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Old 06-05-2021, 11:51   #1
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Dummy Alternator - Myth or Fact?

Hi all,

I have a 1988 Wellcraft St. Tropez 32' cruiser with twin 1988 Mercruiser V8 454's 340HP inboards, Borg Warner transmissions, Thunderbolt IV Bluewater ignition system, distributor (not points).

I have 1 starting battery (Group 29, I think) that feeds both engines via my battery selector switch. (Yes, really, only 1 battery wired to two engines.) My other battery is a 4D deep cycle house battery. Of course, from my one and only battery selector switch I can select 1, 2, or all batteries, and whatever I select will feed BOTH engines simultaneously. My point here is that with the current setup, I have no way of specifying which battery goes with which engine.

The port side has a working alternator. I know because when tracking down an issue last year I broke part of the housing on the old (probably original) alternator and replaced it with a new alternator. I also know because when revving the port side I can see the voltmeter (on both engines, mind you!) go from around 12v to more than 13v indicating it is putting out a charge.

The starboard side alternator (the one in question) seems to not do anything at all. When I rev the starboard engine, the voltmeters don't move, at all. I've also checked with a multimeter and there's no detectable output from the alternator. Of course this means that if my port side motor ever fails and I have to get home on just the starboard side, I'm generating spark from my battery and can only go as far as it will take me until it dies. That's not a risk I like, especially with older motors...

There's disagreement among my boating and mechanic friends as to whether or not the starboard alternator is broken (diode, perhaps?) OR if it is a "dummy alternator" -- a fake there just to have the pully for the belt to go around.

Friends who think it may be a dummy alternator defend their position by saying they think it would be "wrong" (and perhaps dangerous) to have TWO alternators on separate engines simultaneously charging ONE battery -- and possibly each other! I can understand what they're saying here.

Dummy alternator or not, I've been thinking about replacing the non-working starboard alternator with a new, working one. But I'm a bit concerned about having both feed the battery, AND/OR each other.

A close boating friend who is an electrical engineer thinks my boat's electric setup is "weird." I'm not sure I disagree. But, it is what it is, at least, until I change it. He thinks I should add a second starting battery and isolate the two engines each with 1 starting battery of their own, AND put in a crossover switch in the event that one side's battery dies and I need to start it. (While still having the deep cycle 4D house battery, but I'm unclear as to how that would get hooked up.)

What do my fellow CruiserForum readers think? Have I described enough of the story here? What would you do? Just replace the starboard alternator? Is there any risk? Would you rewire anything as part of the job? Should the two motors be electrically isolated in some way that they're currently not? Should I add a battery and isolate the two motors?

Looking forward to hearing what you all think. Thank you in advance!
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Old 06-05-2021, 12:04   #2
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Re: Dummy Alternator - Myth or Fact?

Starboard alternator is likely broken. If it's been intentionally disabled, it was done by someone with no electrical knowledge. It's perfectly fine to have multiple alternators charge 1 battery. One may work harder than the other, but it won't hurt anything.

Personally, with 2 engines and a generator, I have my batteries configured into 3 banks: start 1 (S1), start 2 (S2), and house. There are 4 battery switches. On/off for the house, and then a 1-2-both-off for each engine and 1 for the gen. Those 3 select where each engine gets starting power from. Normally I have them set for the port engine to use S1, stbd and gen share S2.

For charging underway, each engine's system has an ACR that links to the house bank (but is ignition interlocked so it only operates when that engine is running but charging stays independent the rest of the time). This way when the engines are running, each engine charges its start battery as well as the house bank. When the engines are off, the house bank is charged from solar and the charger in my inverter/charger. There's a separate charger that handles the start batteries.

I'd want a separate switch per engine, but as long as you have 2 available sources of starting power, it won't hurt anything to have the engines share a battery.
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Old 06-05-2021, 12:21   #3
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Re: Dummy Alternator - Myth or Fact?

On that kind of boat I would fix the starboard alternator (for the reasons you state) and call it good. Turn the switch to "both" before starting engines, turn it back to "off" after shutting down engines.


