[RE-wrenches] Low array voltage stumper on legacy system

[Allan Sindelar]

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*
allan at sindelarsolar.com <mailto:allan at 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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