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Old 04-08-2015, 03:06 PM   #1
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200 Amp alternator. 100 Amp panel.

We're taking care of a number of deferred maintenance items on our new to us Selene. As is the case with many projects, one thing leads to another. One of the things we are doing is replacing an aftermarket alternator installed by the PO.

Can anyone explain why you would have installed a 200 Amp alternator when the electric panel is rated for 100 amps?
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Old 04-08-2015, 03:11 PM   #2
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The size of the alternator may have been based on battery bank charging rather than the panel board rating. I would have thought that a 53 foot Selene would have a DC panel larger than 100 amps. My 40 footer has more than 100 amps. More like maybe 200 amps.
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Old 04-08-2015, 03:50 PM   #3
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Is the alternator 200 amps (DC) charging your batteries and the 100 amp panel is AC for shore power? More info please.
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Old 04-08-2015, 04:03 PM   #4
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What you describe is totally normal. Your main DC panel breaker is rated to supply most everything you need powered simultaneously. But if you add up the rating of all of the individual breakers it almost certainly will exceed 100 amps. This is the same situation as your home's AC breaker.

And don't forget, the big DC power hogs, the windlass and bow thruster are not wired through the main DC panel. They have their own big breaker (or fuse) fed directly from the batteries.

The alternator is sized to charge batteries quickly and has nothing to do with the main DC panel breaker. The alternator is wired directly to the house batteries, not through the main DC breaker.

Having said that it will take a big battery bank to absorb all of those amps. Most flooded cell manufacturers, Trojan particularly, recommend charging at no more than 25% of the battery bank's amp hour rating. Some battery manufacturers allow more than that for AGMs. So you would need an 800 amp hour battery bank to safely use all of that alternator capacity.

If you are replacing that alternator, then you need to think about how you use the boat, the battery capacity and whether you need such a big alternator. Some do, many don't.

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Old 04-08-2015, 04:05 PM   #5
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You are confusing the power output of the alternator with the max power draw of your electrical panel. Your alternator can put out 200 amps to charge the battery (if required). Your electrical panel is designed to accept a total load of 100 amps. That means the total draw of everything that is attached to that panel shouldn't exceed 100 amps.

100 amps DC is a pretty high load. The largest Blue Seas DC panel is rated at 100 amps.
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Old 04-08-2015, 04:07 PM   #6
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Quote:
Originally Posted by djmarchand View Post
What you describe is totally normal. Your main DC panel breaker is rated to supply most everything you need powered simultaneously. But if you add up the rating of all of the individual breakers it almost certainly will exceed 100 amps. This is the same situation as your home's AC breaker.

The alternator is sized to charge batteries quickly and has nothing to do with the main DC panel breaker. The alternator is wired directly to the house batteries, not through the main DC breaker.

Having said that it will take a big battery bank to absorb all of those amps. Most flooded cell manufacturers, Trojan particularly, recommend charging at no more than 25% of the battery bank's amp hour rating. Some battery manufacturers allow more than that for AGMs. So you would need an 800 amp hour battery bank to safely use all of that alternator capacity.

If you are replacing that alternator, then you need to think about how you use the boat, the battery capacity and whether you need such a big alternator. Some do, many don't.

David
Better explanation than mine.
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Old 04-08-2015, 06:47 PM   #7
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All of the above. Plus, just like your engine (s), if you need 100 amps, it's better to pull it out of a 200 amp alternator than a 100 amp alternator. Alternators generate a lot of heat when reaching full capacity. Many can only do full capacity for short periods of time before the amp output drops off. The new house bank alternator that I installed is 220 amps and cost $500 +/-. A 400 amp alternator was around $1,200. As much as I wanted it, I just couldn't see the need for that much reserve capacity. I certainly wouldn't downsize your unit.

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Old 04-09-2015, 11:17 AM   #8
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Right so the onboard charger is 100 amps.

With everything on we use 30 amps DC while running. If we need anything more, say the washing machine, or AC we have to run the generator for it anyway.

the onboard battery banks are:
6 4D AGM 630 Amp/hrs House Bank
1 8D Flooded 185 Am/hrs Main Engine
1 4D Flooded 135 Amp/hrs Gen & Aux Engine
2 4D AGM 210 Amp/hrs Bow Thrust
2 4D AGM 210 Amp/hrs Stern thrust

The 200 amp alternator has been an ongoing maintenance issue per previous owners records, not to mention its mounting bracket is bent.

The way we see it:
docked, then we have shore power and the alternator size is a nonissue obviously.
anchored and need to recharge the batteries, we're of course using the generator, not the main engine, btw the generator has a 100 amp alternator.
running the main engine will likely run 8-10 hours, at min run 4 hrs.