Confirm that the connections from the starboard alternator are clean and tight and check them with an IR temperature gun while charging with the new alternator and replace any that get hot. Bad connections can lead to diode damage and could be why the existing alternator failed.



You can buy an ACR and not have to turn the switch. Instead you will wonder whether your ACR is broken, and someday it will be.
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Old 06-05-2021, 12:25   #4
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Re: Dummy Alternator - Myth or Fact?

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Originally Posted by Jammer View Post
You can buy an ACR and not have to turn the switch. Instead you will wonder whether your ACR is broken, and someday it will be.

That's one of the big things I like about how my setup is wired. You get confirmation of alternator and ACR function at every start. Basically, right after you start each engine, watch the volt meter. It should rise indicating that the alternator is putting out power, then after a few seconds it'll twitch downward a bit as the ACR links up to the house bank. The only gotcha would be if an ACR sticks engaged and doesn't open on engine shutdown.
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Old 06-05-2021, 13:53   #5
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Re: Dummy Alternator - Myth or Fact?

No problem with two alternators charging one battery. I have delivered a couple of twin engine boats which had no working alternators. They were Florida boats, and had to run the genset underway for air conditioning. On the Sabre 42, both alternators had the excitation wires unplugged. Plugged them in, they both worked, turned off the genset when we got north..
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Old 06-05-2021, 18:25   #6
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Re: Dummy Alternator - Myth or Fact?

you should have 3 battery banks and 3 switches... each engine and house.

dummy alt is dumiest thing I've ever heard.

you have 2 engines and 2 batteries. each engine should be charging one battery.
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Old 06-05-2021, 18:45   #7
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Re: Dummy Alternator - Myth or Fact?

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Originally Posted by donradcliffe View Post
No problem with two alternators charging one battery. I have delivered a couple of twin engine boats which had no working alternators. They were Florida boats, and had to run the genset underway for air conditioning. On the Sabre 42, both alternators had the excitation wires unplugged. Plugged them in, they both worked, turned off the genset when we got north..
My 73 Lamborghini had 2 alternators and one battery. No problem.
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Old 06-05-2021, 21:21   #8
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Re: Dummy Alternator - Myth or Fact?

Quote:
Originally Posted by Harmen View Post
My 73 Lamborghini had 2 alternators and one battery. No problem.
My 1971 VW Superbeetle didnít even have a single alternator.



VW was still running generators then.
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Old 07-05-2021, 05:48   #9
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Re: Dummy Alternator - Myth or Fact?

Aquasition, you asked me what I thought of this thread. As a general answer I think that rslifkin, in the first reponse to your question, gave you a complete and competent explanation.

Let me describe a similar situation. I have two diesel engines, one propulsion and one a genset. They each have very simple generic Delco internal regulator (single wire) alternators that charge the single type 27 batteries that start each engine. The genset can also charge the larger house bank through the sophisticated battery charger that also works off shore power. The main engine has a second alternator, a sophisticated Balmar with its fancy voltage regulator that charges the house bank whenever the engine is running. Overall, this works well because the house bank, being big, would burn out the simple Delco alternators and the house bank gets tender loving care from either of two sources.

THEN, I can cross connect at will with jumper cables. I can start either engine off the house batteries. I can parallel the two starter batteries if one is weak or the morning is cold. I can even put the starter batteries in series for a 24v jolt to the engine starter. Such situations are rare, so I have not felt compelled to install switches to do the job of jumper cables.

Broadly, separate your batteries and their charging sources. That protects you against waking up with no power at all, and makes the charging circuit compatible with each battery.
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Old 07-05-2021, 09:25   #10
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Re: Dummy Alternator - Myth or Fact?

Slightly off track, I would recommend connecting each alternator to the relevant engine oil pressure switch so the alternator does not load the engine until it has oil pressure. The diodes can be checked with just a cheap AC voltmeter that can read to about 1 Volt. Start the engine and apply a heavy electrical draw. Read the AC on the alternator or connected battery. If it is less than about 1/2 Volt AC the diodes are good enough.
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