M75 Siemens low voltage problem [RE-wrenches]

[Bob Maynard, Energy Outfitters]

Hi David,
One or both diodes are blown.  Replace them both and odds are excellent that the
module will be back up to near spec.  I just went through the same thing about 6
months ago.
Bob Maynard
Energy Outfitters

"David Palumbo, Independent Power & Light" wrote:

> 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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