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Old 30-05-2010, 08:35   #1
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Two Alternators on Cat, or Four ?

Good progress is being made on my Atlantic 57 and we're starting to install the electrical system. I am wondering if it makes sense to punt on the stock alternators that come with the Volve D55's that we plan to use for 12V start batteries charging. The high output alternators are 110A/24V Mastervolt alternators for the house 24V system.

Does it make sense to use a 24V-12V DC converter and simply use the big alternators for charging everything?

Pro's

1. More efficient engines (don't have to turn another alternator)
2. Less weight
3. less wear/maintenance on fan belts
4. Converter more reliable than alternator?


Con's

1. Slightly less redundancy?
2. Have to use same type of batteries? (AGM/Lead/LiFePO4 require different charging characteristics.)
3. Stock alternators are a more reliable and separate system?

Thanks!
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Old 30-05-2010, 10:38   #2
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Old 30-05-2010, 10:51   #3
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Our resident 'lectrical experts will give you a more definitive opinion, but I don't think you want to try to charge 12v batteries from a 24v alternator. What a Rube Goldberg setup -- adding splitters, then transformers. I don't think the 24v regulators will be able to understand a 12v battery.

Why in the world would you make your crucial starting batteries dependend on that setup? With one stroke you ruin what would otherwise be a completely separate start battery circuit, a very desirable design value. Any malfunction in the charging system can kill your starter battery.

Of course you want the separate 12v alternators. They will not use much engine power because they will be idling most of the time. You don't need any splitter or any complicated regulator -- start batteries charge just fine with a regular car-type regulator.
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Old 30-05-2010, 12:17   #4
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I work on one of those "mixed power" vessels where the engine start is 12VDC and the house is 24. Since we're a powerboat and run the big Cats, we've got a serious need for robust 12V recharge. We bought several 24-12V converters and have had zero problems for more than a decade.

Out converters are 30A monsters but you probably won't need that much capacity. While it is one more thing in the failure chain, its proven far more durable than the alternators on the engines. I would make sure the converters are well protected from moisture but you're probably ahead of me on that.
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Old 30-05-2010, 17:16   #5
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Interesting, one call's is a Rube Goldberg setup and another say's they have had zero problems for more than a decade and has proven far more durable than the alternators on the engines.
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Old 31-05-2010, 06:05   #6
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The stock alternators also run your tach. Knowing the RPM is useful. Having additional options for charging batteries comes in handy.

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Old 31-05-2010, 06:51   #7
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Interesting, one call's is a Rube Goldberg setup and another say's they have had zero problems for more than a decade and has proven far more durable than the alternators on the engines.
Well, you pays your money and takes your choice. There is no contradiction between the two statements. A needlessly complex system might happen to be, in some cases, reliable. That doesn't mean it's a better approach.

I think that, fundamentally, two simple systems which do not depend on each other, are objectively better than one complex system, as a matter of system architecture. Any fault in the single complex system takes down the whole system. And on top of that there is more to go wrong because the complexity of splitters and voltage converters has been added.

The two simple systems have no splitters or voltage converters to fail (no voltage converter will always be more reliable than the most reliable voltage converter). A fault in one system (already less likely) will not take down the other system. It's just better.

When you add one more layer of complexity -- two engines, so you have to combine the two big alternators anyway -- then the phrase "Rube Goldberg" ceases to be an exaggeration, describing a system where on top of everything else you're trying to charge 12v start batteries off an already complicated dual alternator 24v charging system. Ekh.

I wish we had dual engines -- that is the single best thing about cats in my opinion. Full power redundancy (not to mention great manuevring). We are a single engine mono, and we have 12v start (x2) and 24v house. As designed from the factory, the 12v start banks (engine and generator) are entirely separate from everything else and from each other, each with their own alternator. So there are three alternators on board: one 24v x 110 amp charging the house, one 30amp x 12v charging the engine start battery, and another 12v alternator charging the genset start battery. That means if something goes poof in the house, you can still start your engine (and generator). I will run a spare bilge pump off the engine start battery, too, to get some redundancy in power supply for bilge pumping. All this seems really valuable, to me, and I don't understand why anyone would throw it away, who already has it.
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Old 31-05-2010, 07:06   #8
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The starting batteries shouldn't require much charging at all - they are only used for a very short time. How about a small dedicated solar panel for them?

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Old 31-05-2010, 07:12   #9
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Personally, I would be more comfortable with fully independent starting circuits, unless of course your engines are designed to be hand-cranked. In other words: each engine keeps its tiny 12V alternator to charge its own independent starting battery and run the engine computer, with nothing else (well, maybe a bilge pump) connected to that circuit. Thus, even with a complete failure of the house system (unlikely, but possible), you'd still be able to run the engines.

The house bank, then, would be charged by the big 24V alternators, independent of the engine start circuits. (Providing the ability to do an emergency start from the house bank is probably not a bad idea.) DC-DC converters would then be used only in a few cases, mainly to provide clean, buffered 12V (or perhaps 13V) power to nav computers and radios. The converters will help to isolate these sensitive devices against voltage fluctuations in the house bank.

