I replaced a system with them. I think realistically given the price of
modules, the age of the Shell modules, bless and recycle the Shell
modules, and replace. The system owner for this project was astounded
at how much more energy he had. Off grid system with generator. Chris
On 7/15/2026 5:16 PM, Zeke Yewdall via RE-wrenches wrote:
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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Christopher Warfel, PE
ENTECH Engineering, Inc.
PO Box 871, Block Island, RI 02807
(401) 447-5773
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