Wrenches,
Here's one I haven't seen before. 1997 grid-tie battery backup system
that was one of my first, although actual installation was by another
before I was licensed. 16 BP 275, 36-cell 12V modules, the standard of
that time. Early Outback MX60 replaced C40 around 2005; array rewired to
four 48V strings of four modules at that time, serving a 24V battery
bank. Tested with an end-of-life 9-year-old AGM battery bank in the
system, but in Sell mode with a 26.4V Sell voltage setpoint.
At 11 am, 68ºF, 900W/m^2 on the Daystar, with the array under charge, I
measured 3.5A, 3.4A, 2.4A, and 3.6A. Short-circuiting each string, I
measured 4.0A, 3.7A, 3.8A, and 3.7A. Original rating was 4.45A, so
except for the third string, current is what I'd reasonably expect from
20-year-old module degradation. However, watching the periodic MPPT
sweep from the MX60 at the array, the maximum power point voltage for
the entire 48V array settled at 14.6A at 37.2V, or about 540 watts from
a 1,200W (originally rated) array.
I have commonly seen the current reduce substantially with age-related
degradation, while the MPP voltage remains fairly close to original. I
have never seen the voltage drop this far - an average of 9.3V MPP per
each 12V module. All of the cells are seriously browned. I have other
systems out there with the same modules of similar age that don't
exhibit this weird behavior. Can anyone help me understand why the MPP
voltage would drop so far below what is typical?
There is a slight possibility of corroded buried input conductors,
because of a serious but corrected issue of galvanic corrosion from a
ground fault (a Romex connector pinching a conductor and allowing
seepage to ground). This was corrected years ago, but the input
conductors were direct burial for ~100' and there was never a bonding
conductor between the array and the house, so there is a potential for
corrosion damage. However, the voltage measured at both ends while
charging was identical, so I think that would indicate that high
conductor resistance isn't the issue.
As usual, thank you for the collective and individual wisdom shared here.
Allan
--
*Allan Sindelar*
al...@sindelarsolar.com <mailto:al...@sindelarsolar.com>
NABCEP Certified PV Installation Professional
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder (Retired), Positive Energy, Inc.
*505 780-2738 cell*
**
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