Isolating transformers

[Guest]

I have recently heard that if you use an automotive battery charger to recharge your batteries it is possible to increase the electrolitic corrosion due to the fact that the transformer in the charger will probably be an autotranformer. The article that I read sugested that the marine chargers all have isolating transformers which ensures there is no direct electrical conection between the shore and boat electrics.
I have an American Shumacher Ship'n'shore charger input 120V. If I use an isolating 240v to 110v isolating transformer do you think this will be as effective. p.s. My boat is steel

 

[jfkal]

Most importantant is to have a GROUND isolator installed.

 

[HaraldS]

I don't know your charger but here are some answers:

An isolating 240V to 120V transformer in front of your charger will always block any DC between boat and shore and is as good as it can get with respect to electrolysis.

Not necessarily all automotive chargers have such a problem. Classic automotive chargers have a line frequency transformer and if that is isolating or not is another question, but it makes them heavy. If it is around 2 lbs per DC Amp, it is one of those. These classics usually have a simple output stage, that just delivers the current you select and if you don't disconnect them in time, your batteries will start gasing and suffer.

In addition they may have the problem you mention, by creating a DC connection to shore. Some have an isolating transformer, but still connect shore ground AND DC negative to the chassis.

If they do have an autotransformer, then it is impossible to connect any of the DC outlets with the ground, because it would create a short. AC-'polarity' would be a necessity or the DC outlet could become hot, relative to ground. And worse, it could only deploy a have wave rectifier, not a bridge, making it quite inefficient. While this would work on an isolated car, standing on its tires, you would short one half of the bridge rectifier on the boat, since DC negative is usually grounded. And the associated current would be through the water and electrolytically very damaging. Very old chargers, with a single selenium rectifier were of that kind.

Modern boat chargers, and also some newer automotive chargers, do not have heavy line frequency transformers, rather work like switched mode power supplies. Creating their own higher frequency AC, much smaller transformers can transfer a similar amount of energy, they always are also isolated and regulation between the two sides is via optical couplers. So they usually are perfect in galvanic isolation. If you had no other AC users on board, a modern charger would isolate perfectly and some even adjust to differnt shore voltage automatically.

With uncertain AC devices and chargers like yours, certainly an isolation transformer is by far the safest solution.

 

[VMALLOWS]

HaraldS:

A nice concise reply. But do you think battery chargers (of any sort) were ever made with auto-transformers??.. I sincerely hope not. The only thing I can think of is that perhaps an auto-transformer was supplied to convert 110v/230v (or vise versa) infront of an otherwise normal charger.

(for the bemused, an 'auto-transformer' has only a single winding, and although it 'transforms' the voltage just like any other transformer, it provides absolutely no separation between the incoming supply and the output. Your risk of shock, electrolysis due to stray currents, etc , is the same as if it hadn't been there).

 

[HaraldS]

To be honest, I have never seen one made that way. I did see old ones with half wave rectifiers. But where I come from, hot and neutral conductor are not coded on the receptacles and can be freely exchanged, which would render the autotransformer to a lethal device.

I just thought in some place people might have been crazy enough to do that to safe a few onces of copper ;-)