anyone know a way to slightly reduce the voltage of solar panels?

[fafrd]

2 panels in series is 62v when producing. You will need a high wattage resistor to bring it under 60v. Hopefully it can also bring down voc under 60v too.

You can put any size resistor you want in series but when no current is flowing, the resistor won’t prevent 2xVoc = 76V from reaching to the input of the SCC (and even higher on cold winter mornings).

If you just want to focus on getting 2S string voltage under 60V when Imp = Pmp / Vmp = 8.9A, you’d want 2 volts dropped when 8.9A which means a 225mOhm resistor.

I^2R losses would be 17.8W which is less than 3-1/4% of 2S string output but the real problem is that input voltage can still reach 76V when there is very little light and no current flowing..,

 

[schmism]

this feels a lot like.... just not worth trying to fit the round peg into the sq hole....
and the inevitable... yeah but its what I have and dont want to spend money...
to which the reply is something like... . shure give it a shot, when it goes up in smoke you'll have the opportunity to get 2 things that actually work together the intended way.

 

[DIYrich]

You can put any size resistor you want in series but when no current is flowing, the resistor won’t prevent 2xVoc = 76V from reaching to the input of the SCC (and even higher on cold winter mornings).

If you just want to focus on getting 2S string voltage under 60V when Imp = Pmp / Vmp = 8.9A, you’d want 2 volts dropped when 8.9A which means a 225mOhm resistor.

I^2R losses would be 17.8W which is less than 3-1/4% of 2S string output but the real problem is that input voltage can still reach 76V when there is very little light and no current flowing..,

Ahh, I finally understand voc (at least a little more). So if battery is full and little load on inverter, the mppt will start throttling pv, and input voltage will rise until it reaches voc. If you use a 60v zener diode, it has to be able to take the full wattage, not just a nominal current.

 

[TomC4306]

It will probably work with them in parallel, it just won't deliver full power because the voltage is low and the current is limited. I would try it before adding diodes that dissipate power as heat. The controller should protect itself from overcurrent, but it can't protect itself from overvoltage.

Right. And he already applied 76 volts open circuit to a 60 volt device.

 

[efficientPV]

Buy more panels! The solution to everything. An extra 12V 100W (don't know if they make something slightly over that) panel in series with each 275W panel would meet your requirements. It would be limited by Isc, but you have 200 more watts and a higher voltage than two i275 in parallel. Reliable, low tech, not failure prone, off the shelf, and better low light capability. Three 12V panels in parallel with the two 275 in parallel. Used panels may have less watts.

 

[gnomie]

I have 2 solar panels, they are 275 watts, 38 VOC and VMP 31
what I NEED (for a specific solar generator) is:
watts: 550
VMP > 36
VOC < 60

if I connect them in series, my volts reading is about 64
so I assume that covers the requirement "VMP > 36"
but violates "VOC < 60"

is there any way I can make them work? maybe a way to slightly lower the volts to just under 60?

I have a 12V brushless pump motor that I run off of (1) 100W panel that is rated at about 18V. I'm wired direct from panel to pump so to attenuate the voltage down and keep it from frying my pump I went completely "old school" and taped some cardboard over part of the panel until I got a meter reading of 12V in full sun. Since it's a brushless motor it will still run (slower) at voltages <12V so clouds/partial shade etc... rarely shut the pump down but "hobbling" the panel protects it from spinning up beyond its design parameters and burning out.

 

[texasjames]

I have a 12V brushless pump motor that I run off of (1) 100W panel that is rated at about 18V. I'm wired direct from panel to pump so to attenuate the voltage down and keep it from frying my pump I went completely "old school" and taped some cardboard over part of the panel until I got a meter reading of 12V in full sun. Since it's a brushless motor it will still run (slower) at voltages <12V so clouds/partial shade etc... rarely shut the pump down but "hobbling" the panel protects it from spinning up beyond its design parameters and burning out.

interesting. I'll try it

 

[ncsolarelectric]

the reason it doesn't work in parallel is because the unit's threshold of 36 mvp is never reached

Did you try it? Vmp is not a "start up" threshold. It should still work at 31V. I doubt that would reduce the power by 67% as others say. It's not that big a deal.

 

[Pappion]

Power a small incandescent light bulb with it. It won't be Open circuit any more. Maybe a night light will do. Verify with a meter.

