Mysterious stray current

[mtb55]

I have a mysterious stray current problem I was hoping someone might be able to help me with. I was checking circuits with my multimeter at home trying to find a voltage leak and I discovered I have between .20 and .50 volts on most places around my deck and bilge area. Now here is where it gets interesting all battery cables are disconnected from batteries and anything that might have a capacitor has been removed and im still getting these voltage readings!! Below are some facts about the boat.

-Boat is on the trailer when im getting these voltage readings.
-Boat is a 24' 1998 Alumaweld (heavy gauge welded aluminum)
-Engines are 2.5l 150 merc Black Max and 15 hp merc 2 cycle


After doing exhaustive google searches I have found a few other boaters that have reported similar problems and they never seem to come to conclusion why this happens, I hope someone here might have some ideas whats going on.

 

[Another retired guy]

Re: Mysterious stray current

It's probably due to galvanic reaction between dissimilar metals. Check this discussion which contains a detailed explanation of this.
Stray Current? - The Hull Truth - Boating and Fishing Forum

 

[UncleWillie]

Re: Mysterious stray current

I... I discovered I have between .20 and .50 volts on most places around my deck and bilge area. Now here is where it gets interesting all battery cables are disconnected from batteries and anything that might have a capacitor has been removed and I'm still getting these voltage readings!! ...

We will assume you are also disconnected from shore power.

When you measure the voltages, what, specifically, are the meter leads connected to?
What does it indicate when you connect the leads to the same object?
What does it indicate when you connect the meter leads to each other?

Remember to keep your fingers off of what you are measuring.

 

[mtb55]

Re: Mysterious stray current

We will assume you are also disconnected from shore power.

When you measure the voltages, what, specifically, are the meter leads connected to?
What does it indicate when you connect the leads to the same object?
What does it indicate when you connect the meter leads to each other?

Remember to keep your fingers off of what you are measuring.

The pictures below should answer your questions.

I think the meter is good......I have checked this meter on the battery and it read 12.6 volts and the OHMs read 0.00 when I cross the leads... I have a new multimeter coming tomorrow, I just want cross reference to make sure this meter is not sending me on a wild goose chase with inaccurate readings.

 

[mtb55]

Re: Mysterious stray current

Here is something I forgot to add in my first post that might be pertinent.... I got high readings from a silver silver chloride reference electrode the last time I had the boat in the water. The manual that came with the electrode advised a user with my numbers to check for voltage leaks.

 

[Fleetwin]

Re: Mysterious stray current

In the first picture, the cleat has a steel screw/bolt. The hull is Aluminum. That in itself may set up a small current.

 

[blackhawk180]

Re: Mysterious stray current

Dang, now I'm going to have to check my alumaweld! Mine is newer and all cleats etc are welded so no dissimilar metals there. Worth a few minutes with my multimeter though. I'll be interested to see what you come up with.

 

[mtb55]

Re: Mysterious stray current

Dang, now I'm going to have to check my alumaweld! Mine is newer and all cleats etc are welded so no dissimilar metals there. Worth a few minutes with my multimeter though. I'll be interested to see what you come up with.

I would strongly advise it, also invest in a silver/silver chloride reference electrode especially if you go in the salt water, its the only way to really know whats going on with you hull. What lead me to buy a silver/silver chloride reference electrode is that my parents 03 Boice jet sled had to be totally repainted due to bunch of corrosion (paint bubbles) that appeared quickly after the occasional trip (not moored) in brackish and salt water, and yes everything was grounded back to the battery and it had anodes. To this day their not totally sure what caused it despite having aluminum boat builders and mechanics inspect it. I'm learning that the popular belief in the Pacific Northwest that aluminum hulls are super low maintenance is wrong, its just a different kind of maintenance.

 

[mtb55]

Re: Mysterious stray current

In the first picture, the cleat has a steel screw/bolt. The hull is Aluminum. That in itself may set up a small current.

Good point. I did not expect a number that high, but then again im no electrical engineer lol....If I run that same test on the starboard side cleat I get no reading. The other picture of the wood deck creating voltage blows my mind.

