Building a 12 Volt DC Genny : LUSENET : TimeBomb 2000 (Y2000) : One Thread

(This article was published in Home Power Mag back in 1994....the article in it's entirety including color photos and schematics is available for free download in Adobe's PDF format at our website in the download section as MARK8.PDF at

Build Your Own 12 VDC Engine/Generator Richard Perez

This small, easy to build, engine/alternator is the answer to a burning RE (renewable energy system) question. What do we do when the sun doesnt shine, the wind doesnt blow, and the creek dries up? This generator is a backup power source for times when our RE sources dont meet our demands. It is optimized to do only one thing  properly recharge batteries on demand.

Engine/Generator Overview I have built a dozen versions of this power plant in the last twenty years  three for myself and others for neighbors. Over the years the design has evolved, but the purpose remains the same  on-demand battery recharging and equalization. A version of this article first appeared in Home Power #2  our most requested out-of-print back issue. Here is a revision of this information with an updated regulator design. In the early days (19821985), we used this type of engine setup as a prime mover. It supplied almost all of the energy for our system. We only had two PV modules at the time. As our PV/wind system grew, our dependence on the engine faded. Now we only use it during the depths of winter to meet those cloudy, all-night deadline sessions. From this experience we learned that while an engine is still a great energy back-up, it is a miserable prime mover for the system. These units are most effective if used less than 200 hours yearly. Using the generator as the primary power input will yield 1,000 to 2,000 hours of engine operation yearly  a nightmare of expense, maintenance, and pollution.

Source Capacity and Flexibility for Battery Equalization Every RE system should have at least one power source capable of recharging the batteries at between C/10 to C/20 rates of charge. For example, a battery pack of 700 Ampere-hours periodically needs to be recharged at a minimum of 35 Amperes (its C/20 rate). To figure the C/20 rate for your pack, simply divide its capacity in Ampere-hours by 20. The resulting number is the C/20 rate in Amperes. The C/20 rate is optimum for equalizing charges. An equalizing charge is a controlled overcharge of any already full battery. If your RE sources are not powerful enough, or flexible enough, to equalize the battery, then this engine-driven source can do the job.

Power Source Control Energy sources which recharge batteries need to be controlled. If the charging source is not controlled, then the batteries may be overcharged or recharged too rapidly. The most common method of control is voltage regulation. This works fine in cars and in batteries with shallow cycle, float service. Voltage regulation alone is not enough for deeply cycled batteries. They must also be current regulated to prevent too rapid recharging.

Voltage Regulation Voltage regulation only is OK for batteries that are very shallowly cycled. In shallow cycle service the battery refills almost immediately since it has only had a small amount of its stored energy removed. In deep cycle service the batteries have had about 80% of their energy removed before recharging. If deep cycle batteries are recharged from a source that is voltage regulated, they will be charged at the total output current of the source as it struggles to bring the batteries immediately to the set voltage limit. If the charging source has say 55 Amperes available, then it will charge the batteries at this 55 Ampere rate. If the battery is a 100 Ampere-hour battery, then the C/10 rate for this battery is 10 Amperes. The 55 Amperes from the source would recharge the 100 Ampere-hour battery at a rate over five times faster than it should be charged. This will result in premature battery failure, higher operating costs, and much lower system efficiency.

Constant Current Constant current charging means that the batteries are recharged at a fixed amperage rate until they are full. The voltage of the batteries is left unregulated until the batteries are full. The rate of charge is usually between C/10 and C/20. Constant current charging assures that the batteries are not charged too rapidly. Rates of charge greater than C/10 produce heat which can warp the thick plates of deep cycle batteries. Too rapid recharging wastes energy in heat and hydrolysis, and gradually ruins the batteries.

Engine-driven Power Plants The engine-driven source has the distinct advantages of delivering large amounts of power when you need it. This is very different from wind and solar systems, where you have to take it when you can get it. Its major disadvantage is that it requires fuel and maintenance. Engines do not usually suffer from being undersized. If the power source is capable of delivering between C/20 and C/10 rates of charge to the batteries, then the system is happy.

