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Old 05-09-2007, 16:46   #1
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Home Grown DC Genset

Has anyone on here ever taken an AC genset and rectified its output to produce whopping amounts of DC that could then be handled by a charge controller to charge batts?

Any of the electric/electronic experts want to share any info on how to do this?

I've noticed how much more expensive the DC gensets are compared to your run of the mill AC genset. If it's not too expensive to rectify the signal, I'm thinking I can use the genset for what I want - charging my batteries - without going the route of battery chargers and multiple conversions.

Or... possibly taking an inverter style genset and popping the inverter off and using the raw DC power? (getting a free inverter in the process?)

Any input on this idea? Has it been done? Is it mad?
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Old 05-09-2007, 17:04   #2
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Here are some "junk yard" solutions...

Convert a lawn mower into a generator!

2, 3 and 4 wire Rectifiers

Lucas used avery simple rectifier and zenier diode setup for many british cars and motorcycles. It is about as simple as it gets with COTS parts.
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Old 05-09-2007, 22:22   #3
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When you say an AC genset, what size are ya talkin?? The rectification is your draw back. The bigger the current, the more expensive the Rectifier diodes. It maybe cheaper to take the Iota's with you or buy a couple more. I think the Iota's would be cheaper than diodes and then you have the proper charge controlling. Plus you may get the odd oportunity to plug into AC power somewhere instead of running the Genset everyday.
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Old 05-09-2007, 23:19   #4
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Originally Posted by ssullivan View Post
Has anyone on here ever taken an AC genset and rectified its output to produce whopping amounts of DC that could then be handled by a charge controller to charge batts?

Any of the electric/electronic experts want to share any info on how to do this?
You've just describe the function of a battery charger very nicely. AC input converted to high current DC at a varying voltage as needed to efficiently charge the battery.

So, yes, I do that all the time with my battery charger.

So, I wonder: What benefit do you want to achieve that is not satisfied by a COTS battery charger?
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Old 06-09-2007, 05:17   #5
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A man after my own heart except that I was thinking of a permanent magnet motor as the generator source. Either DC or rectify. Drive it with a Kubota Z602, $2,600 AUD. Add a high pressure pump for desal total about 100 Kg.
Only drawback is the outback charge controller is limited to 60 A, could use 24 to 60 volts for batts and use a dc to dc converter for the gadgets.
Get some high current diodes from an old alternator.

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Old 06-09-2007, 05:22   #6
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You've just describe the function of a battery charger very nicely. AC input converted to high current DC at a varying voltage as needed to efficiently charge the battery.

So, yes, I do that all the time with my battery charger.

So, I wonder: What benefit do you want to achieve that is not satisfied by a COTS battery charger?

I suppose I was looking at dollar figures.

A good AC genset is maybe $3500.

A good DC genset is more like $7000

Battery chargers are $1000 for the capacity I am looking at and involve losses through conversion. DC gensets are more efficient and vary the engine speed according to load rather than just running at 3600 RPM to light a 100 Watt light bulb.

Just exploring an idea here that maybe I could produce a nice, efficient DC genset by rectifying the output of an inexpensive AC genset, then putting it through a charge controller.

For the record, I'm talking around 5KW on an AC genset or a 416 amp DC genset.
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Old 06-09-2007, 10:53   #7
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"A good..." Let's compare apples to apples, as Wheels mentions it may just be the difference in price is because of the difference in AMPERAGE ratings. High power rectifier diodes and whatnot are always more expensive than low power. But in general, AC gensets are more expensive than DC because they need to control the RPM of the genset to match up the 50/60Hz output frequency---if you can find a situation where "power" and amperage are equal.

I'd disagree with Mike about scrounging old alternator diodes. They are usually not designed for continuous use at their rating, and being old and used, sometimes they are also abused and ready to give you a surprise one day. Of course if you can get them cheap enough and by the gross....<G>...

Then there's the entire issue of generators versus alternators, a traditional generator is regulated by dumping power. A real alternator "throttles back" to produce less power when needed. The cost of the regulation/control system can also factor in.

