Floating DC system - unexpected voltage

[MerMike]

I'm looking at an aluminum sloop, euro wired with 230VAC, 24 VDC start and house banks and a 12VDC aux bank for nav equipment.

The DC systems are floating - no connection (or should be no connection) to the hull or engine. The alternators (24,24 and 12 VDC) are all isolated as is the starter.

I was poking around and got a spark on a fan circuit from the DC ground to the engine. Measured -26v between the battery ground and the engine. I was unable to measure any current. The voltage disappears when the house bank is disconnected.

I can't think of why there would be any detectable voltage between the DC negative and the hull/engine if they're properly isolated.

Am I missing something?

[team karst]

Originally Posted by MerMike

….



I can't think of why there would be any detectable voltage between the DC negative and the hull/engine if they're properly isolated.



Am I missing something?

Interesting. But, of course if objects are NOT wired together, then each is free to reach a different potential.
With that recognized, your above sentence is valid if you write “properly bonded” rather than “properly isolated”.

[Bowdrie]

Might the engines gages or start/stop/preheat or gage panel lighting be sharing any connections with the house batts?
The engine and its starting battery, (along with an alternator that only charges that starting battery,) need to live in their own little world with zero connections to anything else to be truly isolated.
A "Drivesaver", (or equivalent,) to break any electrical connection to the prop shaft is desirable as well.

[wholybee]

At a marina? Maybe through the water?

[MerMike]

Originally Posted by team karst

Interesting. But, of course if objects are NOT wired together, then each is free to reach a different potential.
With that recognized, your above sentence is valid if you write “properly bonded” rather than “properly isolated”.

I thought the point of a floating system was that it was not bonded to the hull in any way. I might expect some potential but not 26V.

[MerMike]

Originally Posted by wholybee

At a marina? Maybe through the water?

It's at a marina. It's in fresh water. Not plugged in to shore power.

[MerMike]

Originally Posted by Bowdrie

Might the engines gages or start/stop/preheat or gage panel lighting be sharing any connections with the house batts?
The engine and its starting battery, (along with an alternator that only charges that starting battery,) need to live in their own little world with zero connections to anything else to be truly isolated.
A "Drivesaver", (or equivalent,) to break any electrical connection to the prop shaft is desirable as well.

Those are all worth checking. I was hoping to hear whether a voltage reading was expected at all before I started trying to track it down.

I was expecting maybe a small difference but not 26v.

[Bowdrie]

Originally Posted by MerMike

The alternators (24,24 and 12 VDC) are all isolated as is the starter.

So there are 3 alternators?

[wholybee]

Originally Posted by MerMike

It's at a marina. It's in fresh water. Not plugged in to shore power.

So, if another boat or the marina itself is leaking into the water, your engine will be at that potential. Your DC system is floating at wherever potential it happens to be.

The fact that you measured 26 volts between the engine and the battery ground proves that they are isolated from each other. That is the point of grounding or bonding items, to hold them all at the same potential. I am both not at all surprised that voltage difference is there, but also in no position to recommend if it is bad or what to do about it. Especially on an aluminum boat.

[team karst]

Originally Posted by MerMike

….



I was poking around and got a spark on a fan circuit from the DC ground to the engine. Measured -26v between the battery ground and the engine. I was unable to measure any current. The voltage disappears when the house bank is disconnected.







Am I missing something?

True that the engine block is bonded to the hull?
But the design calls for no engine electrical attached to block.
So a reading of -26Vdc batt NEG to block (loaded) tells me that +24v is shorted to hull, somewhere….

I would suggest a 24v test lamp be used. A DMM will easily lead astray.

[MerMike]

Originally Posted by team karst

True that the engine block is bonded to the hull?
But the design calls for no engine electrical attached to block.
So a reading of -26Vdc batt NEG to block (loaded) tells me that +24v is shorted to hull, somewhere….

I would suggest a 24v test lamp be used. A DMM will easily lead astray.

Not sure if the engine is bonded to the hull, I will check.

Thanks, I'll test that next. I also think there is a short somewhere, but looking for clarity.

[MerMike]

Originally Posted by Bowdrie

So there are 3 alternators?

Yes. 3 alternators. Going to 2x24v alternators. Dropping the 12v, once it's accessible. Which in itself is a project.

[Bowdrie]

Originally Posted by MerMike

The voltage disappears when the house bank is disconnected.

Originally Posted by MerMike

I also think there is a short somewhere,

I think you have a starting point to work from.