Lawnmower Engines and Car Alternators The idea here is to use a lawnmower engine (or any other small horizontal shaft engine) to drive an automotive alternator. The alternator puts out between 35 and 200 Amperes (depending on its size) of 12 to 16 Volt DC power to recharge the batteries. The first engine we used actually came from an old lawnmower we bought for $35. We got a 35 Ampere Delco alternator from a dead Chevy in the junkyard for $15. We bolted the entire works to a thick wood slab, and used an old oven heating element as a crude resistive field controller. The unit ran and charged our 350 Ampere-hour battery for two years before the engine died.

Type and Size of Engine Weve since tried many different combinations of engines and alternators. Small gas engines between 3 and 8 horsepower work well. We found that the Honda small engines will run more than 5,000 hours without major work, Tecumseh engines about 800 hours, and Briggs & Stratton engines about 600 hours. The particular Honda G40 (170 cc, =4.5 hp) engine pictured here has run for over 13,000 hours with the same rings, bearings and valves. The Honda also has the advantage of a 100 hour oil change interval, compared with 25 hours for both the Tecumseh and the Briggs & Stratton. If you consider the operating life and operating cost of small engines, then the higher quality engines are much less expensive despite their higher initial cost. The engines size is determined by the size of the alternator. This assures a balance between system efficiency and cost. A 35 Ampere alternator can be driven by a 3 hp engine. A 100 Ampere alternator needs at least a 5 hp engine. For alternators between 100 and 200 Amperes use the 812 hp engine. See Access at the end of this article for a source of small gasoline engines.

(photo captions)

Above: The engine powering this generator is a Honda G40 model displacing 170 cc with a rated output of 4.5 horsepower. This particular engine has outlived four alternators and now powers a 100 Ampere Chrysler alternator. I estimate that this engine has run over 13,000 hours in the last fifteen years. It still has its original valves and piston rings.

Right: Proper pulley alignment is essential for efficiency and long belt life. This particular generator would produce 60 Amperes of current for five hours while consuming about three-quarters of a gallon of gasoline.

Type and Size of Alternator Just about any automotive alternator will work in these systems. What really counts is the size of the alternator. Its current output (amperage rating) should be sized to match the capacity of the battery pack. The more capacity the battery pack has, the bigger the alternator must be. The alternator must be able to deliver at least a C/20 rate of charge to the batteries. We have had good results with 35 Ampere Delco alternators for battery packs under 700 Ampere-hours. Batteries up to 1,400 Ampere-hours are fed with the 100 Ampere Chrysler alternators. Packs larger than 1,400 Ampere-hours should have a 200 Ampere rated alternator. The higher amperage alternators are measurably more efficient than the smaller ones. The higher amperage alternators are more difficult to find. Try your local auto electric shops, they may have a source for these high amp jewels. Regular alternators up to 70 Amperes are usually available from junkyards at less than $20. Alternator rebuilders can provide rebuilt units (new bearings and brushes) from $40 to $150. These alternators are a good investment. They are designed to run under the hood of a hot car on a summer day. In the type of service we give them, they run cool and last a very long time. Ive seen these alternators last over 10 years with just the replacement of bearings and brushes. The more modern alternators contain their voltage regulators within the alternators case. These internal regulators need to be disabled and/or removed before these alternators are useful in this system. If you cant do this yourself, then take the alternator to an alternator shop for help. Some alternators have what is known as an isolated field. These need to have one field connection grounded to the alternators case and simply feed positive energy to the other field connection. The older Delco types are very simple and straightforward to use. They require no modification. Every alternator is a little different, so if youre not sure what you have, then go to the library and look it up in an automotive manual.

