Diodes or not diodes

[ZooT]

I'm planning on hooking up both a set of 3 HF 15 watt panels, and a 200 watt genny this weekend.

I've got a xantrex c-35 that I'm going to be using in diversion controller mode.

I'd suppose that using a single source might negate the need to use blocking diodes to keep the battery from feeding the panels or genny but when using more than one source, should I use blocking diodes to keep the genny from trying to feed the panels and the panels from trying to feed the genny?

[kurt]

well i would put a blocking diod in the positive  wire going to the battery of the solar panels and as far as the wind mill goes every type of wind turbine i know of eather requires a blocking diod or some bridge rectifiers in the line someware in eather case that should do fine.

[boB]

A PV module will not start conducting from an external DC source

until the voltage goes above the modules open circuit voltage.

Then is starts to look like a forward biased diode (actually, a

bunch of diodes in series)...

So, a 12V module will take up to, and probably more than

around 21 Volts before it starts conducting.  This voltage

of course depends on the temperature of the PV module and

how many cells it has in series.   The Voc is a fairly

good estimate though for a typical mono/poly crystaline

module.

boB

[Slingshot]

What is Voc at night?  

[wooferhound]

I am building the almost the exact system that you are. I said that I was going to put this system in last weekend, well, so far I have put 4 full days into this project with at least 2 more days work before it is completed. Next week I'll be posting pictures and descriptions of my system.

http://www.fieldlines.com/story/2007/9/3/22483/29925

I personally put a single diode on the output of every single panel of my system. When finished there will be 5 total panels. The wind Generator will require a Diode Bridge Rectifier on it's output.

[TomW]

Slingshot;

Uh, Volts Open Circuit at night should be zero or possibly a few millivolts from bright moon / stars. Anyway, well below daylight volts for certain.

Cheers.

TomW

[Ungrounded Lightning Rod]

But the Voc that's significant is the one when there's actually light.  That's a good measure of the voltage drop as a string of diodes - which doesn't change when the light goes away.

[boB]

>> What is Voc at night?  

OK... OK...

What "would be" the open Voc...  This can be measured with a current

limited power supply, even in the day.

Place the module face down, like on a rug (OR, just wait till dark)

and apply voltage to it until the current starts to rise.

That will be approximately Voc.  Temperature dependent, just like

Voc with light.   I think it's just slightly higher than real

light induced Voc.

boB

[Opera House]

I have two 75 watt panels in parallel and the leakage current at night is 65ma if I remember the test right.   Panels are basically stacked diodes, but very imperfect ones.  I don't see what VOC has to do with Chinese apricots being a glut on the market this year.

[Slingshot]

In that case, "dark" panels should not draw current from the batteries, even without a blocking diode.  Is that true in practice?  

Do the so-called "self-regulating" panels, which are merely a lower number of cells to reduce overcharge, require anything to prevent dark discharge?

[ghurd]

They draw a little, but not a whole lot.  A blocking diode won't hurt anything because of the power curves on standard PVs.  Every controller I can think of has nighttime discharge protection built in.

There were a lot of different kinds of "self-regulating" PVs.  Probably forget the diode and get a decent controller.  Decent solar controllers have a much lower forward voltage drop than a diode, and with self-regulating PVs, you need all the voltage you can get.

Self-regulating PVs tend to be small, so a ~$32 SG-4 would do a decent job of it.

G-