Can you parallel two alternators?

[IamBarry]

A little preamble:
I'm the new owner of a small cruiser. It has twin mercruiser V6's that are circa 1989, and two batteries, it looks like one is house the other a starter. The wiring is a mess. It looks like the alternators are in parallel. When both engines are running volts on both console voltmeters jump up to 18 volts and then back to normal periodically. I think the alternators are 80 amp motorola units with internal regulators. I've checked the alternators (disassemble, clean, test) they seem to be OK. I'm wondering if the internal regulators are fighting each other and making the voltage go nuts. Of interest, I've checked many of the suggested links in the various electrical threads including the Blue Sea calalog - so far no one SHOWs how twin alternators are wired.

In a world full of badly wired boats I'm hoping one of you has dealt with a similar problem and can make some suggestions.

Thanks and regards

Bazz

[TeddyDiver]

I'm not an expert in this but have somehere in my head a picture where double alternators are isolated. Other running with starting battery circuit, and other with house battery circ. Jump switch in case of mal function with either one..

[Fanie]

The alternator has it's own internal regulator, and you should not get the 18V jumps. That jump sounds like a loose connection, possibly a dry joint somewhere in one of the alternators.

If the alternators are in parallel they will be able to supply double the current (160A !!) and if the one regulated slightly higher the other should just switch off.

It is always better to have two starter batteries, one for each motor but with a high current switch (solenoid) between them in case one is flat, you can jump start the other motor. The two will act kind of as a backup for each other.

[Tony E]

Bazz

I've seen many a circa 1990's twin engine boat with a mess for wiring. The two engines, and corresponding control gauge panels should be electrically isolated from each other. If port battery goes dead, and you try to start it, the current will come from the stbd battery. If the engine systems are mistakenly paralleled say at the helm, the starter will pull 100 or so amps up to the helm and back to the motor in an attempt to start. I have seen the effects of this type of damage as well.

It is important to keep them separate, and it will solve your two alternators feeding one battery problem.

TE

[CDK]

The common construction in European boats with twin engines is two starter batteries and a manual 3-way switch, offering the choice of starboard, port or both batteries.
Like Fanie pointed out, both alternators will try to charge the batteries when switched in parallel until one of them reaches the threshold voltage.

[IamBarry]

Thanks Gents;

In addition to everyone's useful comments, I found a book showing the connection referred to by CDK. Tony E thanks for your mention of "back feeds" - I'll be very careful to be sure there are no unintended paths.

Although I've never seen it, I've heard of a condition called "load dump" that occurs in cars if the battery connection is lost with the engine running - It causes the alternator to lose it's reference and rapidly climb towards 60-80 volts and destroying all onboard electronics. The "dry connection" comment of Fanie's may be a huge clue. A high resistance, intermitant connection between one or both alternators and the battery(ies) could be involved. The boats' in shrink wrap for another month or so but I'll keep chasing the voltage jumps and add my findings to this thread.

again thanks all.

Barry

[CDK]

An open circuit between the alternator and a battery could be very destructive if the alternator was supplying a high current immediately before the circuit opened.
The internal regulator immediately interrupts the field current, but it takes several milliseconds for the magnetic field to decay because a lot of iron is involved. The result is a high spike on the output that could destroy the regulator itself and any electronic circuit connected. How high the spike is, depends on the load that is present at the time; worst case would be the absence of any resistive loads.
The spike is too short to blow any fuses....

[masalai]

Thanks all a very interesting and timely read...

[IamBarry]

To wrap this one up a little, after lots of checks and way too much thinking (it's easier then working) I pulled both alternators, took them to a repair shop and had them tested. BOTH had the same intermitant problem - once they really warmed up - the regulator jumped the output to 18 volts. The technician had never seen the problem before and couldn't believe his eyes.

In an earlier email I said they were motorola 80 amp units. I truth they are Mando (Korean) 60 amp alternators. New regulators were only $40 each plus a tip to the technician. The problem is now resolved. Thanks again to those who assisted.

Barry

[Landlubber]

Just one more issue here when using twin engine installations, ALWAYS, tie the two engine negatives together using heavy battery cable, this makes sure that Tony E story above does not happen.....common sence when you think about it