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RE: Solar Panel damage



This is only true  for series cells and is because
you are breaking the circuit.  If you have parallel
cells or strings of cells, it decreases the shadowing
problem.  Then the problem becomes the low voltage or
matching.  Always a problem somewhere!

Bob

-----Original Message-----
From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
Behalf Of Bob Bruninga
Sent: Wednesday, October 22, 2003 12:43 PM
To: amsat-bb@AMSAT.Org
Cc: TAPR APRS SIG
Subject: [amsat-bb] Solar Panel damage


I learned something new today about Reversed solar cell damage:

I think everyone who works with solar panels is aware of the shadow effect
which means that a shadow across one cell reduces the power out of the
entire panel as if the whole panel was in shadow (the current out of the
entire series string is no better than the worst illuminated cell).

But what I never realized before is that the shadowed cell then is driven
in reverse, and reverse bias heats the cell and at voltages over 5 volts
can do permanent damage to the cells.  This is usually not a problem in 12
volt systems where the difference between the open circuit voltage and the
battery charge voltage is only a few volts (say 18 and 14).  But in 28
volt systems, where the open circuit voltage is say 36 to 40 and the
battery is 28, then the shadowed cell will see 8 to 12 volts reverse
voltage, and that is enough to permanently damage the cell.

Fortunately, it takes a while to do it, and since satellites in orbit and
the moving sun for terrestrial cells nominally keeps the shadow moving,
this is not that big a problem.  But a drop of bird byproduct that
permanently blocks a cell for long period of time will kill the cell and
the panel...

I knew about the shadow effect, but never realized the potential for
permanent damage.

Just FYI
Bob, WB4APR


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