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Build Your Own 12 VDC Engine/Generator
©1994 Richard Perez This small, easy to build,
engine/alternator is the answer to
a burning RE question. What do we do when the sun doesn’t shine, the wind doesn’t blow, and the creek dries up? This generator is a backup power source for times when our RE sources don’t meet our demands. It is optimized to do only one thing — properly recharge batteries on demand.
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 (1982–1985), 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
Above: This engine/generator uses a Chrysler 70 Ampere alternator.
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 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.
28 Home Power #42 • August / September 1994
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