M75 Siemens low voltage problem [RE-wrenches]

[Bill Brooks]

Dave,

I think you may have more than a diode problem. Lightning can certainly
knock out a diode, but it sounds like a portion of the module was damaged as
well. Those cells could be back biasing to provide the current and heating
up--that's often what you find in large arrays with an infrared camera.  It
could be that the battery is back biasing those cells and losing power that
way. A third of the module is acting like a resistive heater. Without the
diode to pass the current around these damaged cells, the current must pass
through them and dissipate energy.

Remove the diode, retest the module. It will probably exhibit low Voc. That
means you need a new module.

Bill.


> -----Original Message-----
> From: David Palumbo, Independent Power & Light [mailto:ipl at sover.net]
> Sent: Tuesday, July 24, 2001 7:34 AM
> To: Wrenches
> Subject: M75 Siemens low voltage problem [RE-wrenches]
>
>
> 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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