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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.
ABYC Certified Marine Electrical Technician
Last edited by jreddington; 11-04-2008 at 11:26 AM.