12vdc generators - any good ones out there?

[pbiJim]

Thank you very much to all the people who posted noteworthy options for me to consider. The good ideas & good information seem to keep coming in even greater quantities.

Thank you also to the thread drifters for the touch of comic relief that they provide.

I'm just looking to put together something small & cheap to row out to a bare bones boat every now & then. The bank on this particular boat is quite small & will power little more than a few running lights, a 400gph bilge pump & a radio. The boat has no motor & no AC system.

Since I really only need about 50 amps, that means that 12v x 50a = 600 watts. If I figure 50% efficiency from the alternator, that means that I would need about a 1.2kw motor. That's roughly 1.6hp. For the sake of keeping things small & low weight, I'm going to skimp on the motor a little & take my chances that I may only get 40 amps output if the motor slugs down a little. I think that I'm going to use a little 35cc 4-stroke that is rated for 1.3hp @7,000rpm and only weighs about 7 pounds. It's basically a glorified weed whacker motor from Honda.

Since the power output of the motor will be less than twice the potential power output of the alternator, I'll be able to test the assumption that alternators run at only 50% efficiency by monitoring the actual current output that I get. That 50% number has been thrown around be a few people, but I haven't found any documented references. I'll be interested to get some empirical data on the subject.

The Balmer literature seemed to indicate that with most of their alternators, you got the lion's share of output by the time they hit 3,000 rpm & after 4,000rpm, almost nothing was gained. I'm going to risk making the assumption that an inexpensive 60-amp Bosch alternator will respond to RPM in a similar way. Based on that information, I should be able to gear my 7,000 rpm motor down about 2:1. That means that I'm not going to be able to simply run a flexible shaft coupler as I had hoped. I will be running a belt & some shivs.

I may go with a 1/2" wide timing belt to reduce mechanical losses & reduce side loads on the drive line components. I need to look up the loading capacity on those little belts.

I also intend to build a small sound enclosure to put this noisy little bugger inside of. I'm thinking that if I lay it out correctly , I may be able to use the normal aspiration of the motor to move the necessary volume of ventilation air, without needing to add a fan or blower.

I may add a small motorcycle battery inside the enclosure, just to ensure that the alternator will never get accidentally unloaded. I don't want to have to worry about blowing out the diodes or the regulator. I'm pretty sure that I can find an inexpensive motorcycle battery that weighs less than 5 pounds if I look hard enough.

I'm going to try to keep the total weight below 30 pounds & the total cost in the south half of the 3 digit range. That's my goal anyway. I like to give myself a bit of a challenge.

The funny thing is, if I wanted to build one of these to put out 100-150 amps, it would not really cost a whole lot more. 5hp engines are less expensive than 1 hp engines. The only reason that I'm not going for the better bang for the buck is because I want small & low weight this time around. If this was for a bigger boat with more gadgets aboard, I think that I would certainly go with everything that I could ring out of a 5hp.

[Cheechako]

Originally Posted by skipmac

Bet they weren't using a giant stack of Marshalls for the PA. I'm thinking that would choke a 2000i.

Of course your example is completely invalid anyway. If you play in a Blues Band how much hearing do you have left? Could you even hear the big gen set from the first year?

This is an excellent point..... It is amazing hoe much stuff that little unit ran though!

[tom1263]

Originally Posted by pbiJim

Since I really only need about 50 amps, that means that 12v x 50a = 600 watts. If I figure 50% efficiency from the alternator, that means that I would need about a 1.2kw motor. That's roughly 1.6hp.

Sounds reasonable, but beware of the rpm stabilization of the engine, the alternator needs more power at the beginning ...

[noelex 77]

Originally Posted by pbiJim

I'm just looking to put together something small & cheap to row out to a bare bones boat every now & then. The bank on this particular boat is quite small & will power little more than a few running lights, a 400gph bilge pump & a radio. The boat has no motor & no AC system.
.

For this sort of application solar would be much better.
With the modest electrical needs and the opportunity to replace some of the power when you were away from the boat the solar panel could be quite small.

[Dockhead]

Originally Posted by tom1263

Sounds reasonable, but beware of the rpm stabilization of the engine, the alternator needs more power at the beginning ...

Indeed. In any case, don't size the engine so it can barely produce, while screaming its lungs out at redline, the demand of the alternator! That will not make a happy or long-lived generator, if it even works at all. Demand of the alternator should be fulfilled at a sustainable RPM and output, a lot less than max. So the engine should be rated at 3x or better 4x the output of the alternator.

If you want to do it scientifically: design the generator so it runs at that RPM at which the engine has its torque peak, not horsepower peak. A 4kw engine will produce probably 2 or 2.5kw at its torque peak - that's your realistically usable, sustainable engine output, which will run a 1 to 1.25kw, roughly, alternator.

Or better yet, as Noelex suggested, solar! Now that we know exactly how you're using your boat, I agree with him. Solar will be great for your application.

So in summary, in my opinion, your best options are, in order of desirability:

1. Properly engineered home-brew DC generator, good.

2. Honda or Yamaha 1kW suitcase with battery charger, better

3. Solar, best

[pbiJim]

Originally Posted by noelex 77

For this sort of application solar would be much better.
With the modest electrical needs and the opportunity to replace some of the power when you were away from the boat the solar panel could be quite small.

This is a very valid suggestion for the given parameters. The only reason why I am shying away from solar at this time is because I don't want to leave anything more than necessary on the boat, as it will sit unattended for long periods of time. Theft of equipment is a consideration. I want something small & lite that will charge things back up in a few hours when I am there & then come home with me when I leave the boat.

