[Elecraft] Folding Solar Panel

[Don Brown]

Hi

I use 14 volts times the current delivered to calculate the wattage of a 
panel. The open circuit voltage is only useful to get an idea of how much 
overhead you have and is determined by the number of cells in series. Each 
cell will produce about .5 volts at a current determined by the size of the 
cell and the amount of light falling on the cell. This folding panel has 40 
cells that are 1 1/4x 1 3/4 each. I measured 380 mA into a fully charged 
battery at 14 volts. .38 times 14 volts is 5.32 watts. This is what I based 
my statement of an output exceeding 5 watts.

 I have panels that output as low as 18 volts and some as high as 30 volts 
open circuit but as solar panels are constant current devices the open 
circuit voltage is really not critical as long as it is somewhat over the 
batteries maximum terminal voltage plus any circuit drops in the wiring or 
controller. My controllers use power fets with very low on resistance and 
shottkey blocking diodes so the drop is less than 2 volts at full rated 
power.

I would prefer the panels were rated in current but that is not the case. 
Most of them are rated in watts and I have found that using 14 volts 
(assuming a 12 volt system) as the charging voltage gets you fairly close to 
the rated current the panel can produce in typical situations. This allows 
you to at least compare panels from different sources as well as give you an 
idea of what your charging current will be before you spend money on a 
panel.

Don Brown
KD5NDB



----- Original Message ----- 
From: "Mike Harris" <mike.harris at horizon.co.fk>
To: "Elecraft" <elecraft at mailman.qth.net>
Sent: Friday, January 06, 2006 4:40 PM
Subject: Re: [Elecraft] Folding Solar Panel


> G'day folks,
>
>SNIP

> Calculating the wattage of solar panels by using the open circuit voltage
> and short circuit current does not represent a real world operating
> condition, albeit a common method used by manufacturers to mug the unwary.
> >From bitter experience (hill top solar powered microwave telephone 
> >network
> repeaters) I reached the conclusion that once you have got the nominal
> solar panel/system voltage matched all you are interested in is current
> into and out of the battery.  Batteries operate in a world of current, Ah
> capacity, watts are virtually meaningless, only giving a rough guide to
> panel dimensioning for a given load.
>
> Regards,
>
> Mike VP8NO
>