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Author Topic: Looking For Gas Powered 12V DC Generator/Charger Advice  (Read 20947 times)

Posts: 166

« Reply #15 on: November 09, 2012, 07:13:52 PM »

The problem is that almost all of the current small gas generators are 110v AC systems, with very small DC outputs in the range of 8 amps.  I cannot tell by the literature of any of them if the 8 amp charger is "automatic" or if I would need to keep an eye on it to avoid over charging, which is not what I want to do.  8 amps would take 6 hours to charge a half-depleted 100AH battery, longer than what I would like to listen to and wait for.

Here's a unit with 700A output:   WIll run $5000.00 easy

Best bet is to find a local welding shop to make up frame to hold a horizontal shaft motor and large frame 12V alternator.  The guy that made mine a decade ago found lovejoy couplings to tie the alternator shaft  directly to the motor shaft without using belts .   I don't get full output (only get 90 amps out of a 120A alternator) as the engine is governed out at  1800 RPM)

Posts: 101

« Reply #16 on: December 04, 2012, 07:34:51 PM »

I am surprised at some of the posts here frankly.   I owned a welder at one point that was nothing more than a 150 amp car alternator with an external diode pack with the field windings being fed from the output through a rheostat for current adjustment.  I stick welded and dry line TIG welded with this unit for several years.

That being said, a 5 to 8 HP gas engine with the proper belt drive ratios WILL run a car alternator without problems.
The key to this is what can be reasonably expected for output current and voltage Otherwise know as WATTAGE. 
1 hp = 746 Watts.  So with a 5 hp motor the math works out to 3730 watts.  Now before you get all giddy you need to remember that NOTHING is 100% efficient.  So we look to figure this out redneck style.  Call it 50% and you are going to be close.  That's 1700 watts or so.  Now we take the working voltage and divide by the watts to get current.  Works out to 142 amps.  Now unless you are running a 140 amp alternator, or think you need that sort of current, you drop down to a reasonable alternator size.  Say 80 amps.  So you have 80 amps of charge ability at 12 volts.  Here's where the fun starts.  You need to run a regulator on the system just line on a car.  This is easy, get an 80's GM internally regulated alternator.

Once that is addressed, the last issue is alternator speed.  The drive ratio on a car's alternator is about 4 to one.  So if the cars engine is turning 1000 RPM, then the alternator is turning 4000 RPM.  This is important, as you will need to turn the alternator fast enough to operate correctly.  A car engine's speed range runs from idle at about 700 RPM to a max of about 4500 RPM.  A 5 hp small engine run wide open with it's governor properly working runs at 3600 RPM.   you actually want to avoid running the engine at that high of a speed, so we will look at 2700 RPM as a good number to work from.  At 2700 RPM a 1 to 4 drive would net a drive speed of the alternator of near 10000 RPM.  Assuming a 1 to 4 drive on car at cruising speed of 2000 RPM the 1 to 4 drive would be spinning the alternator at 8000 RPM so that is our target speed.   The divided difference of 8000 to 2700 gives us a number of 2.9 and change.  So a 1 to 3 drive ratio will be fine.  Take the diameter of the alternator pulley and multiply by 3 and that number will give you the drive pulley diameter that you will need on the gas engine to properly drive the alternator.

Yes, it's really that easy.  The actual hard part is finding the max allowable charge rate of your battery bank and not exceeding it.  That will boil the batteries and ruin them.  Have fun and don't get your hand stuck in the pulleys. 

Posts: 4356


« Reply #17 on: December 05, 2012, 11:26:44 AM »

Yes, it's really that easy.

Until you actually try and do it.  A 5HP engine really isn't 5HP, except under very controlled conditions.  At 2700 RPM the output will be much less than "peak/surge" HP these things are advertised at.  3600RPM would get you a little closer.  At slower RPM's the governor is much slower to react to load changes and a sudden load could stall the thing altogether.  My 6HP Briggs is nearly full throttle running 3600RPM at my altitude with a measured 45A draw @ 14.2V (639W, <1HP) output .  Next you discover that the pulleys you can readily get to fit the output shaft of a gas engine use a different type of belt than the automotive pulley on the alternator.  After you figure out a way around that, now come up with a frame that keeps the alternator and engine in alignment, spins the alternator in the right direction (or replace the fan), and provide some means of tension adjustment.  When it's all said and done you've invested a ton of time, effort and possibly not a little money for a nominal 1kW DC generator.

