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Old 11-19-2015, 06:47 AM   #1
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Big dc alternator

For folks with massive battery banks that can require 200- 300+ amps of DC to have a reasonable charge time, getting an alternator can be expensive.

For those that prefer to "Roll their Own" and want to create a noisemaker that will operate at slow speeds for minor loads (inverter required) a big DC alt is also required.

This is a video of belting a coach direct drive unit.

These come in 12 & 24V up to 300AH output and rare available rebuilt , or just used , operating , at modest cost.

http://youtu.be/WvuT2ZSO2QI
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Old 11-19-2015, 09:11 AM   #2
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Can't make out much from that vid, but it is clearly a Detroit Diesel. One neat feature of those is that the alternators can indeed be direct/gear driven. The big bus alternators are even more unique in that they had step-up gears internally so it really made some serious rpms and thus power, even at idle. Should be a neat feature for a trawler, but never seen one installed on a boat. Not sure if that alt is available with belt drive??? If so, could be an option for non Detroit boats....
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Old 11-19-2015, 10:43 AM   #3
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With out getting out a calculator or looking it up...
300 amps at 12 volts is around 4.8 to 5hp to keep it spinning.
I'm not sure if you could make it work with a 6 rib serpentine belt.
I'm thinking a high performance notched drive belt would be the way to go.
I have more thoughts on just the HP and starting side under load, but think it might be better to keep my moth shut.

To me the question would be what the proper charge rate for the batteries your charging OR just say “charge it” and pump in as many amps as possible and fry your battery bank.

I have a large house battery bank and I use a 100 amp 24 volt permanent magnet alternator.
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Old 11-19-2015, 12:11 PM   #4
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Lots of us have large alternators on our main propulsion engines for charging a house bank. All it takes is a external multistage regulator.

What is interesting is what was inferred in the original post. You could use a engine to spin a dc alternator. The challenge there is that you would have to come up with a governor that would sense the need for current and increase engine speed to match.
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Old 11-19-2015, 07:42 PM   #5
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Originally Posted by Ski in NC View Post
Can't make out much from that vid, but it is clearly a Detroit Diesel. One neat feature of those is that the alternators can indeed be direct/gear driven. The big bus alternators are even more unique in that they had step-up gears internally so it really made some serious rpms and thus power, even at idle. Should be a neat feature for a trawler, but never seen one installed on a boat. Not sure if that alt is available with belt drive??? If so, could be an option for non Detroit boats....
I actually looked into mounting one of those on my boat. Would have been perfect for my 6-71. Only problem was real estate. It wanted to go right where my turbo was.
It was a great thought on paper.
Ended up with a big as@ Balmar up on the front instead. Seems to do a nice job.
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Old 11-19-2015, 08:19 PM   #6
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Elelctrodyne makes some very heavy duty units that wiill put out impressive watts at low RPMs. We installed a 350A unit in a boat with a 12 x group 31 bank that would put out about 75% at barely over idle. Ask for the output vs. rpm curve for any alternator you are considering -- they vary quite a lot.
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Old 11-20-2015, 07:32 AM   #7
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"To me the question would be what the proper charge rate for the batteries your charging OR just say “charge it” and pump in as many amps as possible and fry your battery bank."

The fiels current draw is low enough so a normal aftermarket cruising boat V regulator will do the job.

The std bus V reg is as bad as the std auto or truck V regulator and should be avoided.
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Old 11-23-2015, 05:33 PM   #8
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Lots of us have large alternators on our main propulsion engines for charging a house bank. All it takes is a external multistage regulator.

