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25 posts

Geek


#265624 1-Feb-2020 14:54
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Hello all,

 

 

 

I have an off grid powered house powered by 6 x 12v batteries all hooked up in parallel. I also have 4 x 150w 12v solar panels all hooked up in parallel too. Things were good however I decided to move up to 24v. I rewired the batteries so that they were now in three pairs so they were 24v. I did the same with the solar panels, putting them into two pairs so they were also 24v.

 

 

 

My mppt can switch between 12 and 24v with auto detection.

 

 

 

Hooked it all up and everything worked.

 

 

 

Happy days.

 

 

 

HOWEVER

 

 

 

I recently bought a new 290w 24v solar panel and added it to my array. The others being functionally 24v and this one being 24v I thought there would be no issues. So I hooked it up and was surprised to see no increase in power. Checked all of the connections and they were fine. I even removed the other panels to just have this one running and still no amps would flow (confirmed this with fluke clamp metre). Checked the volts and it was as expected approximately 30v. I connected just the original panels up and everything was fine, around 15 amps. However as soon as I attached the new panel it dropped down to about 9 amps as if the panel was drawing amps instead of adding them.

 

I contacted the supplier and they sent out a replacement panel. However after hook it up today I am having the same issues. The amount of amps going to my mppt DROPS when ever I add the new panel to the array.

 

 

 

Any help would be much appreciated as I am very confused at this stage.

 

 

 

Thanks!

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196 posts

Master Geek

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  #2411172 1-Feb-2020 15:30
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The panel needs to be in parallel with existing array of panels, positive to positive and negative to negative.

 

 

 

Try the new panel on its own.

 

 

 

Diagrams?


196 posts

Master Geek

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  #2411173 1-Feb-2020 15:32
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Also, MPPT can have issues with local maximums with different panel types. I think this should be less of an issue with a 24V system, but you may need a separate controller.


 
 
 
 




25 posts

Geek


  #2411235 1-Feb-2020 18:12
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Hello there,

 

Please find attached my arts and craft project. 

 

Ok here is a picture of the original 12v system:

 

 

This is my upgraded system, as you can see I have taken the 4 12v panels, put them into series of two and then put those two pairs into parallel creating a 24v system, this works fine

 

 

Here is where there becomes an issue, all I have done is added a new 24v solar panel into parallel however for what ever reason it does not increase the amount of amps, and for what ever reason sometimes it decreases the amount of amps. With a 290w increase I should be getting a noticeable increase in power.

 


196 posts

Master Geek

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  #2411238 1-Feb-2020 18:19
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Yeah, that probably should work. My concern is that the slight difference between the panels means it's not pulling the voltage low enough for the existing banks of panels to start supplying current - the voltage at MPP of the new panel is higher than the open-circuit voltage of the old panel pairs.

 

Can you measure the voltage on the MPPT input of the existing panels only (diagram 2), both sets (diagram 3), and the new panel on its own?

 

Try putting maybe 1/10th to 1/20th of an ohm of resistance in series with the new panel. This should reduce its output voltage enough for the old panels to start conducting. You'll need about a 15W resistor or a large coil of fairly small wire.

 

I think a second MPPT would be your best option.




25 posts

Geek


  #2411239 1-Feb-2020 18:35
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Cheers for getting back to me so quick

 

So earlier today I checked the older panels, they were at about 34v and at the same time the newer one was outputting about 30v. This was a consistent trend throughout the day with the newer panel always being a few volts lower than the others.

 

What would this indicate? Do they need to be almost the same voltage to work, I assumed that it would just drop to the lower voltage and put through the amps at that rate.


696 posts

Ultimate Geek


  #2411242 1-Feb-2020 18:45
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" I even removed the other panels to just have this one running and still no amps would flow (confirmed this with fluke clamp metre)."

 

This is the problem that needs solving. The panel doesn't work with your MPPT!

 

Does the panel supplier have any suggestions, or test instructions, rather than just giving you another one?

 

Was the 30v open circuit, or with the panel connected to the MPPT?

 

What are the make, model, and part numbers of the panels and MPPT?


196 posts

Master Geek

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  #2411248 1-Feb-2020 19:06
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I didn't see that bit of the OP, sorry.

 

 

 

Does your clamp meter measure DC amps? Most are AC-only.

 

 

 

I can't think of much that would intrinsically prevent the panel and MPPT working together.

 

 

 

Can you measure the short-circuit current and open circuit voltage of each set of panels?


 
 
 
 


3854 posts

Uber Geek

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  #2411273 1-Feb-2020 21:10
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NectarBomb:

Cheers for getting back to me so quick


So earlier today I checked the older panels, they were at about 34v and at the same time the newer one was outputting about 30v. This was a consistent trend throughout the day with the newer panel always being a few volts lower than the others.


What would this indicate? Do they need to be almost the same voltage to work, I assumed that it would just drop to the lower voltage and put through the amps at that rate.



Are these open circuit voltages or voltages at the input to the MPPT. I'm assuming you've measured at the MPPT.