I don't see we need the 200 amp alternator. I think replacing it with a 100 amp one will work just fine.
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Old 04-09-2015, 11:43 AM   #9
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When i am running my refrigerator, navigation lights, deck lights, navigation, cabin lights and fans and alarms could easily add up to more than 30 amps. I have 100 amp alternators on each of my two engines.
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Old 04-09-2015, 01:25 PM   #10
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Sounds like a problem to me, what do you want for the alternator?;-)
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Old 04-09-2015, 01:37 PM   #11
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Your local big truck or DD dealer will have a fine HD alt of about 130-135A (frequently a Leece Neville mfg) with remote V regulator for $125 -$150 or so..

These work hard in large trucks for long periods of time.

The remote V reg allows the marine unit of your choice to be installed easily.

If you want to keep a large alt I suggest the

http://delcoremy.com/Alternator-Mode...lternator.aspx

that can be driven by your engine.
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Old 04-09-2015, 05:28 PM   #12
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Quote:
Originally Posted by No Mast View Post
Right so the onboard charger is 100 amps.

With everything on we use 30 amps DC while running. If we need anything more, say the washing machine, or AC we have to run the generator for it anyway.

the onboard battery banks are:
6 4D AGM 630 Amp/hrs House Bank
1 8D Flooded 185 Am/hrs Main Engine
1 4D Flooded 135 Amp/hrs Gen & Aux Engine
2 4D AGM 210 Amp/hrs Bow Thrust
2 4D AGM 210 Amp/hrs Stern thrust

The 200 amp alternator has been an ongoing maintenance issue per previous owners records, not to mention its mounting bracket is bent.

The way we see it:
docked, then we have shore power and the alternator size is a nonissue obviously.
anchored and need to recharge the batteries, we're of course using the generator, not the main engine, btw the generator has a 100 amp alternator.
running the main engine will likely run 8-10 hours, at min run 4 hrs.

I don't see we need the 200 amp alternator. I think replacing it with a 100 amp one will work just fine.
Sounds about right, Harry. External regulation can provide a float charge if it's needed for very long trips.
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Old 04-09-2015, 06:58 PM   #13
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No inverter? Part of the reason I have a large alternator is to run the inverter continuously while under way. If I or my guests are pulling 10 amps 110 ac that equals about 100 amps from the inverter. I try to avoid under loading my generator by running it for nuisance loads. Some of things that fall in this category would be: running the microwave for a snack, cooking dinner all day in a crock pot, charging or running laptops tablets etc, watching tv / movie, and any other personal appliances that require 110 ac.

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Old 04-09-2015, 07:52 PM   #14
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Quote:
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Right so the onboard charger is 100 amps.

With everything on we use 30 amps DC while running.
Does that include your blowers? And bait tank pumps, if you have/use them? Inverter for refrig/freezer, TV, sounds system?
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Old 04-10-2015, 10:49 AM   #15
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Does that include your blowers? And bait tank pumps, if you have/use them? Inverter for refrig/freezer, TV, sounds system?
Yes, it includes the blowers. We don't have live wells, so no pumps. You are right that it does not account for the fridge/freezer. Perhaps I've failed to account for inverter loads adequately. Let me dig into that a bit.
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Old 04-10-2015, 11:17 AM   #16
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How many watts is your inverter rated for? Does your fridge/freezer run on both ac and dc or just ac?
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Old 04-10-2015, 12:50 PM   #17
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The inverter is 3000 watts (maybe larger). but best I can see the only thing it'll run is all the ac outlets and the one fridge in the cockpit (which we usually don't run anyway). The galley fridge/freezer is ac/dc, so that'll switch over to DC when running, and its accounted for in our 30 amps DC load.

We're looking at it like the last alternator was way more than what the boat needs. the difference we will see is just a slower charging from the main engine. We're ok with that. Fact is it'll keep up with any demand we put on it.

If the batteries are really low we're charging them off shore power or the generator. All the engine has to do is maintain and top off.
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Old 04-10-2015, 04:33 PM   #18
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Guess the question to be asked is what do you perceive to be the down side of a bigger alternator? If there was nothing wrong with your current one, would you still want to change it? One last thought, if you were cruising and your generator broke down, would the 100 amp alternator provide enough capacity for several days to get back to civilization? With the exception of the electric stove and the air conditioning, I can comfortably cruise for weeks motoring every other day and stopping every night without running the genset. I don't see a downside to a big alternator, just better options if the genset has a hiccup.

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Old 04-10-2015, 05:08 PM   #19
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I agree, I don't see a downside to a 200 amp alternator on a boat with your battery capacity. Even at half load, your inverter will draw 125 amps/hour by itself. If you are going to pull out the old alternator and put in a new one, why not put in a 200 amp alternator again.
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Old 04-10-2015, 05:16 PM   #20
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Thank you all for your input. Wha I've confirmed with everyone thru the thread is there is no downside to replacing with a new 200 amp one, and if we choose to downsize, we can go to 100 amps without any unforeseen consequenses.
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