 

[dingbat]

Re: Mysterious stray current

A voltmeter will tell you that the 'potential' difference between those two points.
There's no absolute zero voltage so your zero reference is wherever the black lead of your voltmeter is hooked.

What are you measuring from these points to system ground? What do you measure between system ground and "earth" ground?

 

[mtb55]

Re: Mysterious stray current

A voltmeter will tell you that the 'potential' difference between those two points.
There's no absolute zero voltage so your zero reference is wherever the black lead of your voltmeter is hooked.

What are you measuring from these points to system ground? What do you measure between system ground and "earth" ground?

A voltmeter will tell you that the 'potential' difference between those two points.
There's no absolute zero voltage so your zero reference is wherever the black lead of your voltmeter is hooked.

What are you measuring from these points to system ground? What do you measure between system ground and "earth" ground?

Questions;

1. When you say "system ground" are you referring to the negative terminal on the battery?

2.When you say- "What do you measure between system ground and "earth" ground?" Are you asking me to measure between the hull and the cement driveway??? That test would have a ton of electrical resistance making it useless, right??

 

[dingbat]

Re: Mysterious stray current

Questions;

1. When you say "system ground" are you referring to the negative terminal on the battery?

2.When you say- "What do you measure between system ground and "earth" ground?" Are you asking me to measure between the hull and the cement driveway??? That test would have a ton of electrical resistance making it useless, right??

When a voltmeter is connected between two different types of metal, you are not only measuring the difference in electric potential. You are also measuring the potential difference at the molecular level. For instance, the electrical potential may be zero, but the molecular difference 0.25.

The quantity measured by a voltmeter is called electrochemical potential or fermi level. The terms "voltage" and "electric potential" are a bit ambiguous in that they can refer to either of these in different contexts.

In your case, I seriously doubt there is any ?stray current?. That is why I asked what you read from each point to battery (system) ground. Then measure from battery ground to ?earth ground?. Sticking your VM problem into wet earth will work as an earth ground. A screw driver driven into the ground some distance away with a wire attached to connect your meter is even better.

Better yet, connect a 12 VDC light bulb between your two points. No light = no stray current

 

[UncleWillie]

Re: Mysterious stray current

... The other picture of the wood deck creating voltage blows my mind.

When measuring very small voltages the measurement itself can produce a voltage.

You are measuring the wood decking at a damaged spot which certainly has gotten wet at one point or another.
The Wet Wood becomes the electrolyte in a virtual battery.
The aluminum hull is screwed into the wood at some point.
For the sake of argument lets assume the test leads are zinc galvanized and the screws are Stainless Steel.
So starting at the POS Lead going to the NEG we have a galvanic stack of...

Zinc > Wet Wood > Stainless > Aluminum > Zinc

That make 4 areas of dissimilar material contact and 2 of them are at the Meter leads.
It would be unusual to if there were No voltages created in all of that!

That doesn't mean there is or isn't a problem. Reverse the leads and you will likely get a very different number.
If you get the same number but the opposite polarity, The reading is more likely real.
If you get wild numbers that are not repeatable, The measurements should be viewed with suspicion.

I have some meters that measure to 4 decimal places.
I can rarely get repeatable readings at the third decimal place.

 

[mtb55]

Re: Mysterious stray current

When a voltmeter is connected between two different types of metal, you are not only measuring the difference in electric potential. You are also measuring the potential difference at the molecular level. For instance, the electrical potential may be zero, but the molecular difference 0.25.

The quantity measured by a voltmeter is called electrochemical potential or fermi level. The terms "voltage" and "electric potential" are a bit ambiguous in that they can refer to either of these in different contexts.

In your case, I seriously doubt there is any ?stray current?. That is why I asked what you read from each point to battery (system) ground. Then measure from battery ground to ?earth ground?. Sticking your VM problem into wet earth will work as an earth ground. A screw driver driven into the ground some distance away with a wire attached to connect your meter is even better.

Better yet, connect a 12 VDC light bulb between your two points. No light = no stray current

Ok I did these tests, pictures are worth a 1000 words so I will let them speak.




The other end of the wire is connected to a large screw driver that is shoved into the ground up to the handle, its about 6 feet behind the boat.