It's my guess the newer panel is absorbing power from the older panels since it is at a lower voltage. Current always flows from a higher voltage source to a lower voltage source. In crude terms the older panels are supplying power to the MPPT and the new panel.

As has been already suggested your best option is to used another MPPT and parallel their outputs. Before you buy another MPPT it might pay to check that both MPPT's you plan to use are compatible to be paralleled with each other.

I'm guessing they should be compatible but then again if one is set up at a slightly higher output voltage and there is no method for both MPPT's to harminise their output voltages you could end up in a situation where only one MPPT is supplying the load. You won't get the benefit of having the extra solar panel.




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196 posts

Master Geek

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  #2411279 1-Feb-2020 21:44
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I'm guessing they should be compatible but then again if one is set up at a slightly higher output voltage and there is no method for both MPPT's to harminise their output voltages you could end up in a situation where only one MPPT is supplying the load. You won't get the benefit of having the extra solar panel.

 

That shouldn't be an issue. If the load is larger than one MPPT can supply, then the battery voltage will sag until the second MPPT kicks in. If it doesn't sag, then your batteries are fully charged and there's nowhere for the power to go anyway.

gzt

11450 posts

Uber Geek

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  #2411281 1-Feb-2020 21:52
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Some random ideas. Don't know a lot about solar installs.

Panels are often diode protected so current can flow only one way and avoid damage. I expect your old panels are because you would have problems by now.

Is the new one? With the panel 100% dark and disconnected I expect your multimeter will show a low resistance in one direction and a higher resistance in the other direction if it is diode protected.

3854 posts

Uber Geek

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  #2411283 1-Feb-2020 21:55
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SomeoneSomewhere:
That shouldn't be an issue. If the load is larger than one MPPT can supply, then the battery voltage will sag until the second MPPT kicks in. If it doesn't sag, then your batteries are fully charged and there's nowhere for the power to go anyway.


Fair comment, that will work.

Though I still think it's better, if possible, to have both outputs harmonised.




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687 posts

Ultimate Geek


  #2411312 2-Feb-2020 00:22
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If current is really flowing from one panel/group of panels to another parallel wired panel/group of panels, then perhaps it's worth adding a series diode, maybe a Schottky power diode for low voltage drop.





#include <standard.disclaimer>


696 posts

Ultimate Geek


  #2411385 2-Feb-2020 10:20
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Not wishing to offend anyone, but from my earlier post.

 

"I even removed the other panels to just have this one running and still no amps would flow (confirmed this with fluke clamp metre)."

 

This is the problem that needs solving. The panel doesn't work with your MPPT!

 

 

 

The only thing I would add is that I assume that the OP connected the second new panel to the original panels.

 

Could the new panels have been damaged by the higher voltage from the original panels when connected in parallel?

 

 

 

My suggestion would be to disconnect all the panels, get another new panel and connect it to your MPPT on its own. This would confirm if these panels work with your MPPT.


196 posts

Master Geek

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  #2411386 2-Feb-2020 10:25
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k1w1k1d:

 

Not wishing to offend anyone, but from my earlier post.

 

"I even removed the other panels to just have this one running and still no amps would flow (confirmed this with fluke clamp metre)."

 

This is the problem that needs solving. The panel doesn't work with your MPPT!

 

 

 

The only thing I would add is that I assume that the OP connected the second new panel to the original panels.

 

Could the new panels have been damaged by the higher voltage from the original panels when connected in parallel?

 

 

 

My suggestion would be to disconnect all the panels, get another new panel and connect it to your MPPT on its own. This would confirm if these panels work with your MPPT.

 

 

 

 

Yes and no. As I said in my previous reply, a standard clampmeter won't measure DC.

 

There's no good reason why a given panel of the right voltage won't work with an MPPT controller.

 

 

 

It could be outright faulty; the best way to check that would be to test both the short-circuit current and open-circuit voltage while completely disconnected from the MPPT.




25 posts

Geek


  #2411974 3-Feb-2020 16:42
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Hello again all,

 

Thank you for the many replies. I have been struggling to test any of the hypothesises as it has been cloudy. However it seems like both of the 24v panels do work however I have not been able to retest them with the ones in series. I may have been too quick to write off the first panel.

 

I have been measuring both the closed circuit and open circuit voltages, I will try and list them here for consistency. I am using a clamp meter which measures DC amps.

 

Anyways I will conduct the additional testing and find out what is going on.

 

I guess my main question is that if I have panels of slightly mixed voltages how will they even out? Do they just drop down to the lowest voltage and if so should I still not have more eventual amps / watts?

 

Eg

 

If I start with:

 

32v * 10 amps = 320w (leaving out the mppt magic)

 

Then get a:

 

30v (lower volt panel) * 8 amps = 240

 

Add them together with the voltage drop

 

30v * 10 amps = 300w
30v * 8  amps  = 240w

 

I should end up with 540w, much more than the initial 320w...?


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