Why shorting?

[Madscientist267]

One thing I keep seeing post after post is a controller shorting the solar panels when the batteries get full.

I understand the brake force thing with a windmill to prevent self destruction, but what is the point in a solar charge controller? Would seem to me all this does is creates heat and stresses various components along the way. Why not just disconnect them from their load instead?

Steve

[boB]

Steve, I agree with you.   I suppose that one thing it might help is for the Voc not to go too high... That is, if the controller has more current capability than voltage capability.

I myself have not seen a controller that does this, and it would seem to be harder to make than one that just opens up the PV when battery is full etc...

Most controllers just open up and see open circuit voltage when they don't need any more energy.

boB

[Madscientist267]

It made sense to me, although I hadn't thought about the controller not being able to handle the Voc levels. Apparently, there are both schools of thought on the subject, but I just keep seeing it all over the place. It made more sense in my mind to just let 'em disconnect. I know that a PV cell can never produce enough current to spontaneously self destruct (at least with current technology), but does it have an effect on overall life span due to heat dissipation? Solder joints, etc.

I suppose there is a bit of paranoia on my part, as I'm one of them that believes 'take them out of the weather when theres no usable sun', which of course is impractical for large installations. I want to know about this though, as I plan to do a full blown installation here in the near future, and want to cover all the bases. Learning on a small panel is one thing, but real world where hundreds of watts are in action is another thing entirely.

Steve

[ghurd]

Post after post is about home brew controllers?

It is cheaper and easier to short the PVs with common parts.  Meaning N-fets.

Several IC manufacturers show simple PV shorting controller circuits in various application notes.

I thought most factory made solar controllers open the circuit.  'Series Controllers'.

I know of a factory made shorting controller that was still working about 5 years ago.  I do not recall the brand name.  The owner believed it was 15 or 20 years old at the time.  It used a small relay.  Old School stuff.  At least I think it shorted the PV...

I read some of the early solar controllers used a Zener for voltage control, and transistor to short the PV.  Higher battery voltage meant more current through the transistor.

I don't believe it will harm the PV or properly rated components.

The PV's Isc is not much higher shorted than when charging a low battery.

G-

[boB]

It seems to me that it would be harder to make a shorting controller instead of one that just opens up the PV.  Wouldn't you have to first open the PV, THEN short the PV, terminals, (break before make), then reverse that... On and on, ad-nauseum (until you throw up.)  More careful timing AND another switch to add to the circuit.  You don't want to short the battery out... It makes a mess.

Also, relay controllers had (and have) the problem that you have to snub the arc across the contacts (either  open or shorting types).

boB

[fungus]

"Wouldn't you have to first open the PV, THEN short the PV, terminals, (break before make), then reverse that..."

Not really, you just have a diode in front of the controller and all you need is a FET or whatever between the two PV terminals that'll switch on when the voltage gets to the set point ..

[boB]

>>Not really, you just have a diode in front of the controller and all you need is a FET or whatever between the two PV terminals that'll switch on when the voltage gets to the set point ..

Yes, a diode would certainly be a simple way around that.  I try not to add any kind of losses in the system if possible, but a diode would be minimal.  Just costs more.

Good thinking  Mr. Fungus !

boB

[Flux]

I can't think why they short panels either it doesn't make sense but several well known European controllers do it. Possibly it's easier to drive a mosfet referenced to ground rather than use a high side driver for home use but it makes no sense for a commercial device.

Flux

[boB]

They could of course just switch the negative PV terminal on and off and forget the high side drive... That is, if it's not against the rules, which it may very well be in Europe.  It certainly would be here in the USA.

boB

[ghurd]

Yes.  What Fungus said.

Take my "ghurd controller", put the power fet on the solar side of the diode. Done.

There are quite a few people doing it that way.  At least one member posted a story.

The diode loss is of no concern for systems without MPPT.

I do understand you think bigger.

Here is one of my world famous MS Paint sketches made for us Regular Joe's.

Easier than it sounds.

G-