Of course, this is just my opinion.... I've worked on projects where we pushed DC-DC converters to (or beyond) their limits in harsh conditions, and generally speaking, they didn't like it very much. Treat them well, keep them cool and dry, and they should last nearly forever.
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Old 31-05-2010, 08:52   #10
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Given that the stock alternators feed the tachometers and the computer I see no alternative but to keep them. And yes, I agree with the simplicity and benefit of a double redundant system. Just trying to save some weight and a few gallons of diesel...

Thanks everyone for the great feedback.
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Old 31-05-2010, 09:32   #11
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I once worked on an Alden that had 12, 24, and 32V DC systems. Now that was complicated.
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Old 31-05-2010, 10:10   #12
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Quote:
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Given that the stock alternators feed the tachometers and the computer I see no alternative but to keep them. And yes, I agree with the simplicity and benefit of a double redundant system. Just trying to save some weight and a few gallons of diesel...
I understand that; of course weight is important on a cat.

But you shouldn't burn any extra diesel (alternators idle when they're not putting out a charge, and hardly use any engine power at all), and the extra weight of the small alternators is very little.

Enjoy all that redundancy! I envy you your dual engines.
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Old 31-05-2010, 21:53   #13
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Quote:
Originally Posted by Agility View Post
Good progress is being made on my Atlantic 57 and we're starting to install the electrical system. I am wondering if it makes sense to punt on the stock alternators that come with the Volve D55's that we plan to use for 12V start batteries charging. The high output alternators are 110A/24V Mastervolt alternators for the house 24V system.

Does it make sense to use a 24V-12V DC converter and simply use the big alternators for charging everything?

Pro's

1. More efficient engines (don't have to turn another alternator)
2. Less weight
3. less wear/maintenance on fan belts
4. Converter more reliable than alternator?


Con's

1. Slightly less redundancy?
2. Have to use same type of batteries? (AGM/Lead/LiFePO4 require different charging characteristics.)
3. Stock alternators are a more reliable and separate system?

Thanks!
Well heres my view....
Start batteries are only for starting and nothing else, so they dont need charging much. I have a 24v house bank and 12v starting. After you have watched your 12v battery monitor showing 2 amps going in for a while you start to wonder why you have a dedicated 12v alternator spinning around for such a small amount of amps.
So this is what I do....
Make sure the 24v alts are big enough to charge your house bank efficiently, if you have 800ah then you need at least 200a of 24v charging.
Remove the 12v alt and get one of these for example Adverc Battery Management - OSCA-P Battery charger 15 amp step down 24v to 12v
It is a 24v dc to 12v dc 15a charger. When the 24v battery is being charged then the 12v battery is charged from the 24v bank and it stops charging the 12v when the 24v bank stops being charged.
Remove the 12v alternator, it takes space that might be better used by another 24v alternator. Remember efficient charging of the 24v house bank is very important.
Make sure you have a small 15amp AC charger that can run off your inverter to charge your 12v battery, just for redundancy
Disadvantages
The only disadvantage of this is that you have no dedicated 12v alternator spinning around charging your 12v start battery which is always fully charged and so all it does is try to put 1 or 2 amps in.
Advantages
But the real advantages are as follows....
An engine pulley place is saved for other uses.
The dc to dc charger has no moving parts so should be more reliable.
You have redundancy in the form of the 15a AC charger and also if you have an AC generator then the small alternator on this can also charge the 12v battery.
And the biggest advantage
This cat comes with 4 130 watt solar panels which I guess charge the 24v battery bank.
When the 24v battery bank is charged by the solar panels then so is the 12v battery.... automatically without you doing anything.

(on the point about the tachometer feeds, they can run off the 24v alternators, any marine electrician can do this. It takes no time as only the pulses are measured on the alternator so changing should be simple. The revs then need to be measured and then calibrated on the gauge.)

John
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Old 21-01-2011, 04:49   #14
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I have just found this thread as I am having some of the same concerns; dual voltage. I have decided to change the starter to a 24 volt, and put in a isolator. I also added a Guest switch which allows me to combine the house and start batteries if needed to either start or power someting if needed. And, it cost about the same as another alternator.
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Old 25-01-2011, 05:18   #15
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Agility,
If you can't change the start circuit to 24V (doesn't the D55 come either 12V or 24V?), you may still consider making all four alts 24V (depending on the battery bank size and type...for TPPL or lithium you want all the charging power you can get, to save fuel and engine run time).

As Fuss points out, the alt dedicated to a 12V start batt doesn't do very much after the first few minutes. There are several battery to battery charging or DC-DC options out there, to keep the start batt charged up from the house bank.

Anyway, it's just a thought. This option comes up a lot when we do lithium house bank installations, to get as much charging capacity as possible.

Also, those 110A x 24V MV alts aren't really that "big". There are several very dependable higher output 24V alts on the market. I recently have been supplying a new 160A x 24V by American Power that weighs about 17lbs and does well in high heat/load situations (often a issue when charging large or lithium banks).

Yes, this does add some complexity, as ideally all four alts should be independently regulated.
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