What is interesting is what was inferred in the original post. You could use a engine to spin a dc alternator. The challenge there is that you would have to come up with a governor that would sense the need for current and increase engine speed to match.
Got a 12hp Kubota (600cc) driving a 24V 120A bus alternator via two belts.
I have fixed the stop control (cable) so that it also acts as a speed control and just set it by ear so there's a reasonable load on the engine. I have an external Ample Power SAR V3 regulator with a current limit pot which I've set to 90A. More current than this and the belts shred noisily and messily.
It has been suggested, on this forum, that I could increase the size of the sheaves on both the alternator and the engine flywheel to prevent this but I can live with it as is. 90A at 28.8 V(charging Voltage) is 2.6Kw which is plenty.
Two inverters supply 230V AC for hairdryers etc...this is probably not a solution if you need aircon, but we run a small fan heater in winter OK.
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Old 11-23-2015, 06:21 PM   #9
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Got a 12hp Kubota (600cc) driving a 24V 120A bus alternator via two belts.
I have fixed the stop control (cable) so that it also acts as a speed control and just set it by ear so there's a reasonable load on the engine. I have an external Ample Power SAR V3 regulator with a current limit pot which I've set to 90A. More current than this and the belts shred noisily and messily.
It has been suggested, on this forum, that I could increase the size of the sheaves on both the alternator and the engine flywheel to prevent this but I can live with it as is. 90A at 28.8 V(charging Voltage) is 2.6Kw which is plenty.
Two inverters supply 230V AC for hairdryers etc...this is probably not a solution if you need aircon, but we run a small fan heater in winter OK.
230 volt hair dryer? This the European version or is it the Whrlpool brand for Texas Big Haired Women?
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Old 11-23-2015, 07:04 PM   #10
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The 50DN is available as a belt driven unit. It was primarily gear driven on the DetroitDiesel powered busses, but was converted to belt when turbo's were introduced on the 6V92s. It is pressure lubricated from the engine (cooling it also) and the stepup gearing is internal to the rear bellhousing on the gear driven units. A big gear on the cam and small gear on the alt. kinda like a sun gear arrangement. On most of the belt drive units the tensioner is a hydraulic cylinder actuated by engine oil pressure to give adequate belt tension. These are very robust and durable alternators, far exceeding anything one could get thru normal channells. And can be picked cheaply if you know someone in the bus bidness. I just happen to have friends in that area. If you can use one it would be a great investment.
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Old 11-24-2015, 07:23 AM   #11
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"I have fixed the stop control (cable) so that it also acts as a speed control and just set it by ear so there's a reasonable load on the engine"

Lazier folks install a speed control from a welding unit to make this automatic .

Not "marine" labeled so no stoooopid pricing.
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Old 11-24-2015, 07:50 AM   #12
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With out getting out a calculator or looking it up...
300 amps at 12 volts is around 4.8 to 5hp to keep it spinning.
I'm not sure if you could make it work with a 6 rib serpentine belt.
I'm thinking a high performance notched drive belt would be the way to go.
I have more thoughts on just the HP and starting side under load, but think it might be better to keep my moth shut.

To me the question would be what the proper charge rate for the batteries your charging OR just say “charge it” and pump in as many amps as possible and fry your battery bank.

I have a large house battery bank and I use a 100 amp 24 volt permanent magnet alternator.
Simple answer is to go with an additional 8 groove crank pulley as opposed to multiple belts. The specialty shop truck and bus shop I use for alternators etc., told me the below 8 groove serpentine belt setup would be fine for up to a 350 amp alternator. 220 amps with an external sterling regulator should meet my needs nicely with out cooking the battery bank.

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Old 11-24-2015, 08:33 AM   #13
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Simple answer is to go with an additional 8 groove crank pulley as opposed to multiple belts. The specialty shop truck and bus shop I use for alternators etc., told me the below 8 groove serpentine belt setup would be fine for up to a 350 amp alternator. 220 amps with an external sterling regulator should meet my needs nicely with out cooking the battery bank.

Attachment 46805

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Ted, are you using the alternator temp monitoring on the Sterling regulator? I have smaller frame 120/150 amp alternators. The Sterling alternator shut off turns them off when they reach 194F. When the batteries are low and getting charged hard the alternator is turned off and on as the temp rises and then cools off. Your big frame alt may not have the issue. I have dedicated blowers on my alts and it still does it.