The one I built with direct drive was "easy", on paper.  With the lovejoy coupler the mechanics were straightforward and with a battery connected to soak up any transient loads, the engine governor has no problem maintaining the nominal 3600RPM or so.  Even with a free engine and alternator, I still had to buy the hardware and a marine battery for it, plus the time and effort to make up the frame.  If one is interested in a construction project that's fine, but if it's a source of reliable power you're after, in the long run a ready-made AC unit is really a better choice.

Mark K5LXP
Albuquerque, NM

Posts: 101

« Reply #18 on: December 12, 2012, 06:31:08 PM »

Uhhh yeah... K5LXP...  Roll Eyes

It is that easy. 
Pay close attention so that you will know how you got schooled here on all counts.   I realize that there are those hams out there that get all confused trying to assemble a kit dipole with the length cut sheet, but they usually don't try commenting on technical things. 

First off, if you are intelligent enough and understand any of this, the first thing you are going to know is that if you simply fire up a loose 5 horse Briggs motor, it will run all over the place, tip over and generally not sit in a single position.  So it HAS to have a frame under it.  Engine mounts to frame, alternator mounts to frame, makes it all simpler to get it aligned.

Second, he specifically said he was going to use it to charge a battery, so I am curious how "a sudden surge current" is going to stall it out?  What's more if he tries to dump 45 amps into a 100 Ah battery, it's probably going to boil off in about 30 seconds and be ruined.  The last batteries I worked on that would take that sort of charge current were electric forklift batteries and they are a bit more than 100 Ah. 

Third, the reason for running it under max RPM is trying to get better fuel consumption, especially since it's only going to be charging a battery anyway and not being used to power anything directly.  If it were trying to direct power something, then the math is all different and static and surge draws need to be accounted for.  That was not what was ask for, and it was not provided. 

As far as pulleys, specifically, in a pinch you find a steel pulley that will fit the shaft and weld it to a crank pulley off a V-8 car engine.  It's a dirty work around, but if you don't have a metal lathe and the ability to produce a pulley, it's a solution.
If you can't weld, then the whole project is done before it started.  I suppose you could grab some metal and drill some holes and bolt everything together.  But again, it's rather difficult, and one assumes that if you are asking a question, you have some idea what is involved and the ability to do some of this basic stuff like stick welding.  I never indicated he would need to TIG weld, machine, mill or otherwise create parts that were unavailable.  So the final answer to the pulley question is rather simple, The Grainger catalog will have what you need if you want to spend the money.  If not, be creative.

Lastly, I never said that a 110/220 generator isn't a better choice, because it is.  But I failed to see where that was ask for in the original post.  More over, it's also MUCH easier to simply run a fused power cable from a car to the battery and start the car.  Again, cures many problems, gives you a way to transport the battery, and anything else.  Hell, I see you EMCOMM fools building "Go Kits" all the time and talking about what to put in them and how you need to pay attention so it's not overly heavy.  My kit is heavy, in fact it's over 2 ton.  But, it's air conditioned, I have multiple radios and antenna's (some that I carry in a bag to deploy at a site) that cover from 160 all the way through to 900 Mhz, including Smart Zone, Smart Net and LTR trunking.  A laptop and programming cables to make needed adjustments to the 8 radios.  A tent, Sleeping bag (or I can lay the seats down in the back and sleep in it if the weather requires it.  It's 4 wheel drive and with studded snow tire chains will go in snow, ice and mud.  I can patch any of the Radios together via an NCS 200 box.  So it's LOTS simpler and more effective than some heavy  pack or pelican case with a radio or two and a battery that will go flat in a few hours. 
Oh, I forgot that I can carry supplies for a week, including a camp stove, and fire ring, cooler full of water (2 cases of water).  It's all in all a better choice than a gym bag with a roll of coax and an old 2 meter radio.  But again, it's not what the guy ask about, so I didn't bother to mention it.  And I didn't bother to talk about the trailer mounted generator / welder, air compressor and additional storage area for more water and food if needs be.  But that's for another post.

