Hi Allan, I would try turning off string #3 and see what happens to the vmp. I'm guessing it'll go up to a more normal voltage.
I just had a very similar experience, also BP modules, very brown. 2 sets of 4 billion n series to a 12v system. One set was totally dragging down the VMP, I disconnected it, all normal. I tested voc of all modules after they were replaced,(190 watt 72 cell) customer wanted more watts, and all were normal and the same. I didn't spent the time to do a ISC. Replacement is my thought. Jay Peltz power. > On Aug 25, 2016, at 7:04 AM, Allan Sindelar <al...@sindelarsolar.com> wrote: > > 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/m2 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 > NABCEP Certified PV Installation Professional > NABCEP Certified Technical Sales Professional > New Mexico EE98J Journeyman Electrician > Founder (Retired), Positive Energy, Inc. > 505 780-2738 cell > > > _______________________________________________ > List sponsored by Redwood Alliance > > List Address: RE-wrenches@lists.re-wrenches.org > > Change listserver email address & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out or update participant bios: > www.members.re-wrenches.org >
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