Can you mix panels of differnt voltages using a MPPT charger?

System
System Posts: 2,511 admin
in Solar Beginners Corner #1
I guess this question belongs here, sorry if this is a bonehead question. ;)

The general rule I understand is that its best to never mix panels that throw voltages that differ more than 1 volt. But I am curious, does this still apply when using a MPPT charger? I imagine it does, but could you run panels with say 2-3 volts difference, or would that negatively affect the overall combined array?

Right now I am using a 175 Watt Mitsubishi, the largest panel I could find at the time that could be used with a regular PMW charger in a small system.

But now I may have access to some older, smaller panels, who throw voltages about 2 volts lower, could these be mixed or no?
· Share on Twitter

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,637 admin
    #2
    Re: Can you mix panels of differnt voltages using a MPPT charger?

    Take a look at the IV curves for the panels and, especially those curves that include power output (you may have to enter the IV data and do your own P=VI graph...

    I will quote myself from another thread:
    BB. wrote: »
    Here is a data sheet for a crystalline silicon panel that includes a couple of graphs/curves that you don't see on most specifications (however, other Si products should behave pretty much identically).

    For a panel with Vmp~35 volts--The 5% loss of power can be estimated from the graph to be:

    +/- 2.5 volts at 1,000 Watts/sqmtr
    +3/-8 volts at 600 Watts/sqmtr

    So, if the panels are within +/- ~5%-10% of "ideal" Vmp--your output will be within 5% of optimum...

    That is a pretty big variation in the IV curve that gives a relatively small Pmp change.

    To get a 25% power improvement--the "micro inverter" would have to do better than:

    Vmp= outside of 42-25 volts => ~25% power reduction (Vmp=35 volts)

    Vmp does not (appear) to need to be to the nearest 1/10th of a volt... Large swings in "computed" Vmp do not make a very large change in the optimum Pmp output.

    And if is one panel in a string of 10--then it would contribute only a 2.5% overall loss of power (assuming 25% out of spec.). Again, no way a distributed inverter can claim an overall 25% possible system improvement.

    Was in context of the Enphase Micro Inverter discussions (one inverter per solar panel)--but you can easily take the PDF link I gave above, change the scales to match your array (Vmp / Imp / Watts) and it will probably be within a few percent of your generic panels (silicon panel properties are primarily defined by the physics--so different vendors should have very similar curves--just offset by the number and size of the cells).

    And, from other places I have read, Thin Film panels have an even "softer/wider" peak Pmax power curve than silicon (so mixing thin film and crystalline panels should not be an issue there either).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
    · Share on Twitter
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: Can you mix panels of differnt voltages using a MPPT charger?

    with 2 or 3v difference from older pvs i'd recommend getting another separate controller for them that way problems are isolated. the controller does not have to be an mppt, but it should have a battery temp sensor.
    · Share on Twitter
  • System2
    System2 Posts: 6,290 admin
    #4
    Re: Can you mix panels of differnt voltages using a MPPT charger?

    Thanks guys! I had a feeling adding these old panels might be a problem.

    Niel- I wish I could add a different charger to the system, but my array is over 150' from my house and Battery bank(the closest place to get full day of sun), the price of the cable for this run was way killer, and running a second line is out of the question for the foreseeable future, DARN! I guess I was hoping a MPPT charger might let me cheat on the voltages for some reason :blush:

    BB- It will take me a while to digest your post, I'm still getting familiar with the terms and math you more advanced Solar folks use, but I am determined to figure it out and understand.

    Thankyou to both!!!
    · Share on Twitter
  • BB.
    BB. Super Moderators, Administrators Posts: 33,637 admin
    #5
    Re: Can you mix panels of differnt voltages using a MPPT charger?

    Don't let the math confuse you... I was just trying to give a generic answer because solar panels come with so many Vmp / Imp ratings...

    Basically instead of saying Vmp=17 volts +/ 1.7 volts or 32v+/-3.2v -- I was just using 5-10% of Vmp/Imp because the numbers scale this way.

    For your system... A true MPPT controller is your answer. They do a very good job of voltage conversion. For your array of high voltage/low current, the controller will down-converter to low voltage / high current at better than 90% efficiency.

    Forgetting the 5-10% losses in the controller. Assume you have a 12 volt battery bank and a 96 volt Vmp solar array of 800 watts.

    The panels will be:

    P=I*V; I=P/V=800 watts / 96 volts = 8.33 amps from your array

    The MPPT solar charge controller will convert to 14.2 volts:

    I=800 watts / 14.2 volts = 56.3 amps

    So, this controller will allow you to run 1/7th the current your 150 foot run, and give you the exact current and voltage you need at full current for the last 3 feet to your controller.

    This allows you to run much smaller wire from the array (less copper, or almost 1/6th the wire costs--remembering you need 2x150'=300' of cable) to your battery house. Then use only the last 2x3'=6' of heavy expensive cable for the final connections.

    Of course, as in all things, there are the details (what is your battery bank voltage, what panels you have, what you need, can you justify a 60+ amp charge controller, etc.)--But there can be a huge advantage to you if an MPPT controller is used.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
    · Share on Twitter

Categories