Monday, March 10, 2008
Homebuilt DC Generator/ Battery Charger
This generator looks pretty rough. Here's the story: a few years ago I was living offgrid on a tract of land I own, and the Tecumseh-engined piece of junk of a generator I had used all through the previous summer had finally thrown a rod just as the weather was cooling off. This was not a huge loss, considering that it vibrated so much that I had to use bungee cords to hold the air shroud on after it broke the bolts and ripped out the mounting holes; and I had a very short piece of pipe held on the exhaust port by two bolts and a piece of baling wire, because anything with any more mass (such as a muffler) would break. The pipe did nothing to quiet the beast, but it was just enough to keep cold air from warping the exhaust valve. Needless to say, this thing was an unbalanced piece of junk, and I was almost glad when it finally bit the dust.
I made it through that winter sans electricity, using a kerosene Aladdin lamp, a propane lamp, and some standard oil lamps for light. I had a homebuilt woodstove for heat, cooking and water heating, so the lack of electricity wasn't a problem.
For entertainment I read and listened to a crank-up AM/FM/SW radio. When spring came, however, I knew it was time to start thinking seriously about power production. It still wasn't too bad, because the nights were pretty cool; but summer was on the way with daily highs in the 90s and occasionally over 100 degrees Fahrenheit, and with the humidity that keeps temperatures high even at night, air conditioning is pretty much mandatory. I didn't want to spend $500 on another junk portable genset that would last 1-2 years max, and I didn't have the money to buy a good generator, so I decided to take the DIY route. More fun that way, anyway.
Build your own generator-based home power system
So I went to The Epicenter and bought one of the steel plates detailed in their "Tip O'da Week" to mate a lawnmower engine with an automotive alternator. Then I bought a used Briggs & Stratton lawn mower engine for 20 bucks or something like that, and a cast iron pulley to fit the shaft of the engine. It had to be cast iron, not only for longevity but also because low-horsepower vertical-shaft lawn mower engines have aluminum flywheels, and rely upon the mass of the blade for a further flywheel effect. When you remove the blade, it becomes nearly impossible to start and run unless you replace that mass somehow.
Lastly, I went to Autozone and bought a rebuilt 60-amp Delco internally-regulated alternator for 20 bucks plus whatever the core charge was, maybe another $15 or so. I also picked up a v-belt to tie it all together. Then I assembled all this into the solution to my problems.
Or so I thought. Until, that is, I discovered that the tiny little tank on the B&S only held enough gas for about 30-45 minutes of runtime, and the design of the carburettor was such that you couldn't just replace it with a larger tank. I gave the matter some thought, but couldn't come up with a solution that was reliable and didn't carry a high risk of creating the necessity of skin grafts to my face, so I found a Tecumseh mower engine, because they have a carb that has a built-in bowl and a separate tank that can be replaced. I never got around to putting that one together though, because a few days later I was in the Harbor Freight store and saw that they had their Robin-Subaru horizontal-shaft engines on closeout. They later changed their minds and continued to carry the Robin-Subaru line after all, but not before I scored a 7 horsepower engine for $200.
So I took it home, mounted it on a scrap of treated 6x6 post, and made an alternator bracket out of scrap iron and bolts from my junkbox. So why does it look so rough? Think about it: no power to run my welder! When I started writing this, I thought about cutting the extraneous parts from the bracket and welding it up nicely, because it is, after all, a pretty good bracket; but I decided to take pics and video first, showing how it really was, after which I could improve it for the next project it is to be used on. Oh, yes: I'm gonna use it to break in an air-cooled diesel engine I bought, run some experiments with it, perhaps convert it to an alternator-based welder and experiment with that for awhile. Then when I have that engine well broken in, it's gonna go into the project I bought it for: a (hopefully 150+ MPG) diesel motorcycle. At that point, the generator will probably be rebuilt using my single-cylinder, water cooled Kubota diesel engine.
Back to the current iteration, though: How did it work, and what did I learn from it? Well, it worked fine, given its inherent limitations. Those limitations, however, are what I'm here to tell you about; because lots of people (myself formerly included) have a pie-eyed view of what an automotive alternator is capable of. For one thing, they are incredibly inefficient. I'll come back to that. The second thing is, rated amperage. I used to assume that a 60 amp alternator would supply 60 amperes of battery-charging current for awhile, then drop to about 50 amps for awhile once the alternator heated up, then gradually taper off as the battery bank reached full charge. Not so. In fact, while it would surge to 60 amps or a bit more upon first starting up, drop to 40 amperes within a few minutes, then 30 amps within about 30 minutes, then within the first hour it would be down to little more than 20 amps. A couple of hours later it would be producing 16-20 amps, and the one-gallon tank would be empty. This was with a depleted 880 amp/hour battery bank, so it was not getting fully charged and tapering off. So we'll call it 35 amp/hours the first hour, then maybe 20 ah times two additional hours, for a total of 75 amp/hours; less than a tenth of the battery capacity. To look at it another way, I got a total of about 1000 watt/hours out of three hours runtime, and one gallon of fuel.
To put this into perspective, my Indian Lister-pattern diesel engine driving the Chinese ST generator head, which is what I built to replace this generator, will put out 2000 watts continuously for 5 hours on one gallon of fuel (which can be filtered, preheated waste cooking oil, btw); that is 10,000 watt/hrs or 10 TIMES the efficiency.
Between those two extremes though, I did run a 140 amp alternator with the Lister for awhile: it also used a gallon in about 3 hours, but produced approximately 2000 watt/hours during that 3 hours.
From the standpoint of running an air conditioner, if you put a big gas tank on this generator and ran it 24 hours a day charging your battery bank, you would be able to turn on the air conditioner in the evening and run it, a couple of lights, and perhaps a stereo or TV for a few hours, then turn off the lights and entertainment and run the air conditioner overnight. I'm talking about a small, 5000 btu air conditioner. You could only do this if you had a sizable battery bank, and of course an inverter big enough to power the air conditioner and TV/lights/etc. Say about a 2500 watt inverter (my Trace 2500 watt inverter powers those loads just fine). You would have to turn off the A/C in the morning and leave it off all day, while leaving the generator running to recharge the battery bank. This would use over 8 gallons of gas per day. The bigger alternator would allow running only when the loads (A/C, etc.) are running, for considerable fuel savings.
The best, though, is to get a diesel engine and power a 110 or 110/220 generator head, like I eventually did. With that setup I can power that same air conditioner, a couple of lights, TV and stereo, while also using the 3-step 120 amp battery charger that is built into my inverter to charge the battery bank, and still run 4-5 hours per gallon of fuel. What a difference efficiency makes!
That doesn't mean the setup depicted here is worthless, though. For one thing, it is very portable and useful for short-term, low-demand use. For example, if you are camping or boondocking where you only need a little power intermittently, and you don't need air conditioning. Or as a portable means of jumping off vehicles. A buddy of mine has another good use for it: he has a flat-transom canoe with a trolling motor on it, and he likes to set up camp by the lake and go motoring around in his canoe. He has a second battery in camp, and when the first battery is almost depleted he brings it back to camp, connects it to his generator/charger, grabs his second battery and heads back out while the depleted battery is recharging.
Another useful thing about a generator like this is, you can get rid of the regulator and use it as a welder. I haven't tried it yet, but lots of other people have. I intend to try it eventually, and report my results here.
And one last thing. I've noticed that these alternators never seem to get very hot in use, so by losing the regulator and adding some switched power resistors to manually regulate it, including a high-power setting for welding and bulk charging, it should be possible to maintain current output at a much higher level than the automatic regulator will allow.
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