Not sure if you wanted the battery ground cable attached to the battery or not for this test so I did it both ways (see picture below). The other end of the wire is connected to a large screw driver that is shoved into the ground up to the handle, its about 6 feet behind the boat.



Screw driver test with battery ground wire connected.

 

[mtb55]

Re: Mysterious stray current

That doesn't mean there is or isn't a problem. Reverse the leads and you will likely get a very different number.
If you get the same number but the opposite polarity, The reading is more likely real.
If you get wild numbers that are not repeatable, The measurements should be viewed with suspicion.

I have some meters that measure to 4 decimal places.
I can rarely get repeatable readings at the third decimal place.

Reversed

 

[mtb55]

Re: Mysterious stray current

As you guys will probably notice I bought another meter so I can cross check readings.

 

[sam am I]

Re: Mysterious stray current

mtb,

I have not read through this entire thread looking at what has or hasn't been explained, so apologies to all if I repeat something you've or someone has already said but, looking at your title and this most resent measuring efforts you've been doing, I thought perhaps a simple analogy might help dis-spell some, if not, all the "mystery".

With your batteries and/or any other current source dis-connected/isolated from equipment/hull and the like so as to eliminate those as a contributing factor ...........

The various materials you're connecting the positive and negative meter leads on your boat(forward or backwards) are following a fairly simple explanation......

When you find a difference in potential(a voltage reading of 0.5V for example), you have then between the meter's probes, a semi-conductive media with a electrolytic present(chemical semi-conductive makeup the current is flowing through).

At certain/differing acidity/ph levels of the electrolyte[various contamination's flavors(internal or external) of woods , gaskets, carpets, rubbers, oxidation's/corrosion's composites, plastics, water, dirt and the like] between dis-similar metals(or wood in one case) plates, creates differing, normally quite predictable voltages. A lead acid battery comes to mind.

To put it bluntly? You discovered how batteries work man!!

For example...Copper(plate#1, red probe), orange juice(electrolyte, nice high ph for conductivity), aluminum(plate #2, black probe)





The Anodic index says copper will produce around -0.35V and aluminum will produce around -0.95V both from base ref of gold at 0.00V. The negative/ref lead on my meter is on the al. plate, therefor, -0.35V - (-0.95V) = 0.6V


I believe what's going on primarily with your differing readings is as you're moving from place to place, object to object in your boat, you're just switching up the metals/plates types/indices along with varying the electrolyte's levels between all the mixes........that's all.

As a test, noting these voltages pictured, repeat your tests but replace the screw driver w a copper rod driven in the ground for example and/or tap a galvanized nail or copper wire in the wood, then clip the probe on the head of the nail/wire. Changing the voltage readings a predicted amount by ONLY varying the plates type/s within the same setup/repeated test should prove this.

If for example the screw driver is like chromium type corrosion resistant steel(-0.50V), the switch to copper(-0.35V) should result in a voltage change of about 0.150V.

Switch your meter to measure current, and it'll be in the low low micro amps and "don't sweat the small stuff"

 

[dingbat]

Re: Mysterious stray current

Well, I wrote long list of recommendations, things to check, etc. and lost it so now the abridged version.

Understand the difference between potential and voltage. Understand there is no such thing as zero volts. All measurement between two points are relative to each other, not "ground". Your "voltage" between the two bolts proves nothing other than they have different potentials which would be expected given the situation. I would be more concerned if they significantly different potentials referenced to ground.

Start looking for voltage drops within circuits. The 0.2 volts that was there at the battery end of the cable but not at the helm end didn't just disappear. It's found a lower resistance ground path to follow. Damaged wire insulation and corroded grounds are the most common causes.

Disconnect the boat ground from the battery and start taking readings. All grounds should be tied back to your battery so you need to use that as your reference point.

The jump in potential between the battery and earth, with and without the ground cable connected is interesting. I would be looking for loose corroded ground connections first. Then start checking circuits for voltage drops. I chased corrode ground connection in a light fixture for almost a 3 weeks last years.

MIL Std allows for 0.25V or less recreational applications or 0.5V for commercial applications using due diligence.