Getting it all together  Assembly We originally bolted both the alternator and the engine to a wooden slab about 16 inches by 24 inches and 4 inches thick. Be very careful with engine and alternator pulley alignment. If the engine pulley and the alternator pulley are not properly aligned (in the same plane), then the unit will wear belts out very rapidly. These engine/alternator combos work best on heavy metal bases. There is a lot of vibration and the wooden slabs give up after a few years. Either add a sheet of 1/4 inch to 3/8 inch thick steel between the wood and the engine/alternator, or make the base completely out of metal. A local welding shop made us a base out of 3/8 inch thick steel plate with a welded one inch by two inch steel square tubing perimeter for $50. You can see it in the photograph. If you can weld, the materials cost about $18. Use heavy bolts with lock washers to secure everything to the base. We coupled the alternator to the engine with an A sized Vee belt. Keep the belt length to a minimum by mounting the engine and alternator close together. We use belts between 28 and 33 inches in total length. The stock pulley on the alternator works well. The best sized engine pulley is between five and six inches in diameter. This pulley ratio gears up the alternator for better efficiency while allowing the engine to run about 2,200 rpm. We have had very poor results with the lightweight cast aluminum pulleys or any pulley using set screws. These light pulleys were not up to the high vibration job and broke frequently. Were now using cast and machined iron pulleys (such as the Woods brand SDS pulleys) that work very well and are extremely rugged. These are available from power transmission product stores and cost about $40. Be sure to get the alternator turning in the right direction. Electrically it makes no difference, but the alternators fan is designed to suck air from the back of the alternator and to exhaust this air in front around the pulley. If the alternators fan is running backwards then the alternator will overheat when heavily loaded. Use large wire to hook up the output of the alternator. Something between 6 gauge and 0 gauge is fine, depending on the length of the runs. Locate the engine/alternator as close as possible to the batteries. This keeps power loss in the wiring to a minimum.

Control Systems The very first engine-driven charger we built worked fine, but we had problems controlling it. We were using a standard car voltage regulator. It wanted to charge the batteries far too quickly and the large load often stalled the engine. We experimented with many forms of control and found two which work well. Alternator controls work by limiting the amount of power supplied to the alternators rotating magnetic field. All alternator control starts with controlling this magnetic fields energy.

Car Voltage Regulators Car voltage regulators will not work well in deep cycle applications. The regulator makes its decisions based only on the systems voltage. This is fine with the average car battery which is cycled to less than 1% of its capacity before being refilled. However, the deep cycle battery is almost empty when it is recharged. The car voltage regulator attempts to instantly bring the systems voltage to 1415 Volts. A 12 Volt deep cycle lead-acid battery will not reach a voltage of 14 Volts until it is almost filled. The net result is that the car regulator dumps the entire output of the alternator into the batteries until they are full. This is almost always too much energy too fast for a fully discharged battery. To compound the problem, the car regulators voltage limit is set too low for deep cycle service. This low voltage limit means that the batteries are charged too slowly when they are almost full, resulting in many extra hours of generator operation to totally fill the battery pack. Since the car regulator is set at about 14 Volts, we are unable to raise the system voltage up to over 16 Volts for the essential equalizing charges.

Resistive Field Controller The simplest control for the alternator is resistance to limit the power to the alternators field. The idea is simple: insert resistance between the batterys positive pole and the alternators field. This controls the intensity of the alternators electromagnetic field and thereby its power output. Resistance of 2 to 25 Ohms works well. Adjust the resistance until the charge rate is between C/20 and C/10. The less resistance in the field line, the higher the amperage output of the alternator. Originally, we used a nichrome wire heating element from an old electric stove. We selected more or less wire (hence more or less resistance) with an alligator clip lead. It worked fine. A better resistor is a 0 to 25 Ohm rheostat (adjustable power resistor) rated at least 25 Watts. This allows smooth adjustment of the alternators output. The illustration above shows the wiring hookup for a resistive field controller. The switch shown in this circuit needs to handle about 5 Amperes and prevents energizing of the field when the charger is not in use. See Access for a rheostat source.

