Battery charger & Galvanic reaction

[Petar]

Can someone experienced with DC circuits provide advice. Last year in July I renewed my zincs which were just about gone. In May this year I've noticed them gone but have hauled my boat only two weeks ago to find out that with both zincs (shaft and propeler) gone one of my Max prop blades got pitting damage which I suspect from galvanic reaction.
I started looking at my boat and just found that my "True Charge 10 StatPower" battery charger is giving 14.4 V DC but also 30.8 V AC?? Is this normal and woukd this indicate that the carger needs repkacement even though it is charging my batteries well. Could this be the cause of my zincs wearing prematurely and eventually damaging the prop?

any advice is welcome

Petar

[brak]

1 year for zincs in salt water in a marina is hardly "premature". I'd say 6 months is about the most any zinc would last in a marina with electric service.

[SEMIJim]

Originally Posted by Petar

I started looking at my boat and just found that my "True Charge 10 StatPower" battery charger is giving 14.4 V DC but also 30.8 V AC??

How and where (between what two points) did you measure this 30.8 VAC?

Jim

[Petar]

Jim,
Excatly wher I measured DC. Charger has two +ve leads to be able to charge two banks of batteries and one -ve. When measuring potential between on +ve and -ve I get 14.3 V DC on and when I switch my multi tester to AC I get 31V AC on each. Batteries were disconnected, so only DC leads exiting the charger were measured.

Regards/Petar

[Fstbttms]

Originally Posted by brak

1 year for zincs in salt water in a marina is hardly "premature". I'd say 6 months is about the most any zinc would last in a marina with electric service.

This is a broad generalization that is not particularly accurate, but certainly 6-9 months can be considered a normal lifespan for shaft zincs in a marina setting. YMMV somewhat.

[KeelHaulin]

Stray AC voltage does not cause galvanic corrosion; but at 31V it might be enough to give you a tingle or a shock if you are well grounded. You need continuous DC voltage (between the engine ground and the water) to promote galvanic corrosion.

You should contact Xantrex to find out if your charger is bad or if there is a solution to this type of problem; or if a problem exists. It could be that if the circuit is "open" (not connected to a battery) the system passes some AC noise.

[Fstbttms]

Originally Posted by KeelHaulin

Stray AC voltage does not cause galvanic corrosion; but at 31V it might be enough to give you a tingle or a shock if you are well grounded. You need continuous DC voltage (between the engine ground and the water) to promote galvanic corrosion.

Correct me if I'm wrong (and I probably am) 'cause you sound like you know what you are talking about, but isn't galvanic corrosion caused by two dissimilar metals being in electrical contact while immersed in an electrolyte? In actuality, aren't we talking about electrolytic corrosion here?

[EO32]

My guess is this is not Galvanic, but ground differentials from shore power.

If the Green ground on your boat or your neighbor’s boat goes through some corroded connections that create resistance, you will get a voltage differential that will eat zincs alive.

I would disconnect your charger, all three wires, and see if it still exists. If so, then this is the typically shore power grounding issue.

[KeelHaulin]

Originally Posted by Fstbttms

Correct me if I'm wrong (and I probably am) 'cause you sound like you know what you are talking about, but isn't galvanic corrosion caused by two dissimilar metals being in electrical contact while immersed in an electrolyte?

Yes; but it's more general than that. Think about how metals are chrome or gold plated. The process is to have two metals in a tank of electrolyte and pass current through so that the anode (the plating metal) 'corrodes' and then plates onto the cathode. This process would happen naturally but it would be very slow so increasing the voltage/current amplifies the effect or in some cases reverses it (getting a more noble metal to plate onto a less noble one for instance). All of these processes are forms of 'galvanic corrosion'. If you increase the DC voltage at your anode it will corrode away that much more quickly; which is why devices like stray current isolators are used. Since AC is cyclic (positive and negative alternating current) it does not cause galvanic corrosion.

[Faster]

It may be worth checking for that stray "AC" voltage with the batteries attached... I suspect that with a load that AC voltage may disappear.