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Old 31-08-2015, 12:48   #1
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DC genset to power boat.

On another thread here there was a link to a supplier of DC gensets. His pitch was the DC was smaller, lighter, and that one could run the entire boat, including ac for 10 hrs off of a suitably sized inverter. He further stated that a one hr charge would bring the batteries back up.

Of course this wouldn't work with flooded batteries because they have limits on charge rates. AGM batteries, if I understand correctly will take a huge charge rate until fully charged.

I have 2 AGM4D and 2 AGM group 27 batteries all new.

I can handle basic wiring, trouble shooting and use a multimeter but this is beyond me.

Do any of you use this setup? Pros and cons? Seems to good to be true as the saying goes

Thanks
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Old 31-08-2015, 13:09   #2
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Re: DC genset to power boat.

It is untrue that AGMs will accept a "huge charge rate until fully charged". AGMs are lead acid and they will take a large charging current up to about 75% full. After that they slow down their charge rate and it take some hours to get to 100% full. For this reason many cruiser stop charging at 80% full. Under this regime AGMs do not last long and thus have gotten complaints from cruisers. Some AGMs can be "equalized" which may increase their life. But equalization also takes several hours and must be done periodically.

The only batteries that accept near full charge rate to 100% are LiFePo batteries. There is a long thread about them here on CF.

It is not realistic to think you can use a battery bank for 10 hours and then recharge it in one hour no matter what battery type you are considering. I think you need to do a lot of research on this before you leap into the DC gen-set concept. Ask a lot of questions, such as how many other cruisers are doing what is proposed? And, how do you plan to use energy on your boat. For example, if you plan to run air conditioning in hot humid climates then you can forget the DC gen-set IMO.
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Old 31-08-2015, 13:20   #3
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Re: DC genset to power boat.

Thanks for the quick reply. You can be sure before I invest in a system that costs as much as a genset, I will do lots of research.

One of the reasons for posting was to see if anyone's tried it. Now if we could just mount a tiny nuke like NASA uses......sigh

Rich


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Old 31-08-2015, 13:26   #4
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Re: DC genset to power boat.

I've considered it rather seriously, I think the "graduate level" of this energy management system of the future will be a rather large, possibly Life-Po bank and a DC generator, using of course an inverter for AC power when necessary.
I can't see any upside for an AC generator, although it is what I just got through installing, I'd ideally rather have a DC generator, one that the RPM would vary depending on demand.
Yes, I know that can be done with AC generators, I have two Honda's, but it's not done on larger units, at least not that I know of.

On edit, someone here has done it, made their own generator I think.
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Old 31-08-2015, 13:46   #5
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Re: DC genset to power boat.

A fellow at the dock is working on a Li battery system for ambulances. I think they are primarily solar powered. I'll have to talk with him.


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Old 31-08-2015, 14:06   #6
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Re: DC genset to power boat.

I am switching over to a DC genset and inverter from a 2.2kW alternator however I very seldom go into marinas and do not run aircon.

The alternator is getting towards the end of it's life, I like the options of being able to access AC for short periods without having to run the alternator and being able to boost house batteries during extended periods of overcast days and also having a high capacity boost charger should I need it on the cranking batteries.

I am also fitting an air compressor to the DC package which is very handy to have on a steel boat.
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Old 31-08-2015, 14:46   #7
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Re: DC genset to power boat.

There's a theoretical answer to this question, and a practical answer. They are different.

In general, there is less difference than you might think, because modern charger/inverters efficiently convert back and forth between AC and DC power. I actually have both AC and DC generation on board (the DC generator is a 2.5kW school bus alternator on the main engine) and they equally well support life on board.

In theory, DC generators offer the advantage of being able to vary speed with load, which makes them handle partial loads better. The difference in fuel economy is not enormous, but the results for health of the engine is significant.

This is a fairly big plus for lead-acid batteries, but not for LiFePo batteries, which don't need a long absorption phase of charging. So if you eventually plan to go over to lithium, keep that in mind.


Now practice gives a different answer altogether from theory. Here you don't care about the last % of fuel economy or efficiency; here the supreme three values are 1. reliability, 2. reliability, and 3 -- reliability, with durability, quiet and smoothness coming in somewhere much further down, but still way ahead of efficiency and fuel economy.

To fulfill those values, you want a very simple, low stress, and very highly developed generator which is made for continuous duty, that is for prime power, rather than something made for standby power. You want a low RPM, three cylinder diesel AC generator by Northern Lights, Onan, or Kohler, which is designed to run and run and run without giving you any trouble, for days and weeks on end. NOT a science experiment or beta test rig, or something overcomplicated or overstressed. You will gladly put up with the greater bulk, weight, and cost, to have a generator which just runs and runs and doesn't break down or require fiddling, and which you can hardly feel while it's running. Believe me! So I guess my advice is -- first of all, define your priorities and values. And then -- choose a generator which suits those values and priorities.
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Old 31-08-2015, 15:43   #8
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DC genset to power boat.

My choices are somewhat limited. The only AC gen that will fit down the hatch is a NexGen 3.5 and even then it will have to go down in pieces. The hatch is only 15 x 16 in. Once past that there is plenty of room.

The DC gen is a one cylinder Kubota and the whole pkg weighs 146 lb. it will be snug but it should fit through the hatch. I read a couple of testimonials but I take them with a block of salt.

