# Restrict output from panel?

Solar Expert Posts: 56 ✭✭
A little background so you understand what I want. I am in the process of buying some acreage and will be building an off grid cabin as funds are available. I don't have any experience with solar but do have a backup battery system that I put together in my current house and have been slowly researching/lurking for a few years now so I do have some knowledge.

Anyways, in anticipation, I bought a 260 watt LG panel to experiment with (got a heck of a deal on it so I couldn't pass it up). I am planning on using it in my current house to keep the batteries topped up and learn.

Now, once I get the land I want to put together a system like the solar watering systems for livestock to slowly fill a tank from a well. I was thinking I would use this 260 watt panel for that since it will be an oddball and order a different set of panels to run the cabin. I was looking at the solar pumps on NAWS and the ones I am looking at use about 150 watts. I then checked out the sunpumps controllers for these and they say maximum load power 150 watts, max load current 5 amps, max open circuit voltage 45 volts. My panel is the LG260S1K-A3. It is 260 watts, max current 8.34 amps, max open circuit voltage 38.2. Is there a way to use something like a resistor to keep the input from the panel to the controller below the controllers max? Or can someone educate me anymore on this? Thanks for any help!

No, there is no easy way to limit the output voltage of a solar panel.

However, there is one trick that may work--If you can open the rear junction box of the panel, you may see several connections and diodes. Take your volt meter, black lead on the negative terminal, and take the red lead and measure the voltage to each other terminal... If you are lucky and there are three bypass diodes/panel circuits... You may be able to see 13/26/39 volts Vmp (Voc will be higher). Connect the positive lead to the 26 volt Vmp (probably around 36 volts Voc) and you can use 2/3rds of the panel's output power--And you can revert back to full power for the next application.

The only other possibility I can think of (assuming you don't want a 12/24 volt battery bank), is to see if you can find a Linear Current Booster--Sort of like an MPPT to DC pump controller (no battery).

http://www.solarconverters.com/index.php/products/83-power-pump-tracker-linear-current-booster/116-ppt-12-24-7

http://www.solar-electric.com/7amplincurbo.html

Not real cheap--But should work for your application (I think--I have not reviewed all the specifications for panel/LCB/pump).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 3,009 ✭✭✭✭
Hi Levsmith, welcome to the forum.
Read over your post a couple of times, and perhaps I still don't really understand, but I'm not seeing what you need to reduce using for instance a resister.
Your controller's max input voltage you have listed as 45 volts. The panel's max voltage is more or less 38, therefore you have a safety margin of 7 volts, which is fine.
The MAX current you have listed for your panel, is 8.34 amps. OK.
You've listed your controllers MAX LOAD current (not the same as the max current available source, which is a different thing) as 5 amps. OK
Now to the unknown - - - - - WHAT is the max current DRAW of your pump? IF it's less than 5 amps, your controller will handle it no problem.
However, it the PUMP draws more than 5 amps, you're in trouble and will need a stronger controller with higher ratings, and depending on pump motor requirements, you may also need more PV panel/s. OH, and what VOLTAGE does your pump require?
Hope this is of some help.
• Solar Expert Posts: 56 ✭✭
Hi Levsmith, welcome to the forum.
Read over your post a couple of times, and perhaps I still don't really understand, but I'm not seeing what you need to reduce using for instance a resister.
Your controller's max input voltage you have listed as 45 volts. The panel's max voltage is more or less 38, therefore you have a safety margin of 7 volts, which is fine.
The MAX current you have listed for your panel, is 8.34 amps. OK.
You've listed your controllers MAX LOAD current (not the same as the max current available source, which is a different thing) as 5 amps. OK
Now to the unknown - - - - - WHAT is the max current DRAW of your pump? IF it's less than 5 amps, your controller will handle it no problem.
However, it the PUMP draws more than 5 amps, you're in trouble and will need a stronger controller with higher ratings, and depending on pump motor requirements, you may also need more PV panel/s. OH, and what VOLTAGE does your pump require?
Hope this is of some help.

Wow, I dont know how I missed that. LOAD is the pump correct? So the only numbers I need to worry about as far as the panel is concerned is the max voltage?
This is the pump I am looking at: http://www.solar-electric.com/wind-and-water-products/sodcwapu/sun-pumps/susdssusopua/sdstsesupu/sdsosupu.html
This is the controller: http://www.solar-electric.com/wind-and-water-products/sodcwapu/sun-pumps/sopuco/pcasepuco/pca-30-m1d.html

So the MAX motor watts for the pump is 146 watts and MAX current is 4.9 amps, which are both under MAX on the controller. And my open circuit voltage on the panel is 38.2, which is also under the max of the controller. So I shouldn't have any problems using that panel? Thank you for clearing that up Wayne, that makes much more sense now.

Now one more question... Looking at LG's website, it shows the Normal Operating Cell Temperature numbers for this panel. It shows 28.3 is the MPP Voltage. Does this mean I would probably have to set the controller at 24 volts rather than 30 or do you think it acts like an MPPT charge controller and decreases the amperage to increase the voltage to what is needed? Not a huge issue but it would bring the pump from about 2 gallons per minute down to about 1.5 gallons per minute if it was switched to 24v.
• Solar Expert Posts: 56 ✭✭
BB. wrote: »
No, there is no easy way to limit the output voltage of a solar panel.

However, there is one trick that may work--If you can open the rear junction box of the panel, you may see several connections and diodes. Take your volt meter, black lead on the negative terminal, and take the red lead and measure the voltage to each other terminal... If you are lucky and there are three bypass diodes/panel circuits... You may be able to see 13/26/39 volts Vmp (Voc will be higher). Connect the positive lead to the 26 volt Vmp (probably around 36 volts Voc) and you can use 2/3rds of the panel's output power--And you can revert back to full power for the next application.

-Bill

Thanks for your response Bill. Looks like I may have been leading you guys astray with my figures, not realizing the load was the pump and not the panel. Anyways, that is interesting about switching leads. I'll have to check my panel just out of curiosity. That info may come in handy some day...