Parallel PWM controllers

gertvanzyl
gertvanzyl Registered Users Posts: 5 ✭✭
I have a tech question wrt application. I live in isolation and have different size panels which is arranged in two arrays, each with its own pwm controller. They WERE charging in parallel tot my 48v battery bank consisting of 16 batteries stacked in 4x 4 12v banks with a 3kw 48v inverter.

HOWEVER a link in the battery bank came loose, dropping the bank to 36v and blowing my inverter. I have a spare 96v 10kw inverter which will have to bail me out for the next 4 weeks. So i made changes to the battery bank.

I've split the bank in two 48v banks and linked them in series to supply the 96v inverter. However now the banks are charging unballanced and one bank is drained at night way before the other. I need to balance the PV arrays.

My idea is to connect the two pv arrays in parallel onto bars and feed from this "balanced" connection to the two PWM controllers in order to charge the banks at equal rate! Will this work? Will the 96v not "feed" back through the controllers and create a dead short die to the series 96v connection on the battery banks? Can this be done? Or is there another way to balance the charging to independent banks?

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    edited June 2016 #2
    Basically, connect one controller plus 1/2 of the array (fix from "stay" to array) to the low 48 volt bank. And the second controller and array to the high bank.

    Make sure that the charge controllers are insulated from everything.

    Many controllers can have the controller negative lead tired to sheet metal and or external communications leads.

    It will work. But be very careful with your connections. Treat the high controller add if it may have +48 volts (minimum) on its metal and other electrical connection.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭

    "HOWEVER a link in the battery bank came loose, dropping the bank to 36v and blowing my inverter."

    I can NOT picture any way this could happen. If they are hooked up as 48 volt and a cell fails, you might have zero, and I guess in an odd case you might have 46 volts. But you can't get to 36 volts unless something is hooked up wrong?

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • gertvanzyl
    gertvanzyl Registered Users Posts: 5 ✭✭
    edited June 2016 #4

    Hi Bill,

    Thank you for the reply. I need to clarify:

    I am not sure what you mean by "low" and "high" bank as both my banks are 48v? They are then linked with a series pos to neg link to deliver 96v to the inverter.

    Also I am not familiar with  the term "stay"?

    One Response on another forum also believe the 96v will loop back through the controllers once the controller switch closes and will create a dead short.....?? See below:

    "Yes, you will get a dead short.  PWM controllers do not provide isolation, so the two negative outputs will remain connected together through the solar array, thus shorting out the lower bank.

    Options are:

    1) Rewire the solar array to provide >96V

    2) Switch the output of the solar system between the two banks, spending an equal time on both banks.  NOT recommended, and difficult to do; it would have to be a custom design."


    also Bill, I am trying to find a way to bridge the next 4 weeks with same hardware (controllers) due to my location and isolation...

  • gertvanzyl
    gertvanzyl Registered Users Posts: 5 ✭✭

    Photowhit,

    I have 4 sets of 4 batteries. Each set of 4 is wired parallel and result in a row tally 12v. thus linking each row in series to the next will then add: 12 to 24 to 36 to 48 total. One of this series pos to neg links came loose resulting in 3 rows of 12v.

  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    gertvanzyl said:
    I have 4 sets of 4 batteries. Each set of 4 is wired parallel and result in a row tally 12v. thus linking each row in series to the next will then add: 12 to 24 to 36 to 48 total. One of this series pos to neg links came loose resulting in 3 rows of 12v.
    I still don't get it... can you post a drawing of how the batteries are connected and where the broken connection is?  I just don't see how you can get to 36 volts.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    "Stay" was an auto correct on my phone... Was supposed to be "array".

    More or less, you have "two" 48 volt battery banks that are connected in series to 96 volts. You attach one 48 volt solar charge controller (and 1/2 of the array) from negative battery bus to +48 volts.

    And you connect the second 48 volt solar charge controller (with another 1/2 of the array) to the +48 to +96 volt battery bank.

    You cannot share a single array between the two charge controllers. Each as to have 1/2 of the array (no connections between the two arrays). If you where to use "one array", you are correct, the two charge controllers would have the same "+ and -" connections to the array and you would create a dead short across the battery bank (i.e., the negative of the solar array would be directly attached to negative or 0 volt battery bus and +48 "tap" from your series battery bank connections). And similar issue with the +48 and +96 volt shorted together on the + side of the array.

    There are no "isolated" standard solar charge controllers (there are a few "isolated" charge controllers, really isolated DC to DC bi-directional converters, but I don't think they are rated to 48/96 VDC of isolation). And they are not cheap either.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭

    Photowhit,

    I have 4 sets of 4 batteries. Each set of 4 is wired parallel and result in a row tally 12v. thus linking each row in series to the next will then add: 12 to 24 to 36 to 48 total. One of this series pos to neg links came loose resulting in 3 rows of 12v.


    One row comes loose and you don't have a connection to the next row, you have zero volts!, if a connection to one of the 4 batteries in parallel becomes disconnected, you have 48 volts at a lower amperage.

    You have a problem in understanding what happened in the past with your connections. I would work first on understanding what happened in the past before you end up with other problems.

    Sorry I couldn't do a diagram, I don't have photoshop on this computer, and most people hook up their batteries in series first and then in parallel, it typically make finding problems easier.


    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • gertvanzyl
    gertvanzyl Registered Users Posts: 5 ✭✭
    Photowit

    I totally agree. I did not pay much attention to this item at first and jumped to this conclusion but when you asked the question i realized exactly what you are getting to. There can only be the two scenarios you stated!

    I am trying to figure out what caused the failure of the inverter. However i have since then totally rearranged the connections to manage my crisis and cannot recall all the connection.

    One possible cause is that my pv array was wired to 94v delivery by the installer way back. I inhereted the solar system with the house and do disagree with this as in my understanding this is senseless as a pwm controller was installed and 48v battery bank. I will agree with the max voltage if a mppt was used. I kept complaining as my ons set of batteries were "cooking" all the time. Also the pwm's installed was very cheap brand and no settings or user interface was possible.

    I have since replaced the pwm's with better quality and can now control float voltage which i set at 56v. Previously it was going up to 60v in extreme cases.

    As mentioned i think that when the battery link came undone while the controller was pushing the higher whatever voltage straight through to the inverter as the controller and inverter was connected at the same terminal points and could cause high voltage failure on the inverter??
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    If you lost bank capacity due to bad connection, a large enough solar array and less than ideal solar charger can overvoltage the battery bank and possibly damage the ac inverter.

    Is that what happened, it was it something else, difficult to know.

    Over voltage (battery voltage, static charge, lightning, etc.) can really weaken electronics and cause failures immediately or days/weeks /months down the road.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    I should add, 60-62+ volts on a 48 volt battery bank are common high end voltage range.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • gertvanzyl
    gertvanzyl Registered Users Posts: 5 ✭✭
    edited June 2016 #12
    Bill the "norm" set by suppliers max voltage for 12v batteries are given at 14,0-14,2v/battery. This in my mind i should be at 56v max?

    Since i changed the controllers and set the float at 56v, I do not have cooking batteries anymore. Thus I believe that the previous "less than ideal" controllers caused the overcharging/cooking issue?

    Why would one want higher voltage than battery max if you use pwm controller as in my understanding it cannot really "do" anything with the eccess voltage?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Very cold flooded cell batteries with temperature compassion and equalization and/or industrial traction batteries will require this high of voltage sometimes during normal charging/maintenance charging.

    Warm agm batteries will usually be much lower voltages. But I would expect a standard inverter to accept the wider voltage range.

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