Isolated vs Negative ground alternators

[Glyka]

My Hitachi 55A alternator passed the way trying to charge half empty 360Ah batteries with a little help from a Sterling booster.

As this is an isolated-ground alternator, spare parts prove to be rare and really expensive. I found out that I can have an even better (higher output) alternator with less than I would pay for a repair, but this would be a "normal" negative ground one. Does anyone know if there is a problem using negative ground alternators on a (GRP) boat? Why Yanmar is originally equipped with an isolated-ground one?

Thanks,

Theodore




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[pvb]

Shouldn\'t be a problem...

Isolated ground alternators have a number of benefits. Firstly, you avoid using the engine block as part of the return circuit, so preventing the odd stray currents. Secondly, some boats are configured with positive ground, and an isolated ground alternator can be used on these as well as on negative ground boats. Thirdly, some boats (usually metal ones) isolate the engine from ground, requiring an isolated ground alternator.

In the case of your Dufour, there should be no problem in fitting a negative ground alternator, as long as it bolts direct to the engine. (I mention this because some secondary alternator brackets utilise rubber-insulated bushes to minimise vibration - these obviously don’t conduct electricity). The existing negative lead from your old alternator could be connected to the alternator body/bracket; if not, ensure that the existing ground lead from the engine to the battery bank is big enough to cope with the potential higher output from the new alternator (it should be because it will most likely be the return for the starter motor. You also need to ensure that the positive lead from the alternator is correctly sized for a higher output.

You can’t effectively drive much more than a 90A 12V alternator with a single belt, and there wouldn’t be any benefit for you in having a bigger alternator than this because of the size of your battery bank. Make sure the alternator pulley matches the engine pulley, and buy the best quality V-belt you can find (Gates make good ones).

Excess heat is a frequent cause of alternator failure, so if you’re fitting a more powerful unit, you might give some thought to ventilation. If combustion air is ducted into the engine compartment somewhere, you could revise the ducting so that the incoming air blows on the alternator, thus giving some extra cooling.



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[steve28]

I had a problem with my 2gm20 at the weekend, it is used in conjuction with a sterling pro and a display, the display would suddenly display an error saying high negative detected , system disengaged.
After several hours and various things tried it turned out to be the negative isolated wire that comes from the alternator and goes to the started motor securing bolt. It seems to me that they just strap out the mechanical joint of the alternator and the engine block in this case.

This doesnt answer your question but may be of interest.



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