Can VOC be tamed?

[good4politics]

If VOC stands for "Voltage, Open Circuit" Why can't we tame VOC (lower it) by wiring a 10k Ohm 100w resistor from positive to negative just prior to the Solar Charge controller? It would eliminate the open circuit and once the charge controller comes online, electricity should flow through the lower resistance path, bypassing the resistor. I must be missing something here...

 

[HighTechLab]

I don't think that would be enough current to pull the voltage far enough down the curve to accomplish much.



Let's say you are connecting your resistor to a 500V string, for example. Maybe you are trying to pull it down to 450v for a solar controller. 10kohms at 500V would give you .05A and would dissipate 25 watts. If you figure the above curve is somewhere between 0-9 amps (typical currents for a panel), this is hardly off the x-axis of the chart

So, you may think, let's use a different size resistor, right? Well, 2.5kOhm resistor would pump out a whopping 100W of heat (note: your 100W resistor won't like this much, better get a fan on it!) and despite the 100W of heat you still are only running .2 amps...still hardly above the axis line of the chart. At this point, you are destroying that precious power that your solar controller could be using to power your system!

 

[Hedges]

Nope, just let Voc do it's thing, and make everything else able to withstand that.

It takes a huge current to pull down I/V curve of PV panel significantly.

You could introduce a voltage drop, but that isn't easy either. Series resistor would drop V = IR, so at zero current you still see VoC. Diodes would drop 0.7V each (approximate) over a wide range of current but would leak up to full Voc (parallel resistor might help.)

You can short out one PV panel in series. or open junction diode and short out one diode.

You can get a controller like Midnight Classic that tolerates Voc = SCC rating (e.g. 150V) plus battery voltage (e.g. 48V), but without operating (waits for weather to warm up.)

Best advice is select PV panels to match SCC, or SCC to match PV panels. Also select series/parallel combination of panels in array, of course.

 

[hwy17]

electricity should flow through the lower resistance path, bypassing the resistor

Electricity flows over all paths proportionate to their resistance, not only the lowest resistance path.

 

[Quattrohead]

A better method would be to bypass a panel until the VOC is within range.
How to do, various methods.
Best method, match your equipment.

 

[good4politics]

Electricity flows over all paths proportionate to their resistance, not only the lowest resistance path.

True, but at even with a 1k ohm resistor, the voltage goes negative and the amperage is less than 150ma. Perfectly acceptable.
VOC is 105v max with these cheap thin film panels. Seems to work out in an online circuit simulator.

 

[good4politics]

A better method would be to bypass a panel until the VOC is within range.
How to do, various methods.
Best method, match your equipment.

Sometimes best = what makes sense financially. You don't buy a $350 Solar Charger for 3 $20 panels. Cheaper chargers are more commonly limited to 100v.

 

[Quattrohead]

Ah well now we know you have low power panels, put a 5/10/25w regular normal bulb across your PV input and it should take care of keeping the volts lower until the unit can pull charge from the panels.

 

[good4politics]

Ah well now we know you have low power panels, put a 5/10/25w regular normal bulb across your PV input and it should take care of keeping the volts lower until the unit can pull charge from the panels.

Not terribly low power, rated for 150w. high VOC, low amps. Problem is, I hate them. Tried 3 just to see what they would do. They do seem to work. Thinking I will gift them to my neighbor for his chicken coup. Obviously, I don't want to put hundreds of dollars into a gift for chickens. I just need them to work without blowing up a decent but cheap MPPT charger.

 

[Rocketman]

Design that system so the panels are in parallel only. Voc of 79v means use like a (assuming a 12v battery):

A Victron 150/35 for 3 or 4 panels
A Victron 150/45 for 4 to 6 panels
…
Up to a Victron 150/100 for 10 to 12 panels.

Just put a fuse on each panel if going over 3p.

No issues…

Good Luck

 

[Ampster]

Best advice is select PV panels to match SCC, or SCC to match PV panels.

I agree. I once bought some 96 cell panels and had to purchase Enphase IQ7X-96 micros to run them. Later I sold the panels and used those same micros to run several pairs of 250 Watt panels. Two panels in series worked well with those micros which could handle the higher voltage of two panels in series.

 

[Tomthumb62]

Not terribly low power, rated for 150w. high VOC, low amps. Problem is, I hate them. Tried 3 just to see what they would do. They do seem to work. Thinking I will gift them to my neighbor for his chicken coup. Obviously, I don't want to put hundreds of dollars into a gift for chickens. I just need them to work without blowing up a decent but cheap MPPT charger.

Yeah I wouldn’t waste my time with panels like that. 110 Voc!! Hope you didn’t pay for them.

 

[Hedges]

Should work fine with a 140V or higher MPPT.
Just connect all panels in parallel.

Some equipment is high enough voltage to take multiple of these panels in series, but price per watt may be higher.

 

[12VoltInstalls]

just let Voc do it's thing, and make everything else able to withstand that.

I contemplated starting a new thread but I read a bunch of stuff and decided just to tag on this one.
I have thirty-nine ~98VOC 60W thinfilm panels. I was hoping to utilize them with on-hand equipment. They cost me $40 of gasoline, round trip.

Other than an incandescent light bulb- which could burn out, negating the drop- I’m basically concluding that there is no other way to limit or suppress VOC:

put a 5/10/25w regular normal bulb across your PV input and it should take care of keeping the volts lower until the unit can pull charge from the panels.

I’m offgrid so microinverters don't really offer me anything.

As has been put bluntly above, they aren’t worth much but they will make power.

I’m thinking of simply doing 3S8P into an lvx6048 for the shop, employing my eight tired fla 100Ah batteries merely as a surge capacitor. It would basically function daylight hours anyway so I don’t really need to know about the limitations I am quite aware of.

However, i ’still’ would have an issue with these panels with their 1.1A spec. Lvx6048 shows 27A max- not sure if that is a hard limit or a useable limit. But at 450VOC it would handle 3S, but low temp 4S wouldn’t be ok. And unclear if the LVX has two mppt’s or just one; two would be great. Then I could add another string if the machine will accept the input amperage above 27.

Should I find these a new home, or use them?

 

[Hedges]

What does your math say about 4s?
With 98Voc and 450V limit, I get 14.8% increase allowed for cold. That covers most panels and temperatures. What is temperature coefficient of these?

I doubt current is a hard limit. If you have some strings of different orientations, that should reduce current.
With 3s, your panels make an array only about half the limit, anyway.

2400W, which can be bought for around $500 and 6 to 10 panels. Keep your mounts inexpensive.

 

[12VoltInstalls]

What does your math say about 4s?
With 98Voc and 450V limit, I get 14.8% increase allowed for cold. That covers most panels and temperatures.

I landed too close to 450V for my comfort with 4S.

What is temperature coefficient of these?

Don’t have it in front of me atm.

2400W, which can be bought for around $500 and 6 to 10 panels. Keep your mounts inexpensive

My existing equipment tops at 150 for one SCC and 200V on another. I was just trying to online them without spending money right now. The 6048 would handle them.

But yes, for thinfilm panels from 1883 the mounts will be extensive.
Secondary benefit is they will make a nice privacy fence on a S-facing portion of my property line, so on a 4x4-post line they are just convenient to ‘save’ on boards for the fence LOL

 

[Hedges]

Close is perfectly comfortable if your pencil is sharp.

For existing SCC, of course 1s works. Why not do that?