VOC is with no loading, the SCC is idle. Many SCC have a switched load output. Try putting a load on that. Use a meter to verify if it works and drags down the panels voltage.

 

[Pappion]

2 panels in series is 62v when producing. You will need a high wattage resistor to bring it under 60v. Hopefully it can also bring down voc under 60v too.

The range of VMP to VOC is not linear current, so it shouldn't take much of a load to drop VOC to 60v. About 5 to 10% of IMP should do it.
Look at the graph: VOC VMP IMP

 

[texasjames]

Did you try it? Vmp is not a "start up" threshold. It should still work at 31V. I doubt that would reduce the power by 67% as others say. It's not that big a deal.

I did try it. no go.

 

[texasjames]

interesting. I'll try it

I tried it. volts now reads 50v - now I need to wait for full sunlight.....

 

[texasjames]

I have a 12V brushless pump motor that I run off of (1) 100W panel that is rated at about 18V. I'm wired direct from panel to pump so to attenuate the voltage down and keep it from frying my pump I went completely "old school" and taped some cardboard over part of the panel until I got a meter reading of 12V in full sun. Since it's a brushless motor it will still run (slower) at voltages <12V so clouds/partial shade etc... rarely shut the pump down but "hobbling" the panel protects it from spinning up beyond its design parameters and burning out.

 

[texasjames]

I am trying your idea. I blocked out a single string on one panel. the volts is now under 60 and it IS charging the unit!. however......I'm getting low amps and watts (see pics)

 

[TomC4306]

Sun needs to be at 90° to the face of the panel. No shadows

 

[Hedges]

Shading a single string could work (needs to be completely opaque, else you will get near Voc at zero current.)
At full current under load, bypass diode may fail, could overheat and crack panel.

Opening up junction box and moving PV wire to skip one section was a better suggestion.
Connecting a lower voltage panel is series with one of these higher voltage panels was another good suggestion.

 

[fafrd]

Shading a single string could work (needs to be completely opaque, else you will get near Voc at zero current.)
At full current under load, bypass diode may fail, could overheat and crack panel.

Opening up junction box and moving PV wire to skip one section was a better suggestion.
Connecting a lower voltage panel is series with one of these higher voltage panels was another good suggestion.

Or one of these in a 1S2P string: https://www.amazon.com/Controller-A...9032079&hvtargid=pla-1808085806380&psc=1&th=1

 

[texasjames]

Or one of these in a 1S2P string: https://www.amazon.com/Controller-A...9032079&hvtargid=pla-1808085806380&psc=1&th=1

currently (pun intended) trying it with one string blocked. it is charging at last. but my numbers are low...

 

[gnomie]

I am trying your idea. I blocked out a single string on one panel. the volts is now under 60 and it IS charging the unit!. however......I'm getting low amps and watts (see pics)

Yep. There is a tradeoff. When you physically attenuate the panel, it attenuates the voltage and amperage which reduces the wattage. My little pump runs on 20 watts, so it's never starved for wattage from the 100-watt panel that is shaded to about 60 watts. There may be a better way to do this but ANYTHING you use (resistors, diodes, etc..) to attenuate the panel is going to attenuate the wattage, voltage and amperage to some extent. Basically, you just have an equipment mismatch that is going to reduce the efficiency of the whole system. You can trade up on your controller/charger or downsize your panels or leave the pancho in place and call it a day. P.S. you can use aluminum foil or an old dashboard shade or plastic matt. I recommend something reflective as the excess heat can damage your panels.

 

[fafrd]

currently (pun intended) trying it with one string blocked. it is charging at last. but my numbers are low...

I don’t know what your budget is to make this work, but it is clear that Boost MPPT SCCs are a thing now: https://www.amazon.com/Renogy-Controller-Compatible-Flooded-Batteries/dp/B08QMS7GMR?th=1&psc=1

• 【Boost Charging】Boosts the voltage of 12V or 24V solar panels to charge 36V or 48V batteries

$160 for a 500W SCC is not too bad but if you want to cheap out and purchase from China, there are much less costly alternatives: https://m.aliexpress.us/item/325680...pclist.product.22&gatewayAdapt=gloPc2usaMsite

There is no question that going with any solution based on an MPPT SCC designed to boost a lower-voltage string to charge a higher-voltage battery is going to be safer that attempting to limit a higher-string voltage to prevent damage to an SCC designed for lower-voltage strings…