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  #11  
Old 08-24-2008
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Quote:
Originally Posted by KeelHaulin View Post
Since AC is cyclic (positive and negative alternating current) it does not cause galvanic corrosion.
I'm certainly no authority, but I suspect that's incorrect. Two things: Yes, it is Alternating Current. So wouldn't the current direction cause the anode to lose material half the time? Given the size of the "bath," I would hardly think it would be restored the other half of each cycle.

Secondly: I believe it was here on Sailnet that, last season, a member reported on a boat in his marina that sank in its slip. Upon investigation it was found that all the boats below-waterline fittings had disintegrated. Of sacrificial anodes that was nothing to be found. They subsequently found that that boats slip neighbour's shore power cord was partially in the water and had a pin-hole breach, creating a fairly significant leakage current into the water. The boat-in-question's fittings corroded-away in less than two weeks, IIRC.

Jim
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Old 08-26-2008
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Well; I'm almost certain that AC current does not increase galvanic corrosion. Corrosion is a diffusional process; and an AC current is switching +/- too quickly for the molecules to break their bonds and travel from anode to cathode.

Two examples that will help convince you of this:

There is no such thing as an AC battery; that is because the galvanic reaction that takes place inside the battery produces DC current only. The process is reversed by applying a reverse current DC and "re-plating" the anode.

Galvanic isolator circuits are used to "filter" stray DC current out of the AC supply. The AC component is not the source of stray current induced galvanic corrosion so it does not need isolation.
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Old 11-04-2008
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Battery Charger A/C Ripple & Galvanic Corrosion

In response to the first post, the A/C you are reading across the battery is "ripple". A charger basically works by taking A/C which is a sine wave with a positive half and negative half, and running it through a rectifier. This "flips" the negative half of the wave so it would look like a series of "humps". If you put a voltmeter across this, it sees the constantly fluctuating voltage as A/C. To smooth out these ripples the output is filtered using capacitors. If you put it on an oscilloscope to see the shape, you'd see a line at 12 V but with some ripple to it. The more filtering you build into the charger, the less ripple (and more cost).

Batteries are pretty insensitive to this ripple so the charger does not need a lot of filtering. However, this ripple can affect sensitive 12 VDC electronics. But most manufacturers do recognize that there will be some ripple while charging and design around that. If you have a piece of electronics that acts whacky while the charger is on but runs fine battery alone, that's probably the culprit. Either the device's own internal filtering circuits have gone bad or you need to add some filters. Depending on the sophistication of your system, you might need a qualified marine electrical technician to help out with this.

By the way, your alternator does the same thing while charging when the engine is running. The alternator produces AC power which is internally rectified to DC.

The galvanic isolator is needed on the AC system, not because of the AC itself, but because your DC system is bonded to the ground of the AC system. When you are plugged into AC power this ground is connected to all the other grounds (and therefore DC grounds) of the other boats on that system. Any DC leakage (either from the boat, or contact of the grounding conductor with water) will set up a circuit that will rot out the fittings on all other boats unless they have a galvanic isolator or isolation transformer.

Technically this is not "galvanic" corrossion since that specifically referrs to the battery formed by dissimilar metals producing the current itself. It is more formally impressed current corrosion, but the effects are the same and caused by current forcing material from the less noble metal.

Jim R.
ABYC Certified Marine Electrical Technician

Last edited by jreddington; 11-04-2008 at 11:26 AM.
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Old 11-04-2008
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quote from original post on AC measurement "Batteries were disconnected,"

Measuring some sort of AC on the charger output with it disconnected from the battery really doesnt mean much since its operating in a mode its never intended to operate in. Some "switching type" electronics dont like to be operated without a load (and can burn out) so I would just make sure the charger still operates correctly when hooked back up the battery.

What you should be checking instead is the boat ground vs the shore power ground and the main place the shore power ground connects to the boat DC ground may be through the charger? There have been some detailed discussion in this forum in the short time Ive been visiting so plenty to look up by search.
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