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Adding mis-matched panels in series?

Woodpens

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Feb 13, 2022
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I started a small installation last summer on my backyard shed to mainly charge my lawn tractor battery. Started with a Renogy kit of 50watt Mono panel, Renogy 10A PMW controller and one deep cycle 12v battery for storage. Worked well and I wanted to expand a bit to be able to use it for a few hours of power outages for the house, so I added another battery in parallel to double the storage. Then I upgraded to a Renogy Elite 20A MPPT controller and added another Renogy 100w mono panel that I hooked in series to increase voltage. Everything seems to be working well and I am showing 44v at the panel on sunny days. My question is the two panels have different specs. The 50w has open current of 21.8V and optimum current of 2.9A. The 100w panel has open circuit of 24.3v and optimum current of 4.93A
Am I in any danger with this system being mismatched? Would I be better to take out the 50w panel and replace with another 100W? I am thinking of adding another 100w panel to what I have now making it 250w. I appreciate any suggestions, thanks!
 
In my signature there is a link to mismatched solar panels.

B893E6C3-E50B-4FDC-85E8-30CABC17D714.jpeg
With a 50 watt panel, and a 100 watt panel I would not be surprised if you end your with a 100 watt output.
 
The 50w has open current of 21.8V and optimum current of 2.9A. The 100w panel has open circuit of 24.3v and optimum current of 4.93A
The theoretical output (for series) should be:
(Vmp panel 1 + Vmp panel 2) x lowest Imp of either panel:

Guessing numbers here:
(18V + 20V) x 2.9A = 110.2W

In parallel the voltages being very similar might show to be better:
(Imp panel 1 + Imp panel 2) x lowest Vmp of either panel:

More guessing:
(2.9A + 4.93A) x 18V = 140.9V
 
Thanks! If I am seeing 44v at the controller with them in series wouldn’t that mean that they are working together well? I am new at this!
 
If I am seeing 44v at the controller with them in series wouldn’t that mean that they are working together well?
What is the amp output? That's the other half of the story. I don't know what "optimum current" means. Is that Imp or Isc?

Either way, multiply what the output amps are (or likely lowest listed Imp), by the voltage and that should be your watt output.
 
I guess I can try parallel and see if it puts out more charging amps when the battery is drained some?
The rub for having them in parallel, is that they may start charging later in the day, when the voltage gets about 5V above battery/charging voltage. Same for later in the day and low light (cloudy) conditions. So a momentary reading in full sun may not reflect the daily harvest reality.
 
Thanks MisterSandals, that’s what I read in some RV forum, that is you have an MPPT controller it may be better to connect in series. My controller is rated to 100v so I think I could add two more 100w panels and still be ok. I am just wondering if I should take the 50w panel out altogether so everything works together better?
 
The current flow will be limited by the lowest panel in the series string so you have a maximum current of 5 amps maximum. It would be much better to use two strings of near equal panels

Two strings....
String one.....2 152 watt panels for 38 volts times 8 amps equals .......304 watts
String two 1 ea. 85 watt and 1 126 watt 35 volts times 5 amps....... 175 watts
"...........................................................string mismatch penalty ........-10 watts
..........................................................................................................469 watts or so

All in series 73 volts times 5 amps .............................................375 watts or so


That would be a gain of approx 100 watts approx....

Your results may vary, but I hope you get the idea here..

Strings of panels in series should be same current
Strings of panels in parallel should be same voltage

Everybody needs to start somewhere
 
I would connect the 50w panel to the PWM and the 100w to the MPPT, there's really no harm is having 2 different charge controllers connected to the same battery.
 
I would connect the 50w panel to the PWM and the 100w to the MPPT, there's really no harm is having 2 different charge controllers connected to the same battery.
That's what I did for my driveway lights. I've got a 100w on a PWM and a 208w on an MPPT both going to the same battery. It helps... stupid PNW winters... Grrr...
 
Testing update, I ran a 900watt, (7.5A )heater today and my panel was putting out 42V and around 8amps charging so it was alm,ost maintaining in direct sun. I’m thinking adding another 100w panel in series and I will be good for that load in emergencies?
 
better yet, buy another 126 watt panel and you will gain 145 watts or so if you make 2 strings in parallel
 
ran a 900watt, (7.5A )heater today and my panel was putting out 42V and around 8amps charging so it was alm,ost maintaining in direct sun.
That’s 336W solar input which is 1/3 unless I’m missing something.
 
I bought another Renogy 100w panel and tried three different ways to connect it. Tried running the two 100w panels in series by themselves with the 50w disconnected, then the same with addition of the 50w panel with a combiner in parallel, then all three panels in series. That yielded the best result: 65volts at the controller, and charging my batteries at 12 amps in full sun, this was pulling a 3.3 amp load for my shop window air conditioner (new Midea 10,000BTU very efficient unit) I was able to maintain the battery voltage in full sun with this load! I have room to add one more 100w panel on my shop roof to max out this system and I am very impressed about this small backup system that has multiple uses! Thanks for all the info on this board.
 
I have taken the recommendations of the contributors to this thread and have removed the 50 watt panel from my system, I am just using it with the original Renogy 10A PMW controller to provide a constant charge to my SLA lawn tractor battery.
I have connected my other three 100w panels in series and it is providing about 68v to my Renogy 20A MPPT controller. I am getting an Ampere Time 100AH Lithium battery in a couple days to replace my existing lead acid deep cycle batteries. I have read that I need to fuse the positive cable from the controller to the battery. Would I use an ANL 30A fuse there close to the battery? Thanks
 
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