M75 Siemens low voltage problem [RE-wrenches]

[David Palumbo, Independent Power & Light]

A fellow brought in a Siemens M75 (vintage 1994) 48 watt module to me
yesterday with a problem that I have never seen before.
He reported that there was a lightning event close by and afterwards this
one M75 (out of a simple 2 module array, daisy chained; the second module is
OK) was draining his batteries. It was warm to the touch on the glass
covering about one third of the cells.

I checked the modules operation with my Fluke and found the following. OCV =
6.05, ISC = 2.95 during a hot, sunny/hazy noon time with no shade on the
module. The factory specs are: OCV = 19.8, and ISC = 3.35

It looks to me like only one third of the cells are now in the circuit. Is
this a good assumption?

I checked the bypass diodes and the one in the positive box has continuity,
it reads .1 Ohms in either direction with no sun on the module and it reads
from 20 to 30 Ohms depending on the strength of sunlight in both directions
(bad diode?). The negative diode has no continuity with no sun and has
continuity in the sunlight in one direction only (which I think is OK). I
put a jumper across the positive diode and the module output was still only
6 volts.

No physical damage is visible to my eye. Does anyone have any idea what is
going on? And suggestions on how to fix it?
Thanks,
Dave

A thank you also goes out to Richard & Karen Perez, and Ian Woofenden for
gracing Vermont's SolarFest with Home Powers presence a couple of weeks ago.
It was great to have Richard speak during our battery workshop, we all
enjoyed (and learned) from his battery sulfation / Dating Game analogy.

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