Importance of open circuit voltage rating

[IdaSHO]

I may have made a bit of a mistake.

Maybe you can tell me if I did, or if it may actually work out okay.

36V solar array. Max open circuit voltage of 45.5V

20A MPPT charge controller spec'd with a max open circuit voltage of 42V


Am I in trouble? Are the specs THAT critical?

Will this totally doom the controller, or is there a chance this might actually work?


Thanks for any help :snorkel:

 

[Haf-E]

I wouldn't try it - you'll likely let the magic smoke out of the MPPT controller and then it won't work anymore...

You could measure the actual open circuit voltage of the solar module to see if its more than 42 volts - but keep in mind that the voltage increases as the temperature of the solar module goes DOWN - so it might work with a solar module that is warm in bright sunshine but fail when its hit with less sunlight on a cold morning. I like to have a safety margin of about 20% on the open circuit voltage levels to allow for this to occur...

Good luck! Don't let the magic smoke out of the wires...

 

[IdaSHO]

Copy that.

Ill toss it on the back porch in the evening and get a reading first thing the morning before temping to attach it to the controller.

Seems winter hasnt quite given it up just yet. This has been a tropical winter for us here, but freezing solid again at night lately.


I guess I just do not understand the threat of over voltage with zero current??

Anyone care to help me understand that?

 

[dwh]

I'd call the manufacturer of the charge controller. There has to be a safety/fudge factor in that max voltage number. You're talking less than 10% over voltage. I doubt it would let the magic smoke out - probably has some built-in shutdown protection feature.

I guess I just do not understand the threat of over voltage with zero current??

Anyone care to help me understand that?

Sure - there's no such thing.

Zero current = zero electricity flowing = zero voltage.


For the MPPT to work it has to measure the voltage and amperage, then compare a couple of readings to find the sweet spot. To do that it has to make a circuit and allow some electrons to run around the race track.

 

[IdaSHO]

I'd call the manufacturer of the charge controller. There has to be a safety/fudge factor in that max voltage number. You're talking less than 10% over voltage. I doubt it would let the magic smoke out - probably has some built-in shutdown protection feature.

I figured as much...

Afterall, everything else in life has a fair safety margin

Sure - there's no such thing.

Zero current = zero electricity flowing = zero voltage.

That helps my sanity.

So what I should really look at is operating voltage maxes??

Looking at those, Im completely in the green.

 

[Dervish]

Zero current = zero electricity flowing = zero voltage.

That's not really correct. Current is the number of electrons passing through a conductor over a period of time. Voltage is the relative concentration of electrons (or charge carriers in a semiconductor like a solar cell) between two points. If there is no conductive path between those two points there is voltage, but not current.

A solar module is like an electron pump. Like a water pump, which has a maximum pressure it can operate to, a solar module has a maximum voltage. Without a circuit to pump it's electrons into, it will just increase voltage until it hits this maximum value. Once at this voltage, the cell basically stalls (in pump terms).

The concern for your MPPT would be whether the presence of this voltage would damage any of the internal components (arcing across a capacitor or IC), or whether they just give you that voltage value to help choose an appropriate module. As mentioned, you'll have to ask the manufacturer about that. My guess is that they'll stand by their maximum.



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[IdaSHO]

Seems the website doesnt have the correct info listed

I downloaded the actual manual for the charge controller and Max open circuit voltage is 50V DC :ylsmoke:

THAT is good news :wings:

 

[wirenut]

50V DC certainly gives you more room. You still may smoke the controller in really cold weather. The Voc of a panel is usually given at 25 degrees C. As the panel temp. drops the Voc rises based on the temp. coefficient for that panel which should be available from the manufacturer. A 25% rise isn't out of the question in really cold weather. That would still put you over the 50V DC spec.
I wouldn't try it for fear of damaging the controller. However, if it's never in the sun when it's really cold you will probably get buy. I guess you could just run it for now and if/when it gets damaged buy a new controller then.

 

[dwh]

That's not really correct. Current is the number of electrons passing through a conductor over a period of time. Voltage is the relative concentration of electrons (or charge carriers in a semiconductor like a solar cell) between two points. If there is no conductive path between those two points there is voltage, but not current.

Theoretically. But if there is no conductive path, then the voltage is only a *potential* and does nothing.

 

[dwh]

So what I should really look at is operating voltage maxes??

Looking at those, Im completely in the green.

No, Voc can't be ignored. The greater the load, the lower the voltage will be, so when operating (loaded up charging a battery) the Vmp is the important number. But when the charge controller just makes a circuit that has almost no load on it (MPPT doing it's "check the voltage of the solar" thing), then the circuit is going to be at Voc.

 

[IdaSHO]

Well, we are cooking now, and all seems to be doing great :ylsmoke:

Two 48V 100W Renogy panels

My custom tilt mounts.

All runs using 10 AWG PHOTOVOLTAIC Wire

All junctions made using MC4 connectors

All pumping through an el-cheapo Eco-Worthy MPPT.
Lots of buzz about these on forums these days.
I rolled the dice. If it works out, it will be a huge savings.
If it doesn't, no real loss :sombrero:

So far it is doing fantastic. Have seen a max of about 9 amps so far.
But nothing has been ideal. Camper is parked in a somewhat shaded area, and its been rainy.


Some pics, still have to tidy up the wires:

 

[dwh]

Nice job. Clean.

Love the snowglobe on the roof.

 

[IdaSHO]

Ah yes, the sun dome.

That thing tosses more light than EVER expected.

This photo shows it performing in the SHADE:Wow1:

Works great for our winter trips.

Not so much in the summer. So far Ive been dropping the interior lens and putting reflective insulation in there to keep the light and heat out.