Besides reliability I'm looking at fuel consumption and sound level. I would like to see a couple of real installs and talk to the owners.

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Old 31-08-2015, 16:09   #9
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Re: DC genset to power boat.

Are you sure the Nexgen won't fit?
I put one through a hatch that was I think 14"? Anyway it was .5" less than the Nexgen with sound shield, I removed the electric relays and I think the starter, 5 min. work, anyway it's sort of like putting a couch in the house, you can tilt the generator and get it to go through a smaller hole than what the dimensions say you can, the coolant cap was what stuck up the most on mine, could have reduced the size by at least another .25" by removing it and several more inches if I took off the heat exchanger, but didn't even have to remove the cap.
If you do go the Nexgen route, double isolate it, they will sell you four more of the isolators they use for not much money, and isolators are "tuned" for a set weight and frequency. They provide almost everything except the thru hulls and exhaust hose, throw away the bilge blower hose they send, it's garbage. The fan has to be run so the hose is under a vacuum, it sucks flat and the airflow goes to almost nothing and the Gen overheats. I used 3" aircraft Scat hose, works great.
I chose an AC generator because I wanted a high output watermaker, and they are pretty much AC only


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Old 31-08-2015, 16:16   #10
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Re: DC genset to power boat.

I really think you need to take a hard listen to the gensets. Unless you can come up with really effective sound insulation, I think you might prefer to be hot.
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Old 31-08-2015, 17:40   #11
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Re: DC genset to power boat.

Quote:
Originally Posted by CarlF View Post
I really think you need to take a hard listen to the gensets. Unless you can come up with really effective sound insulation, I think you might prefer to be hot.

You may be surprised, in the Salon and forward in my boat, you can't hear or more importantly feel the generator if the AC is on.
It is loud in the aft cabin though as it shares a whole wall with the lazarette, where the generator is in.
I say feel as I believe a whole lot of noise is transmitted through structure, and if that's the case you can sound proof and it not do much good, that is why I have the sound shield, but still double isolated it, that is it is mounted on isolators in the sound shield and I mounted the sound shield on another set of isolators, to absorb vibrations.
But the purpose of my generator is not to run air conditioning or at least that is not why I installed it, but I can tell you if the Admiral complains about the heat and bugs, it's an option. I hope to run it for maybe 4 to 8 hours a week, to make water, wash clothes and get my batteries through the bulk phase so Solar can top off the charge. They will be AGM and need to be topped off frequently and to do so I believe I need to be through the bulk phase early in the morning.
This could be done with a DC generator and a large inverter, in fact could be done with a big alternator. I have three sources of power, a 150 amp alt, and currently 750W of Solar and the capacity of going to 1250W of Solar, and the Nexgen 3.5 KW generator. I hope to run the gen only when I need water, but if I'm running it anyway, why not use it's excess power to do other chores like clothes washing?
It might be that I can get by with all Solar, but I don't think so, I'd rather run the generator than I had the propulsion engine.
Now maybe if I go with a DC watermaker, maybe I could?


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Old 31-08-2015, 18:05   #12
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Re: DC genset to power boat.

Quote:
Originally Posted by a64pilot View Post
You may be surprised, in the Salon and forward in my boat, you can't hear or more importantly feel the generator if the AC is on.
Indeed. There is very wide variation between the noise of generators.

Mine is completely inaudible from outside the boat or from the cockpit, and you can barely tell it's running in the salon or in the aft cabin. It runs at 1500 RPM and lives in a sound enclosure which in turn is inside a soundproofed engine room. The sound is no louder or more intrusive than a dishwasher on land.

Two, and even more, one cylinder gensets, especially if they run at 3000 or 3600 RPM, are much louder and cause much more vibration.
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Old 01-09-2015, 13:55   #13
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Re: DC genset to power boat.

One important variation in having a DC genset is that it can run at its optimum RPM and power output for most of the time that it's in operation. That's because, unlike an AC genset, it sees a full load as it charges the batteries. Where an AC generator is seldom run at full capacity, and has to deal with small nuisance loads on occasion, having a large bank of batteries in the circuit allows the DC generator to run flat-out, until the acceptance begins to taper on the battery bank (presuming no inverter loads). Running a diesel at it's intended power output will maximize engine life and fuel economy.

My problem with the solution is that most choices are small single cylinder engines that may not be as smooth has two- or three-cylinder engines on larger AC gensets. So I agree with the person who suggested you listen to it, under expected load.

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Old 01-09-2015, 16:07   #14
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Re: DC genset to power boat.

Am I missing something? I thought every one uses alternators to produce DC voltage from AC.


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Old 01-09-2015, 16:16   #15
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Re: DC genset to power boat.

My new/old boat came with a Balmar DC-750 DC alternator genset. The alternator is 245amps and it is based on a water-cooled 2 cylinder Yanmar diesel. I have only run it to do maintenance so far so don't have any real experience with it. The boat came with 3 8D Lifeline AGMs so the big alternator should make a big difference. It is moderately loud and shares the same engine room as the main engine - which is noisier by far.

Regarding AGM charging I believe that they will accept a higher rate of charge much longer than a wet-cell battery so that they will get closer to a 100% charge much quicker than the same size wet cells. Still need long run times to get to absolute 100% but not as long as for wet cells.
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