A Resistive field controller. Using resistive field control only regulates current. The resistive circuit does not provide any form of voltage regulation. When the batteries are full the system voltage can get high, over 16 Volts. Voltage this high can damage 12 VDC appliances. The highest voltage for most 12 Volt equipment is around 15 Volts. If you are using resistive field control, be sure to monitor the systems voltage and reduce the current output of the alternator to keep the system voltage under 15 Volts when appliances are being used.

Electronic Field Controller We eventually solved the problem of control by desig- ning electronic field controllers that regulate both the amperage and the voltage of the alternator. With this electronic field control, we simply set the desired charge rate, and set the systems voltage ceiling. The battery is recharged at a constant rate until it is full. When the batteries are full, the voltage limit pre- dominates and the system is voltage regulated, thereby protecting the batteries from overcharging, and also protecting all electrical equipment on line. The amperage output is adjustable from 0 to the full rated output of the alternator. The voltage limit is adjustable from 13.5 Volts to 16.5 Volts.

For the intrepid electronic builder, this electronic field controllers schematic is included on page 31. It uses off-the-shelf parts available at RadioShack or any electronics supply store.

Engines for Equalizing Charges This engine-driven source is a good type to use for the equalizing charges and whenever the RE sources are not keeping up with the systems energy consumption. Its voltage output is capable of being adjusted to over 16 Volts in order to accomplish the equalizing charge. The engine-driven source is capable of delivering a C/20 rate of charge for the at least five continuous hours necessary for battery equalization. Remember the batteries must already be full before the equalizing charge is started.

Access Author: Richard Perez, c/o Home Power, PO Box 520, Ashland, OR 97520  530-475-3179 Engines: Northern Hydraulics, PO Box 1219, Burnsville, MN 55337  800-533-5545  612-894-9510 Rheostats and surplus meters: Fair Radio Sales, PO Box 1105, Lima, OH 45802  419-223-2196

-- Don Kulha (, August 18, 1999


Thanks for the amperage regulated charging information. I was going to give up on deep cycle stuff, now I have some hope since you explained the distinction between deep cycle and automotive type cryteria for battery charge maintenance.

I have been trying to recharge a couple dead 100 AH solid batteries with no success, overheating everything. Now I at least know why.

Thanks again,

-- Tom Beckner (, August 19, 1999.

Excellent information - thank you.

You referenced a electronic voltage regulator: <>

Since "page 31" isn't attached to my PC [8<)] - do you still have it available?

I did not see an equivilent diagram in the homepower webb page, but found the solar charts etc very useful.

-- Robert A. Cook, PE (Kennesaw, GA) (, August 19, 1999.

"This article was published in Home Power Mag back in 1994....the article in it's entirety including color photos and schematics is available for free download in Adobe's PDF format at our website in the download section as MARK8.PDF at"

Honest, I just double checked and it's on the website. It includes schematics, parts list, photos, etc. The whole enchilada. You don't HAVE to build a regulator and could just do the same thing using an alternator with internal (or external as in auto type) regulation though the article explains why this isn't optimum. I wanted to test a "one wire" alternator with built in regulator for grins so I mounted one to my platform and Honda GCH motor. Hooked it to a decent 100 ah battery. Upon starting the alternator (65 amp delco) started dumping 45 amps into the batt (I would have preffered no more than 20) and when the batt was 90% full it was putting 4-5 amps in (to little). If I didn't have solar to top off the batt I would be grousing about having to run a 5hp motor to put 50-60 watts into a battery...very wasteful.

Ideally one will charge a lead acid battery (amps in) at a C/10 rate (battery capacity in ampere-hours divided by 10) for maximum life. They will tolerate a C/5 rate when between 20% and 80% state of charge with little or no harm. Voltage doesn't really matter a bunch for the most part (as long as you have enough). A little bubbling is actually good as it stirs up and destratifys the electrolyte.

The best way to monitor your batteries and derive a lot of the info above is to use a "cumulative ampere-hour meter" such as a "Trimetric" from Bogart Engineering or an E-Meter from Cruising Equipment Co. I'll get into explaining those another time (BTW, they have websites).


-- Don Kulha (, August 19, 1999.

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