Posts: 5525

« Reply #19 on: December 13, 2012, 05:39:59 AM »

"....the first thing you are going to know is that if you simply fire up a loose 5 horse Briggs motor, it will run all over the place, tip over and generally not sit in a single position.  So it HAS to have a frame under it.  Engine mounts to frame, alternator mounts to frame, makes it all simpler to get it aligned."

True, but the hard part is getting or making the frame and the mount.  It all has to be measured and checked, then put together.  You just can't bolt it together either, it has to be welded.  Try making one by just bolting the frame pieces together, and you'll get a frame that has to be constantly monitored and tightened and that will wear out in short order.  That really isn't useful--it's a genset prone to failure.  To add to the argument, precision welding isn't easy--or cheap either.  Strike one. 

Now, such a frame has to be heavy as well, and it does have to have enough weight incorporated in it to prevent possible vibration damage to it or it's components--including the battery--or the genset WILL vibrate and 'walk' all over the place.  Add to that the damage that might be caused by the movement of the genset to things around it where it's used.  The alternative is to have the unit designed and balanced so vibration is less of a factor, more than likely by adding shock absorbing features, adding to the costs.   Such a setup if not properly designed isn't really portable--or really safe--now, is it?  Strike two.

Finally, if such a setup isn't really portable, it defeats its purpose, since there ARE small, portable 110 volt gensets readily available that cost much less--AND are useable for more than just charging a battery.  Strike three.  That idea is OUT!

One last thing to consider--if you really need such a charging setup, simply put suitable connectors on your vehicle and charge the battery there.  For the short time you'll probably be using such a setup, the cost of the gas to run the vehicle won't even come close to the amount of time, effort and funds that you'll have to invest to make a good, dependable battery charging genset.


Posts: 101

« Reply #20 on: December 13, 2012, 03:12:15 PM »

I respectfully agree and disagree with the above post.  I can weld.... MIG TIG Stick and gas weld,,,, not a problem.  I can't agree that you would call it precision welding.  I would call welding aluminum elements to a thin wall boom building a 440 beam precision.  Sticking together some angle iron, not so much but it does need to be square and plumb.  Easiest way to weld something like this up would be build the base frame first, drill and mount the motor.  Install the pulleys, get a belt and then fab the alternator mount tabs and once you have everything squared up tack it good.  Fire it up and verify it's square then weld it tight. 

Portable is to a degree personal ability.  For me, a 150 lb item with proper handles is portable for short distances to the trailer.  When I was 20 years old, if I had a sholder sling to carry that weight, it would have been more portable than that.  20 years and 100 lbs later, not so much.  I would build it on a two wheeled cart that transported like a wheel barrel.  But that's just me.   I personally would bother with this at all, as I have several generators that range from 1500 watts to a 25K towable unit that will run a house.  I use it to run a big welder, and a big 3 phase compressor.  It's overkill for most things, but it was reasonable to buy, and at the time I was doing alot of welding and fabrication work and it was needed for that purpose. 

Again, on your last point, I fully agree.  Using a small car as the power source to charge a 12 volt battery bank is the easiest route.  It's totally portable, it will have the ability to transport its self, the battery bank, food, supplies, radios and antennas to any reasonable location.  If remote off road locations are desired, a pickup truck with a cap or an SUV (what I have) is true portable operation, it will go on and off road equally well and do so under its own power, and takes you to the location as well. 

Posts: 5525

« Reply #21 on: December 14, 2012, 01:27:01 PM »

Yes, if you can weld, have the materials and the patience to produce what you need and the time to do it, yes, you can save some money.  On the other hand, the poor guy who has to have such a thing made--even if he has all the measurements and requirements needed, will still have to pay through the nose for such a custom job.  If it needs a bit of tweaking, the cost is even higher.

You may disagree since you can do the job yourself, but I'll stick to my statement.  A decent 110 volt genset and a 120 to 12 volt charger unit is more versatile and costs much less than such a custom built single purpose genset.  73!
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