sma ac coupling

I have a 11kw array with two sma inverters. I want to add 2sunny islands and battery back up. I would like to use nickel Iron batteries. Is there any reason why I couldn't do this myself, installers all want 10 to 12k above equipment to install?

Comments

  • Solar GuppySolar Guppy Solar Expert Posts: 1,963 ✭✭✭
    Re: sma ac coupling

    The SI's alone are 10K, another 20K for battery's to support a 10kw backup

    I'd guess turn key with permits, over current and materials your looking north of 40k
  • SolarLurkerSolarLurker Solar Expert Posts: 122 ✭✭
    Re: sma ac coupling

    The PV is oversized, I only intend to go with 100 amp at 48 v battery bank.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,963 ✭✭✭
    Re: sma ac coupling

    100 ah with a pair of Sunny Islands ( 10kw ) won't work

    You need about 100ah per kw, regardless of the manufacture of the inverter. If the SI's pull there rated load, your looking at close to 250 amps, with a 100ah battery I wouldn't be surprised if the battery's explode under that load, at a minimum you will melt the plates
  • BB.BB. Super Moderators, Administrators Posts: 31,419 admin
    Re: sma ac coupling

    By the way, the 100 AH per 1kW of Inverter/etc. is with 48 volt battery bank.

    A 24 volt battery bank would be 200 AH per 1kW of inverter. And 12 volt would be 400 AH per 1kW inverter.

    Do you have a stable source for Nickel Iron batteries? I thought they are getting pretty rare (maybe one MFG in the US at this point--maybe).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLARCinFLA Solar Expert Posts: 1,351 ✭✭✭
    Re: sma ac coupling

    AC coupling avoids the high ripple current on batteries when pushing to grid.

    100 AH is very light however for any useful off grid useage for 10kW Sunny Island inverter power. The batteries will have to take whatever AC load there happens to be on your aux power box if grid drops out, assuming worse case no PV power generation to help out.

    You will get the ripple current when PV is pushing on an off grid inverter output since the Sunny Island inverter is the voltage reference anchor with grid out. You would have to set up the SMA Sunny Island inverters to control the GT's to no more then 15-20% of battery AH rating in amps of charging from GT inverter.

    I am not that familar with the SMA system and how fast its feedback system works to back down the GT inverters when AC loads plus max charging cannot consume the PV generated power. If it is slow to react (and there will be some finite amount of time) when AC load drops off there could be some high surge currents to batteries that you would want a large enough AH battery to take. A high surge, beside possible negative effect on batteries, could produce a battery overvoltage shutdown of Sunny Island inverter. I think NiFe batteries have higher internal resistance so voltage surge could be worse then a lead-acid battery setup.
  • SolarLurkerSolarLurker Solar Expert Posts: 122 ✭✭
    Re: sma ac coupling

    I appreciate your opinions, I will ask over at sma forums about battery bank sizing.

    I only need a modest bank, most of my loads are during the day. I want to use the full array because in winter months or cloudy weather I need the full array just to get 1000 watts or more.
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