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Old 02-10-2011, 19:08   #1
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Two Diesels: Double the Charging ?

I'm chartering a lagoon 380 and have a dumb question about charging the house batteries. I know I will need to run the engine(s) a couple of hours a day to keep them charged, but will I get twice the charging if I run both engines? Lets assume an rpm of say, 2,000. I guess the other way of asking this is, is there a maximum rate of charging with one alternmator that can't be increased by a 2nd alternator?
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Old 02-10-2011, 19:16   #2
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Re: Two diesels, double the charging?

Generally, only one engine is setup to charge the house bank. The other engine charges a separate start battery.
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Old 02-10-2011, 21:45   #3
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Re: Two diesels, double the charging?

It depends on how your DC system is set up.

I run a twin engine boat for a living. If the batteries are depleted pretty far then running both engines can charge them quicker. If the batteries are not fully depleted then running both engines only splits the load between the two alternators.

The way I wired my alternators is so they share the house load and the start battery is connected to the house load with an isolator so that the house load cannot draw current from the start battery.

Many other people prefer to so as DotDun describes.
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Old 03-10-2011, 05:25   #4
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Re: Two diesels, double the charging?

Do you have a meter you cann watch? Mine will charge on one, but thew second motor kicks in a wee bit more juice as David describes, they are splitting the load......i2f
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Old 03-10-2011, 05:30   #5
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Re: Two diesels, double the charging?

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Originally Posted by DotDun View Post
Generally, only one engine is setup to charge the house bank. The other engine charges a separate start battery.
If I had a boat set up that way, that would be the first thing I would change. You have a whole redundant propulsion system -- and you'd miss the chance to have redundant charging? Seems crazy to me. Of course the regulation is somewhat complex. The Balmar Center Fielder is one way to deal with it.
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Old 03-10-2011, 05:38   #6
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Re: Two diesels, double the charging?

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If I had a boat set up that way, that would be the first thing I would change. You have a whole redundant propulsion system -- and you'd miss the chance to have redundant charging? Seems crazy to me. Of course the regulation is somewhat complex. The Balmar Center Fielder is one way to deal with it.
Not disagreeing with you, simply telling the OP of my experience on how boats are setup from the factory, hence most boats in charter.
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Old 03-10-2011, 05:50   #7
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Re: Two diesels, double the charging?

The answer to your question should be part of your charter briefing.
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Old 03-10-2011, 10:03   #8
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Re: Two diesels, double the charging?

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The answer to your question should be part of your charter briefing.
Yes, I will definitely ask that. The first two times I chartered mono-hulls the house bank was inadequate, and power usage and charging became the most important issue for me. Being the first time I will be chartering a cat, I began thinking about charging with two engines. The Lagoon has 4 house batteries, so it should be much better. I agree, it would be a dumb setup to have one engine charge the house and the other charging the start batteries. Perhaps I will post the question on the Lagoon 380 thread.
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Old 03-10-2011, 10:15   #9
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Re: Two Diesels: Double the Charging ?

I would imagine the charter boat is setup to be simple "start the engine and its done". I think all the charters I've done have had engine drive refrigeration, but I havent done a charter in quite a while now....

On my cat I had two 100 amp alternators, two battery banks and manual switiching to allow charging of either from either. I didnt really use it due to the long cabling involved, but it was there in an emergency. If I remember right, Ample power sold a regulator that controlled two sources but I didnt go that way. My refridge was engine drive on the port engine and my water maker was engine drive on the starboard engine. The one thing I will say about engine drive refrigeration is that you spend your running time directly charging the cold without the battery charging losses. The downside is you must be at the boat to run daily.
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Old 04-10-2011, 21:39   #10
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Re: Two Diesels: Double the Charging ?

I have two diesels charging both the start and house batteries. When started, each alternator tops up the start battery, then switches over to the house bank. The regulators look at what charge is needed to top up the house bank and adjust the output to suit, be it one alternator (engine) or two.
The only time you could get double the capacity out of the alternators is when the house bank is severly depleted.
Rather than two engines running to each provide half the charge load, run one engine to provide 100% of the charge load, it is more economical on diesel useage.

Peter
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Old 05-10-2011, 10:16   #11
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Re: Two Diesels: Double the Charging ?

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I have two diesels charging both the start and house batteries. When started, each alternator tops up the start battery, then switches over to the house bank. The regulators look at what charge is needed to top up the house bank and adjust the output to suit, be it one alternator (engine) or two.
The only time you could get double the capacity out of the alternators is when the house bank is severly depleted.
Rather than two engines running to each provide half the charge load, run one engine to provide 100% of the charge load, it is more economical on diesel useage.

Peter
What keeps one regulator from reading the output voltage of the other alternator and then backing off to float?
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Old 05-10-2011, 11:29   #12
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Re: Two Diesels: Double the Charging ?

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What keeps one regulator from reading the output voltage of the other alternator and then backing off to float?
A regulator reads battery voltage. Battery voltage goes up as the batteries become charged. Theoretically, 2 identical regulators on 2 identical alternators would equally share the load charging the batteries.

Like you postulate, if one regulator is aiming for 14.4v while the other is going for 14.2v, the second one would back down it's output to nil as the battery voltage got to 14.2v.
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Old 05-10-2011, 11:42   #13
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Re: Two Diesels: Double the Charging ?

Yeah, it's always been confusing to me, but if you put a voltmeter on the battery posts while it is being charged, wont it read 14+ volts? How does a regulator know this isnt the battery voltage....?
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Old 05-10-2011, 12:09   #14
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Re: Two Diesels: Double the Charging ?

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Yeah, it's always been confusing to me, but if you put a voltmeter on the battery posts while it is being charged, wont it read 14+ volts? How does a regulator know this isnt the battery voltage....?
An alternator may put out 14.4v, but you won't read that until the battery is (almost) charged.

Watch the voltmeter on your 12v panel during the charging process, that's the same voltage the regulator is seeing.
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Old 05-10-2011, 15:42   #15
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Re: Two Diesels: Double the Charging ?

The issue with two regulators fighting each other is the reason I only use one engine to top the batteries up, unless motoring when both may be running.
It is far better to have one engine and alternator working at optimum levels rather than two working at 50% capacity.

Peter
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