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632 posts

Ultimate Geek

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  #2411981 3-Feb-2020 17:14
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What model MPPT controller are you using?

 

If the controller has a max input voltage greater than 66 volts, you could try

 

One string made up of 1 x 12v panel and 1 x 24v panel wired in series.
and
One string made up of 3 x 12v panels wired in series.

 

Parallel wire the strings to the MPPT controller.
This should give you a no load voltage of about 60v and a loaded voltage of about 54v at the input of the MPPT

 

 

 

 

 

 


2610 posts

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  #2412019 3-Feb-2020 19:39
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Have a good read here:

https://solarpanelsvenue.com/mixing-solar-panels/

 
 
 
 


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  #2412133 3-Feb-2020 22:21
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NectarBomb:

 

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...?

 

 

The only way you'll get to 30 volts on the panels with the higher voltage is when the current draw exceeds what they can deliver. Then the voltage will start to drop. Once they reach 30 volts then the other panel with start to feed current into the system.

 

In other words you will need to run the higher voltage panels above their capacity before the lower voltage panel will contribute anything.

 

Yes, you should end up with more amps/watts but with both panels (32v and 30v) wired in parallel you will never see any current flow from the lower voltage panels on your clamp ammeter until the other panels are well and truly maxed out. 





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4985 posts

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  #2412226 4-Feb-2020 08:49
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djtOtago:

 

What model MPPT controller are you using?

 

If the controller has a max input voltage greater than 66 volts, you could try

 

One string made up of 1 x 12v panel and 1 x 24v panel wired in series.
and
One string made up of 3 x 12v panels wired in series.

 

Parallel wire the strings to the MPPT controller.
This should give you a no load voltage of about 60v and a loaded voltage of about 54v at the input of the MPPT

 

 

 

 

This (I think).

 

What is the model of your MPPT? What is it's rated input current/voltage? MPPT controllers should be able to handle 12,24,36V panels.

 

You do need to work out why the new big panel isn't charging your batteries on its own (unless the batteries are full?).




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  #2412449 4-Feb-2020 13:16
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This is the MPPT I currently have in use.

 

https://www.jaycar.co.nz/12v-24v-30a-mppt-solar-charge-controller/p/MP3735

 

I am not overly fond of it however as it only shows the amount of amps coming in and going out at that point in time. If anyone knows of one which records the input and output over time that would be awesome.

 

It is definitely not because the batteries are full as as soon as I remove the the 24v panel the amps jump up.


1546 posts

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  #2412986 5-Feb-2020 12:27
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(EDIT: sorry formatting of tables got munched ... might be semi readable)

 

 

 

Your MPPT controller is a bit low end for what you want to be honest, the max PV open circuit of 65volts doesn't give a lot of headroom, and I think you would exceed the 30amp maximums if you added the 24v/290W panel to the mix

 

Scribbling in Excel I get your old setup to be (these are rough maximums and a few guesses on your panels and probably shonky maths as I'm also learning so I can upgrade a dodgy setup I got when I bought an off-grid cottage):

 

Original 12v config :

 

String Size 1 Number of strings 4 Array voltage 18.1 Array amps 34.64 Max Open Circuit V 22.40 Charge Voltage 12.00 Amp multiplier 1.51 MPPT charge amps 52.25 Controller Efficiency 95% Array Watts 595.6348

 

- Even that setup is too high for your MPPT controller limit of 30amps.

 

24v setup with old panels

 

String Size 2 Number of strings 2 Array voltage 36.2 Array amps 17.32 Max Open Circuit V 44.80 Charge Voltage 24.00 Amp multiplier 1.51 MPPT charge amps 26.12 Controller Efficiency 95% Watts 595.6348

 

- Not much headroom on open circuit voltage or amps

 

then add in an extra string which is your new panel :

 

String Size 1 Number of strings 1 Array voltage 30.5 Array amps 9.52 Max Open Circuit V 37.80 Charge Voltage 24.00 Amp multiplier 1.27 MPPT charge amps 12.10 Controller Efficiency 95% Watts 275.842

 

- And you'd go past the 30amp mark again.

 

 


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  #2413280 5-Feb-2020 19:08
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I know with inverters it's generally considered OK, and even ideal, for panel capacity to exceed inverter capacity. Most of the time, more panels equals more power, but the additional inverter capacity isn't used.

 

You only lose a few percent capacity, and only for a very small fraction of the time. This is far outweighed by the percentage increase given by more panels whenever there is any sunlight, which you get year-round..

 

 

 

This is doubly important when you consider that the panels will likely only be hitting design capacity during summer, when you've likely got an excess of power or electricity is cheap. During winter, you want every bit of surface area that you can and are unlikely to hit the inverter cap anyway.

 

 

 

This all assumes the MPPT controller can safely back off if it reaches a thermal limit, though.


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