You should be able to find all sorts of web sites talking about homebrewing your own genset, but it might be cheaper to simply buy a military surplus genset that produces what you need--assuming you can find a match. "416 amp DC genset." Not too likely, but then again, you won't conveniently find 600-amp rated diodes on the shelf, either. (You need to rate them higher than the continuous usage.)
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Old 06-09-2007, 14:29   #8
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5KVA at 12V??? 416A!!!! Holy crap batman. Your talking major arc welding equipment here mate. Like something you would weld the Hoover Dam with. The 5KVA I think you have in mind is more likely the 120V rating. That is still capable of 41A which is big for a single phase Genset. I have a 5KVA set @230V which still can produce 21A. and that runs everything I have on the boat all at once.
The next issue is, I have never seen a 12V 416A geny. I doubt you would ever get one. The windings would be something like a mini power station. You would have to tow a trailer with your mini power station built inside :-)
Hey there is a way to get power for free however. Take a large mains transformer like from a welder. Take off the secondary winding. Place the primary winding on the roof of the camper. Now park under High voltage lines. The magnetic feild under the lines induces a voltage in the winding and you have power straight from the grid with no physical connection. As long as you don't stay around the area too long, no one will ever know.
On the seriouse side though, It is illegal, as you are actually stealign power. But cool. We had a Farmer here in NZ do this some years back when I was a kid. He had High tension line run through his property, so he built a little shed under them and did what I just described. He ran his home on the system. But he got caught and was don for theft of electricity. It hought they guy was a geniouse and deserved the power for inginuity.
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Old 06-09-2007, 15:15   #9
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If you just want to charge batteries a much smaller system will do. I bought a inverter style generator (800 watts) for $400.00 new. It feeds my 30 amp charger (30 x 12 = 360 watts). The engine of course only goes as fast as it needs to and as the batteries approach a full charge slows down more and is as efficient as it can be.

The added bonus is a generator to run small appliances and tools including a small pressure washer to clean the boat with a minimum of fresh water
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Old 06-09-2007, 15:25   #10
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Wheels, you guys must have a specific law that makes that illegal. Heck, all the guy is doing is catching radiation that is being sent into his property. Kind of a good incentive for the cheap power companies to use bigger towers, and get their damn cables and EMI way up away from the ground level. < G >

Seems almost like putting up a solar panel--you're just catching what falls on your land, no? < G >

Back on Sean's point...I think he's seeing the reason that Glacier Bay and others are using 10kw gensets and running their battery banks at something like 120VDC in the "electric boat" propositions. Allows them to cut the amperage problems tenfold. Maybe Sean just needs to rethink the RV, and run "native" systems at 120V (AC or DC) and just run the headlights and motor on 12VDC.
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Old 06-09-2007, 15:36   #11
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5KVA at 12V??? 416A!!!! Holy crap batman. Your talking major arc welding equipment here mate. Like something you would weld the Hoover Dam with. The 5KVA I think you have in mind is more likely the 120V rating. That is still capable of 41A which is big for a single phase Genset. I have a 5KVA set @230V which still can produce 21A. and that runs everything I have on the boat all at once.
The next issue is, I have never seen a 12V 416A geny. I doubt you would ever get one. The windings would be something like a mini power station. You would have to tow a trailer with your mini power station built inside :-)
Hey there is a way to get power for free however. Take a large mains transformer like from a welder. Take off the secondary winding. Place the primary winding on the roof of the camper. Now park under High voltage lines. The magnetic feild under the lines induces a voltage in the winding and you have power straight from the grid with no physical connection. As long as you don't stay around the area too long, no one will ever know.
On the seriouse side though, It is illegal, as you are actually stealign power. But cool. We had a Farmer here in NZ do this some years back when I was a kid. He had High tension line run through his property, so he built a little shed under them and did what I just described. He ran his home on the system. But he got caught and was don for theft of electricity. It hought they guy was a geniouse and deserved the power for inginuity.

Wheels! That is a GREAT story! Using induction to get a great power assist! Wow. I like that.

I'm looking at running all DC because I am planning on taking everything off the "land boat" and putting it all in the hull of a real boat after out time on land is up.

Right now, I have a 5+ KW genset on the boat delivering of course, 41.6 amps at 120V (or 20.8 amps at 240). My thinking is to take this same type of generator and rectify the output - this would give me the whopping 416amps output. You make a good point. It is the winding inside the power head that would fail if this were attempted, right? There would be too much current for the windings themselves?

PS: I am looking at a 675 to 900 amp battery bank and I like to charge things in an hour. If the genset is a no-go, I'm putting on 2 large case alternators, hot rated at 200 amps EACH with a charge controller.

