[RE-wrenches] Shell Solar Warranty?

[Zeke Yewdall]

If they are at 30% of rated output, is it current or voltage that's
suffering at Vmp?  And how does Voc and Isc stack up to rated values?  Have
you done a IV curve on the actual module, or just measuring the total
array?  What sort of MPPT are you using on them?  My experience with old
modules is that they are usually still good (80% or better of rated power),
or are completely dead (open circuit, no output).  70% loss while not being
completely dead seems odd.

I have had an issue with some older 36 cell 120 watt modules, which made it
appear that they had precipitously dropped in output, when really it was
only about a 10 or 15% drop in Vmp and Voc.  But this was enough for the
charge controller to fall off the IV curve, and drop the current by almost
50% just because of the position on the IV curve.  In this case, they were
two 36 cell modules in series, charging a 24 volt battery (some
configuration that the old PWM charge controller had used) -- the charge
controller could not track any lower voltage because it had to stay at
battery voltage + a few volts.   So a relatively mild loss in Vmp had
resulted in a serious loss in output power due to the MPPT tracking
details.  When I rewired them in strings of 6 in series instead of 2 in
series, the charge controller was able to maintain tracking on the actual
Vmp, and power was at least 80% of rated again.

I have had issues with shorted bypass diodes dropping voltage of some newer
modules to 2/3 or 1/3 of rated output.   In my cases, they were usually due
to lighting strikes (also took out a bunch of other stuff, and upon
repairing the other stuff, we found some strings of modules were not up to
rated voltage, and upon testing individual modules, we found a couple of
them to be exactly 1/3 or 2/3rds of normal voltage).   A thermal image
while the module is under load can identify shorted substrings of cells in
a module -- the shorted string will be hotter than the portion still
producing power.

And, with some 120 watt Evergreen modules, which had two parallel strings
of cells in them, I had issues with a bad internal connection to one of the
parallel strings of cells -- which meant that voltage was normal, but
current was only half of normal... this was a problem due to being wired in
series in other modules that still had full current.  Both Isc and Voc
measured where they should for the complete string, but the curve was very
much not right.  Measuring individual modules showed that some were only
half rated amperage.

Zeke
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