The 8 groove belt with at least 180 degrees of contact is the ticket. If the contact is less, say due to an idler pully setup they can still slip.
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Old 11-24-2015, 09:29 AM   #14
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Ted, are you using the alternator temp monitoring on the Sterling regulator? I have smaller frame 120/150 amp alternators. The Sterling alternator shut off turns them off when they reach 194F. When the batteries are low and getting charged hard the alternator is turned off and on as the temp rises and then cools off. Your big frame alt may not have the issue. I have dedicated blowers on my alts and it still does it.
Hi Dave, yes I wired both the alternator and the battery bank temp monitoring. Am curious to see if it cycles on an off with the large frame alternator. What is your duty cycle (percentage off versus percentage charging) when charging the depleted battery bank? How long does it take for the alternator to cool down before switching back on?

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Old 11-24-2015, 09:36 AM   #15
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Ted- A larger diameter pulley on the crank will give you better alt performance and better alt cooling. If you have issues with running low engine rpm, consider that.

You'll know how well it works after splash day. Which is when??? Exciting...

You should see the crank pulley driving the alt on a Cat 3176/3196: It's like a foot in diameter!! Alt on that beast spins plenty fast at idle..
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Old 11-24-2015, 09:45 AM   #16
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Ted, it takes about 7-10 minutes for the alternator to cool off and start charging again. At the beginning of hard charging (about 200 amps total, 2 alts) it will charge about 15 minutes and then be off 7-10 minutes. Before I put the blowers on it was on for 5 min off for 10min. If I pull 300 amp-hrs out they put it back in 3-4 hrs. The Sterling guys and the alternator supplier told me to take the temp protection off. The alternator supplier says "Wenormally see between 270 and 330* at the stator core when charging at90-300A." Thats a long way from 194F.
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Old 11-24-2015, 10:05 AM   #17
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Fry- Alts should be able to handle well over 194F, but only alt supplier will know what the actual limit is. Also, you should be able to set current limit, either sensed at stator or exciter, so thermal limit will not be reached.

Is temp limit cutoff adjustable?

I'd find that on/off behavior annoying. Gots to be a better way...
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Old 11-24-2015, 10:57 AM   #18
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Ted- A larger diameter pulley on the crank will give you better alt performance and better alt cooling. If you have issues with running low engine rpm, consider that.

You'll know how well it works after splash day. Which is when??? Exciting...

You should see the crank pulley driving the alt on a Cat 3176/3196: It's like a foot in diameter!! Alt on that beast spins plenty fast at idle..
Hi Ski, thought about going with a bigger crank pulley. Have this same setup on my Cummins 6CT 300 in the charter boat. Alternator really doesn't put out until the engine reaches 900 to 1,000 rpm. With the trawler, I expect the John Deere to cruise between 1,400 and 1,800 rpm (6 or 7 knots), and WOT 2500 rpm. So I'm guessing very little output and heat production at or near idle, and plenty of rpm / cooling at cruise. Guess we will see what happens when I go from theory to reality.

Splash day should be around December 15th.

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Old 11-24-2015, 11:11 AM   #19
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Ted, it takes about 7-10 minutes for the alternator to cool off and start charging again. At the beginning of hard charging (about 200 amps total, 2 alts) it will charge about 15 minutes and then be off 7-10 minutes. Before I put the blowers on it was on for 5 min off for 10min. If I pull 300 amp-hrs out they put it back in 3-4 hrs. The Sterling guys and the alternator supplier told me to take the temp protection off. The alternator supplier says "Wenormally see between 270 and 330* at the stator core when charging at90-300A." Thats a long way from 194F.
Hmmm, was hoping you were getting a better duty cycle. Maybe you need to move the temp sensor to a different bolt, like the mounting bracket.

Actually moving the sensor to a different bolt my allow a moderately higher alternator temperature without it getting too high. Might be worth trying and verifying results with an infrared temp gun.

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Old 11-24-2015, 11:18 AM   #20
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It is annoying. The twins get out of sync when the alt load changes. I may remove the protection on one and see what happens. It may end up as a $350 experiment if the alt fails.
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