[pbiJim]

Originally Posted by Dockhead

Indeed. In any case, don't size the engine so it can barely produce, while screaming its lungs out at redline, the demand of the alternator! That will not make a happy or long-lived generator, if it even works at all.

This is sage advice for someone who needs to build something that he will need to rely upon as soon as it is built. I have a little more leeway in my situation & I'd like to take advantage of that leeway to experiment a little & determine how much I can get away with. I want to design this for low weight & low cost, then determine the reliability of the system experimentally. If it isn't up to snuff, I'll go back & make changes as needed after destructive testing proves out the weaknesses.

I appreciate your concern about not beating the engine to death. With me designing the system with the motor as the limiting factor, I realize that I am planning to do that. My other option is to design the system with the alternator as the limiting factor. In that case I would be beating on the alternator rather than the motor. in many ways, beating on the alternator rather than the motor would make more sense. The alternator is cheaper & easier to replace. In my case, I am going for small & lite. Using the smaller motor moves me further in that direction. Also, I am interested in learning if alternators really are only 50% efficient, as several people have now told me. The small motor will certainly help to test that parameter.

This thing is going to be a prototype. To me, a prototype is a research & learning tool. If the first one happens to work well, that is a bonus. I want to figure out how to make the smallest, most efficient little 12vdc amp producer I can. I plan to push my luck a little in the beginning for academic purposes. I'm not afraid to break a little new ground.

[goboatingnow]

Originally Posted by pbiJim

This thing is going to be a prototype. To me, a prototype is a research & learning tool. If the first one happens to work well, that is a bonus. I want to figure out how to make the smallest, most efficient little 12vdc amp producer I can. I plan to push my luck a little in the beginning for academic purposes. I'm not afraid to break a little new ground.

Keep us in the loop as you build. I didi a design excercise for a client a while back , was always tempted to built it.

(Kubota Mini series/Iskra water cooled alternator, direct coupled, electronic control with variable speed running)

Dave

[tom1263]

Auto-Parts don't need a higher efficiency, because there is always an energy surplus and a reserve in that System. Your system will work, albeit with an manual operation.

Here is a smal kit of gererator to DC from Torcman. The continuous charging current to a 12V battery is in the star-operation (quiet) 15 - 25A in delta mode 30 - 35A, Briefly.

[goboatingnow]

So in summary, in my opinion, your best options are, in order of desirability:

1. Properly engineered home-brew DC generator, good.

2. Honda or Yamaha 1kW suitcase with battery charger, better

3. Solar, best

The big advantage of a high power DC generator, is to convert to all DC operation and generate all needed AC via inverters. This is something that solar cant do ( or even the honda can do).

if you are going to build one , I suggest going for a very high power alternator ( 200 A) you can always throttle it back. ( or one with dual 140A for example).

If I was doing my design again, Id forget about the single water cooled alternator and go for two 140 bus alternators but with the regulators AND the diodes external . Id consider fabricating a small seawater jacket in stainless for the alternators as heat build up is a big issue especially if in a sound enclosure. Rig a big multi belt on the crank and drive the two alts. ( better side loading etc) . Cool the regulators with forced air and design a bit of monitoring electronics to monitor it all.

Optional variable speed is a bonus and saves quite a bit of fuel and cuts noise.

dave

[Dockhead]

Originally Posted by goboatingnow

The big advantage of a high power DC generator, is to convert to all DC operation and generate all needed AC via inverters. This is something that solar cant do ( or even the honda can do).

if you are going to build one , I suggest going for a very high power alternator ( 200 A) you can always throttle it back. ( or one with dual 140A for example).

If I was doing my design again, Id forget about the single water cooled alternator and go for two 140 bus alternators but with the regulators AND the diodes external . Id consider fabricating a small seawater jacket in stainless for the alternators as heat build up is a big issue especially if in a sound enclosure. Rig a big multi belt on the crank and drive the two alts. ( better side loading etc) . Cool the regulators with forced air and design a bit of monitoring electronics to monitor it all.

Optional variable speed is a bonus and saves quite a bit of fuel and cuts noise.

dave

I think you missed his design criteria - light, low cost. He doesn't need big power.

[goboatingnow]

Originally Posted by Dockhead

I think you missed his design criteria - light, low cost. He doesn't need big power.

oh , right, just buy the small Honda, or even a "one hung low" variant, I see he only wants to use it now and again.

Ive seen small ones in Lidl or Aldi for about 300 quid.!

dave

[Dockhead]

Originally Posted by tom1263

Auto-Parts don't need a higher efficiency, because there is always an energy surplus and a reserve in that System. Your system will work, albeit with an manual operation.

Here is a smal kit of gererator to DC from Torcman. The continuous charging current to a 12V battery is in the star-operation (quiet) 15 - 25A in delta mode 30 - 35A, Briefly.

A kit! Very good. Looks like a fantastically engineered unit, by people who do electric motors for aircraft.

Using a kit, which benefits from someone's real engineering work, is greatly preferably to cobbling it up yourself.

Need to know German to navigate their site, however (which I do).

Note that they produce peak 20 amps at nominal 12v DC, probably about 250 watts, out of a one horsepower engine. One datum for sizing the motor.

[tom1263]

Originally Posted by Dockhead

Need to know German to navigate their site, however (which I do).

you can use google translate

[Dockhead]

Originally Posted by tom1263

you can use google translate

Good; use it. There is a mass of interesting information, relevant to this discussion, on there.