Man, building stuff is fun!!!
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Old 06-09-2007, 15:41   #12
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If you just want to charge batteries a much smaller system will do. I bought a inverter style generator (800 watts) for $400.00 new. It feeds my 30 amp charger (30 x 12 = 360 watts). The engine of course only goes as fast as it needs to and as the batteries approach a full charge slows down more and is as efficient as it can be.

The added bonus is a generator to run small appliances and tools including a small pressure washer to clean the boat with a minimum of fresh water


Those are pretty nice, Bill, but this is for my home and wife. Hence, I need to be able to microwave, run a refer, have a strong power system. I currently have 180 amps of charging capacity on my boat going into 440AH of batteries. Works like a charm! The expense of essentially doubling my capacity has me thinking of ways to get around this. Hence, this thread.

The current batteries charge up to near full in 45 mins under normal conditions or an hour if we have used a lot of power. Gotta love that. There is a great economy to be had over time if you size up your battery banks and charging system since you don't have to run the genset even an hour a day. (I live full time at anchor - and will be living full time, unplugged in this "land boat" I'm building).

BTW: The land boat is already bought. The fit out starts next week. We move off the Gulfstar in about a month. Scary stuff! (Has anyone ever mentioned that coming BACK from sailing is as scary, if not more so, than going in the first place was??)
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Old 06-09-2007, 23:21   #13
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Just exploring an idea here that maybe I could produce a nice, efficient DC genset by rectifying the output of an inexpensive AC genset, then putting it through a charge controller.
If "charge controller" here means something like what you would use with a solar panel, then the system you are describing is essentially identical to a battery charger. The solar charge controller is a DC-DC converter. It breaks up the DC input into pulsed DC (or even AC in some implementations) which it can step up or down as necessary to get the desired DC output voltage. One common implementation of a battery charger rectifies the AC input first, then uses the resulting DC to drive the DC-DC converter. The bulk of the efficiency losses are in the converter.

If "charge controller" means the device you typically use with an alternator, then this idea doesn't work at all. That type of charge controller works by directly controlling the magnetic field in the alternator. Without that feedback, the "controller" can't actually control anything.

Quote:
Right now, I have a 5+ KW genset on the boat delivering of course, 41.6 amps at 120V (or 20.8 amps at 240). My thinking is to take this same type of generator and rectify the output - this would give me the whopping 416amps output.
No, it would give you about 41 amps output at 120 volts DC. There are two good ways to get 12 VDC out of this system: 1) use a regulated power supply such as you find in a large battery charger or one of IOTA's devices, and 2) replace the 120 volt alternator with a 12 volt alternator.

Quote:
If the genset is a no-go, I'm putting on 2 large case alternators, hot rated at 200 amps EACH with a charge controller.
You mentioned $1000 for a battery charger in a previous post. How much will a pair of 200 amp alternators plus a charge controller cost?
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Old 07-09-2007, 01:18   #14
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If the genset is a no-go, I'm putting on 2 large case alternators, hot rated at 200 amps EACH with a charge controller.
We're going to have to start calling you "Tim". Oh oh oh oh
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Old 07-09-2007, 07:34   #15
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You're right. I guess there isn't a practical way to do this. Thanks for clearing that up. It was just a thought, so on to the next:

The pair of 200AMP alternators is about $2200. Not so bad, especially considering that driving to the store or work will net me free charging.

I'll go that route.

Thanks for all the input.



Quote:
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If "charge controller" here means something like what you would use with a solar panel, then the system you are describing is essentially identical to a battery charger. The solar charge controller is a DC-DC converter. It breaks up the DC input into pulsed DC (or even AC in some implementations) which it can step up or down as necessary to get the desired DC output voltage. One common implementation of a battery charger rectifies the AC input first, then uses the resulting DC to drive the DC-DC converter. The bulk of the efficiency losses are in the converter.

If "charge controller" means the device you typically use with an alternator, then this idea doesn't work at all. That type of charge controller works by directly controlling the magnetic field in the alternator. Without that feedback, the "controller" can't actually control anything.


No, it would give you about 41 amps output at 120 volts DC. There are two good ways to get 12 VDC out of this system: 1) use a regulated power supply such as you find in a large battery charger or one of IOTA's devices, and 2) replace the 120 volt alternator with a 12 volt alternator.


You mentioned $1000 for a battery charger in a previous post. How much will a pair of 200 amp alternators plus a charge controller cost?
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