Solar Panel Match

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keyturbocars
keyturbocars Solar Expert Posts: 375 ✭✭
in General Solar Power Topics #1
Hello,

Last year, I picked up a 117W solar panel to experiment. The other day, I bought 3 more panels (130W) from the same company. I didn't really think about the mismatch of voltages causing any problem. I figured they all just add up in series and be fine, but now I've read some stuff related to voltages.

The 117W panel is rated around Vmp = 17.2V.
The 130W panel is rated around Vmp = 18.0V.

What exactly happens when hooking up panels like this in series? Did I really screw up going this route?

I already had the 117W panel and I didn't want to exclude it. I am planning to hook up the 4 panels to charge a 48V battery bank. All 4 panels look identical but I guess the 117W panel was just rated lower because of the specific solar cells not giving maximum output.

Thanks for the insights,

Ed
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
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    Re: Solar Panel Match

    Ed,

    What is the Imp of the panels?...

    As it stand right now, I would probably just use the 117 watt panel just like the 130 watt panels in any series/parallel connection you need... They all should be within 10% of the Vmp/Imp. And there should be no "issues" of reduction of power output due to the "small" mismatches.

    Minor changes in panel output ratings probably are a cross between sorting of panels (some batches of cells produce a bit more power than other batches of cells) and "optimizing" specifications (sometimes, they can increase specifications as their production optimizes/stabilizes at the high end of the bell curve).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #3
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    Re: Solar Panel Match

    Thanks for the reply Bill.

    The Imps are around:

    Imp (117W) = 6.89A
    Imp (130W) = 7.38A

    I was worried that I really screwed up and the combination of these panels would somehow really be "handicapped" because of my ignorance. If the performance would be affected severely, then I could just buy another 130W panel.

    In the future, I might add another parrallel string of 4 more 130W panels. Not sure if that would change anything. Just don't want to save a little money by keeping this 117W panel and dragging down the rest of my panels, and in the end not saving any $ after all.

    Thanks for your insights,

    - Ed
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #4
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    Re: Solar Panel Match

    At this point, you have the panels so give it a try... At the worst, the "whole array" is dropped by XX% instead of the lesser energy of the 117 watt panel.

    If the loss is enough--You can still order the replacement panel and EBay off the extra one (or keep it around for other projects). There should not be any damage caused by this experiment.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #5
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    Re: Solar Panel Match

    OK. Thanks Bill. What would you recommend as the best way to test how much the 117W panel affects the other 3x 130W panels in the 4 panel array?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #6
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    Re: Solar Panel Match

    My guess,

    On average, your mid day maximum power output from your charge controller would be around:
    • 71% average nice day (controller out watts / panel STC ratings)
    • 77% on a very good day
    • 86% on a spectacular cold clear day
    Note, the above are just guesses based on my GT system... Your actual power output will depend on battery state of charge and other variables.

    Also, Solar Guppy has seen controller that report solar charge controllers report power output, on the high side, of 5-10% from major brands. And one one brand, it can be upwards of 30% error (controller works very well--it just was not designed to report exact current with a precision shunt).

    So, with all of the variables--unless you are doing the current/power measurements with your own DMM and calibrated solar irradiation monitor--you can only probably tell to a ~20% accuracy what the true power output is for most setups/controllers.

    If you are really concern, I would measure the voltage across each panel (with the charge controller charging a 1/2 discharged battery bank (or other connected load to ensure full charging current output). If the voltage across the 117 watt panel is within 2 volts or so of the other panels--then you are probably not losing much power at all...

    Sorry for the long around about explanation... It is actually quite difficult to get "accurate" power measurements from solar charge controllers without a bunch of lab gear.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #7
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    Re: Solar Panel Match

    Related to hooking up this 4 panel array to my 48V battery bank, do I need to use a blocking diode?

    I found out from the panel manufacturer that they have bypass diodes, but I don't know if this means that I don't need a blocking diode. I was planning to not use a solar charge controller, as my 48V battery bank is regulated with a controller set as dump load at 54V. I heat hot water with my dump load.

    I was just going to make a small box with ammeter, circuit breaker, blocking diode (if needed), and hook up my solar input directly to the battery bank.

    Not sure if I am missing something in my thinking.

    Ed
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #8
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    Re: Solar Panel Match

    Thanks Bill for the detailed information. It is a big help as I'm just learning about this solar stuff.

    - Ed
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #9
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    Re: Solar Panel Match

    If you are going to direct connect--For a 48 volt system, yes you will need a blocking diode (no solar charge controller).

    Also, you will need a series protection fuse to protect the solar panels from shorts/back-feed from the battery bank (fuse from data sheets or around 2xIsc rounded to nearest standard fuse size).

    Usually, for things like blocking diodes, code assumes that one diode will fail, so you need either a second blocking diode and/or a fuse/breaker to protect against a single point failure.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #10
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    Re: Solar Panel Match

    Thanks again Bill!
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #11
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    Re: Solar Panel Match

    I decided to buy another matching 130W panel and save the 117W panel for another project that I have where I need a single panel.

    Has anyone heard anything good/bad about Solartech Power solar panels?

    These are the ones I bought...

    http://www.solartechpower.com/solarpanelsw/solar_panel_130WATT.html

    I can say that they appear to be constructed well and have quality appearance to them. I understand that this doesn't necessarily mean that they will last as long as other name brand panels. I paid $330 each for the 130W panels and that included the freight. I know it's possible to get some blemished panels for a good price too, but this is what I got.

    Look forward to setting them up soon when my other projects and workload allows.

    Ed
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