voltage drop.

[F51]

Originally Posted by sailinglegend

If it is a straight piece of wire with no other connections then there will be no voltage drop.

I believe if you will review your Ohm's Law you will find that is incorrect. E=I x R. Even a straight piece of wire has some resistance.

There is so much misunderstanding on this subject that it behooves us all to check our facts before posting and spreading misinformation. I know a great deal about voltage, current, and resistance because of my background in Electrical Engineering, but I don't know enough to pontificate. Even if I think I have my facts straight, I will often ask CF to do a sanity check. I don't know it all and I know it. How should we handle those who don't know it all and don't know it?

[sailinglegend]

Originally Posted by F51

I believe if you will review your Ohm's Law you will find that is incorrect. E=I x R. Even a straight piece of wire has some resistance. ...

Sorry I should have made it absolutely clear that I was referring to the OPs post #1 where he said there was no load on the wire therefore no current flowing through the 27 ft wire and therefore there could be no voltage drop. But the point was he seemed to have a voltage drop when no current was flowing.

If you had read the whole thread properly you would have realised exactly what I was talking about instead of trying to be clever and point out people's lack of understanding of Ohm's Law.

If you are going to quote Ohm's Law the please use the current form of V=IR.

V stands for voltage and E stands for "Electromotive Force", which is actually not a force anyway, so maybe it changed to correct that.

EDIT:
E was often used to represent the volts across a battery and V the voltage drop in other parts of the circuit. I believe it is difficult for some people to understand the application of Ohm's Law along a piece of wire. Where has the Electromotive Force come from to produce that voltage drop?

[F51]

Sorry you took offense. That was not my intent. My apologies.

[Wotname]

Gentlemen, really.
V=IR
E=IR

E is as short hand version of EMF.

The OP (and the cruising sailor) could be excused for thinking we are dancing on the head of a pin here if we plan to discuss which is (more) correct. Suffice to say, learn one way and stick with it. It makes no difference when working out what is wrong.

I personally prefer to think in terms of PD. Potential Difference and use the term Volts to describe the amount of PD in any part of the circuit.

YMMV

[Ketchgould]

The issue seems to be withy blue sea vessel system monitor 422. The multimeter at the battery reads 12.6 and the wires 27 feet away measure 12.6. The vsm is showing voltage to be 12.7 or something higher than at the actual battery and at every new crimped, heat shrunk terminal.

I'm gonna call blue sea and run this by them.

Thanks for the help.

[Wotname]

Originally Posted by Ketchgould

The issue seems to be withy blue sea vessel system monitor 422. The multimeter at the battery reads 12.6 and the wires 27 feet away measure 12.6. The vsm is showing voltage to be 12.7 or something higher than at the actual battery and at every new crimped, heat shrunk terminal.

I'm gonna call blue sea and run this by them.

Thanks for the help.

Sounds like a calibration issue, either with the VSM 422 or with the multimeter.

IMHO, this is not a biggie - at least if you are aware of the slight difference, providing it remains constant.

[Ketchgould]

There is no load. Vsm reads 0 amps and this part of the monitor works with no issues.

[Maine Sail]

Originally Posted by Guy

In addition to all the other junk we carry on board, we should carry a meg ohm meter, then that pesky battery cable would show it's true nature.

Jeez most boaters squeal when asked to pay more than $1.59 for a crimp tool and now you expect them to spring for a megger.

[decktapper]

Several people have indicated lift your wires at the load end and you should have zero voltage drop as you have effectively zero current. That is the first thing I would do. If you are concerned that you have a connection with 100 mega ohms of resistance, connect any load of one watt or greater to the load end of your wires, and your voltage should effectively go to zero if you have that much resistance. Better yet, connect a known load and measure voltage and calculate your resistance.
Good luck.