Simple configuration question

GekoGeko Registered Users Posts: 6
Hi all,

I have a very simple question for a particular setup. I live in Europe, not that much sun here. I am building a 24v system. I have eight 230W panels (Pmax 230W; Isc 8.30A; Voc 36.6V; Impp 7.75A; Vmpp 29.65V - standard test conditions) and an Outback FM80 MPPT charge controller. I currently have a 24V battery bank of four 150Ah batteries, will expand it shortly with two additional ones.

My question - how to wire the panels for maximum efficiency.

Option one: wire all panels in parallel.
Option two: wire each pair of panels in series (giving me 4 pairs of ~ 70V each) and then wire all pairs in parallel.

The panels are on a roof and I have no shade problem. I am just looking for maximum efficiency. The charge controller can handle both options. The "all panels in parallel" option is going to give me a somewhat lower voltage in typical conditions than the recommended for 24V system in the charge controller manual (which is "36V or higher" - not sure why they say this).

Thanks in advance for your advice.

Comments

  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Simple configuration question

    Hi,

    Option 1 will destroy your batteries over time because of chronic undercharging. The absorb charge voltage should be around 28.8V, with a Vmp of 29.6V and taking into account losses in the charge controller, wiring and then slightly warm weather and the voltage would drop to well below that.
    Your only option is number 2; with 4 parallel strings of 2 panels each.
  • GekoGeko Registered Users Posts: 6
    Re: Simple configuration question
    stephendv wrote: »
    Hi,

    Option 1 will destroy your batteries over time because of chronic undercharging. The absorb charge voltage should be around 28.8V, with a Vmp of 29.6V and taking into account losses in the charge controller, wiring and then slightly warm weather and the voltage would drop to well below that.
    Your only option is number 2; with 4 parallel strings of 2 panels each.

    Thanks. Isn't, however the job of the MPPT to compensate for that?
  • BB.BB. Super Moderators, Administrators Posts: 29,724 admin
    Re: Simple configuration question

    As Stephen says...

    Vmp (voltage maximum power) is sensitive to panel temperature... In very hot weather under full sun, Panel Vmp can drop by as much as 20% from standard test conditions (which are around 25C):
    • 29.6 volts * 80% = 23.7 volts (approximately)
    You need panel Vmp > ~35 volts to be able to consistently charge your battery bank to ~29 volts (25C), and to get >~31 volts to equalize your battery bank (if flooded cell).

    Add a couple volts for wiring voltage drop and controller voltage drop--and you are looking at >~33 volts nominal for minimum Vmp panel voltage (note: Vmp is not a sharp curve--so you can be within ~10% of Vmp/Vbatt-charging and still get fairly OK results).

    Your panels--you need the 2x in series to recharge your battery bank.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Simple configuration question

    The currently available MPPT controllers are buck converters which means that they can only down-convert voltage, they can't boost voltage.
    In any case, going for higher voltage on your array will also let you use thinner cabling.
  • GekoGeko Registered Users Posts: 6
    Re: Simple configuration question

    Oh, I see.
    Thank you so much guys for the input! The thing about MPPT not able to boost voltage is new to me and very useful info.
  • GekoGeko Registered Users Posts: 6
    Nighttime radiation

    Ok, now, maybe a very stupid question, but I have to ask.
    I have noticed that my panels give 10V each during the night (moonless, pitch dark night). They probably pick up some kind of radiation or something. So... what if I make the system so that all of the 8 panels are connected in series only during the night. That should give about 80 volts... I am probably missing something here, but since the MPPT is able to transform volts into amps, wouldn't I be able to use this power at night? I searched the forum and the internet and couldn't find anything related. Maybe I couldn't figure out what to search for.
  • BB.BB. Super Moderators, Administrators Posts: 29,724 admin
    Re: Simple configuration question

    Actually, no, the panels are not producing any measurable energy/voltage at night.

    However, there was a known problem with FM 80 controllers that would have problems not shutting down at night:
    wagoneer wrote: »
    Ok guys!!!

    I have found some info on Outback's Forum relating to system producing power at night

    Apparently as B.B stated it was a known issue from back in 2008.
    • The permanent fix is to send the FM80 back for a firmware upgrade.
    • While the work-around is to use Auto Restart Mode 2 (in Advanced menu).
    dwh - sorry for almost causing you to spew your coffee with the typo :D

    Thanks to all for your support. This is truely an amazing forum.

    Call your local Outback Rep/Distributor and/or check the Outback Forum for details (their forum is very slow right now).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • GekoGeko Registered Users Posts: 6
    Re: Simple configuration question
    BB. wrote: »
    Actually, no, the panels are not producing any measurable energy/voltage at night.

    However, there was a known problem with FM 80 controllers that would have problems not shutting down at night:



    Call your local Outback Rep/Distributor and/or check the Outback Forum for details (their forum is very slow right now).

    -Bill

    Hmm... Thank you, that's interesting. I will try to measure the panels directly tonight. I read about the Outback problem as you suggested. I have the latest firmware, my controller is brand new, and what I observe seems to be a little different from what people are reporting about. Also, my panels are not typical - they use a new technology developed by Q-Cells in Germany. But you are probably right. I will measure precisely.
  • GekoGeko Registered Users Posts: 6
    Re: Simple configuration question

    Well.... Problem with the controller. I don't have magic panels :(
    The same issue like the one reported here http://forum.solar-electric.com/showthread.php?t=4643
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