Subject: Generator 3 of 7



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(3541)   Sun 21 Jul 91 23:25                             
By: Don Kulha
To: All

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Lawnmower Engines and Car Alternators
The idea here is to use a lawnmower engine (or other small 
horizontal 
shaft motor) to drive an automotive alternator.  The alternator puts 
out between 35 and 200 amperes (depending on its size) of 12 to 18 
volt DC energy to charge 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 2 years 
before the motor died.

Type and Size of Motor
We've since tried many different combinations of motors and 
alternators.  
Small gas motors between 3 and 8 horsepower are ideal for this job.  
We found that the Honda small engines will run about 5,000 hours 
without major work, Tecumseh engines about 800 hours, and Briggs 
& Stratton engines about 600 hours.  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 
units are much less expensive in spite of their higher initial 
cost.  
The engine's 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. motor.  A 100 ampere alternator 
needs at least a 5 hp. motor.  For alternators between 100 and 200 
amperes use the 8 hp. motor.

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 
which charges it 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.

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