piclist 1999\03\07\151848a >
www.piclist.com/techref/index.htm?key=solar+panels
BY : Wagner Lipnharski email (remove spam text)

Mike Keitz wrote:
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As far as I understand Ohm's law, the maximum current will be
proportional to the maximum voltage, so... if the batteries
are at 96V, any voltage above that will work. The output power
is a fixed relation to the input power less the losses in the
conversion, so, if the cells generate 0.45Vx110A (50Watts),
take off the conversion losses, let say 15% in the best deal,
the power output will be 42.5W.  It will take a cell size of
aprox 1660 square inches, or a board of 41x41 inches.

It doesn't matter much what the voltage is being generated at
the bust conversion, it needs at least to be higher than the
batteries voltage. The batteries internal impedance would be
so low that this is what will rules about the battery charge
current. Also the solar cells high impedance will drop its
output voltage with the increase of the drained current.

So, imagine the batteries are at 85V, your buster step-up is
trying to generate something around 100V, but its output
impedance is higher than the batteries impedance, so the
current output will be a factor of the power transferred
divided by the battery voltage. 42.5(W)/85(V) = 0.5 A.

I never saw a step-up conversion with a productivity better
than 85-87%... even the small ones produced by Maxim or Linear
Tech, and charge capacitive pumps are not better, so I think
that losing 15% in solar power is a high price.

Solar cells are very easy to cut, plastic, flexible or the
liquid ones right now, so build an array with 300 cells
in series is not a big problem... well, I don't know the
problems you may have to do that, but it would give you at
least 15% more power... aprox 588mA, not talking about the
most economic wiring AWG as well the cost of the buster,
or then 15% less cell overall sizes, what correspond to
a reduction of almost 16x16 inches in the cell area.
--------------------------------------------------------
Wagner Lipnharski - UST Research Inc. - Orlando, Florida
Forum and microcontroller web site:   http:/http://www.ustr.net
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