Sunextender batteries in a minimum battery configuration

HuckMeat
HuckMeat Registered Users Posts: 24 ✭
in Grid Tie and Grid Interactive Systems #1
I installed a 6kw solar system with 2 outback GVFX3648 inverters in a grid intertie system. At the time, I had some used C&D Dynasty batteries I tied into 3 strings of 4 batteries for around 210Ah @ 48V The system worked, but the batteries have failed.

I know that is not enough battery for that system, but the genesis of the system was that the inverter/charge controllers and integration hardware were free, and a local rebate covered the installed cost of the panels and mounting hardware, so I basically paid my electrician and paid for some wire/conduit/incidentals.

The batteries are really only setup to provide a minimal backup role, I find I'm selling 250-450kwh per month back, and using it up at night. The 2 times that I have actually used the batteries, they did do a nice job covering a few lights and a laptop until the power came back on.

Given that I'm not trying to sustain myself for multiple days (I have generators for that), and I have this system already installed, I was thinking about installing a single series string of 4 PVX-2580L SunXtender batteries (available from our sponsor), which according to the data sheet, can handle a higher charge current.

Will these really work for this application? Any suggestions for alternatives?

Thanks,
h
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Comments

  • Seven
    Seven Solar Expert Posts: 292 ✭✭
    #2
    Re: Sunextender batteries in a minimum battery configuration

    How old are the batteries? One of those inverters can output 45a so that would take care of a 450ah bank. I don't know if the second one in a slave configuration would add any charge of it's own. I am not familiar with them.
    Are you only considering AGM batteries? The Trojan L16 are comparable in price with a much larger ah capacity. 370 vs 210
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: Sunextender batteries in a minimum battery configuration

    i don't think he's asking if the agm batteries in general will work with those inverters as the inverters already worked with agm batteries or if he should consider agms over a fla type, but rather if the sunextender pvx 2580 will sub for the older c&d batteries. my guess is that it would suffice as a substitution for the 210ah c&d batteries if that c&d battery arrangement worked fine for you. the agms you had previously had the ability for higher charge rates as do sunxtenders, but sunxtenders are rated for a few full c rates. i doubt you will feed them that much current, but the pvs will feed at a bit more than a c/2 (>50%) rate. the internal chargers on the inverters may kick in if recovering from an outage at a rate of 45a adding to the solar if during the daytime, but the batteries can take it allowing for a quicker recovery time.

    what i would be concerned with is the extreme loading of the small bank size with a large draw at the time of the outage. i think seven is steering you in the right direction with going to a higher capacity. the large loads during brief outages may have strained the c&d batteries and would strain the sunxtenders too taking the voltage quite a bit lower on the bank. agms do better than fla types on charge and discharge, but with the possibility of up to over 6kw loading at the time of the outage it may pay to up the capacity even though you would soon switch in the generators. sunxtenders also come in l16 styles. if the batteries are subjected to colder temps then you better read the manual as there is a design derating for low temps. all batteries derate in their capacity in colder temps for the record.
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  • boB
    boB Solar Expert Posts: 1,032 ✭✭✭✭
    #4
    Re: Sunextender batteries in a minimum battery configuration

    Don't forget to adjust the temp comp to -4.00 mV per degree C per cell for Sun Xtenders.

    boB
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  • HuckMeat
    HuckMeat Registered Users Posts: 24 ✭
    #5
    Re: Sunextender batteries in a minimum battery configuration

    Thanks guys!

    The actual loads on the inverters are generally between 100-250 watts, but I'll look at moving to a bigger bank... I have room/racking for up to 12 8D batteries, so I'll look at a lower voltage AGM so I can keep everything in a single string.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #6
    Re: Sunextender batteries in a minimum battery configuration
    HuckMeat wrote: »
    Thanks guys!

    The actual loads on the inverters are generally between 100-250 watts, but I'll look at moving to a bigger bank... I have room/racking for up to 12 8D batteries, so I'll look at a lower voltage AGM so I can keep everything in a single string.

    why do you have 2 large inverters if the loads are usually so low? you know better than i what you draw and i was worried of more significant loading on the bank when switching over. if you know it'll stay that low then i don't think expanding the battery bank is necessary.
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  • HuckMeat
    HuckMeat Registered Users Posts: 24 ✭
    #7
    Re: Sunextender batteries in a minimum battery configuration
    niel wrote: »
    why do you have 2 large inverters if the loads are usually so low? you know better than i what you draw and i was worried of more significant loading on the bank when switching over. if you know it'll stay that low then i don't think expanding the battery bank is necessary.

    It's a grid interactive system - Most of the time during the day, it's selling back 2+kw per inverter.
    At night, it's doing the outback hybrid 'passthrough' - No load on the batteries, using the grid to power what is on that panel.

    Theoretically, I could draw a lot out of the batteries if power was out and I insisted on running a hot water heater or something silly, but the 'protected load' panel that is tied to the outbacks in the event of a power fail is pretty minimal - a few CFL lights, a 50w radiant floor circulator, and possibly a laptop. When grid power is there, the 'input' side of the outbacks sell through a breaker in the main panel.
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #8
    Re: Sunextender batteries in a minimum battery configuration

    You need 100ah per kw that your selling ... so for a 6kw system you should have 600ah @48v

    Doesn't matter the make, whether is an Outback or Xantrex unit
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  • HuckMeat
    HuckMeat Registered Users Posts: 24 ✭
    #9
    Re: Sunextender batteries in a minimum battery configuration
    Solar Guppy wrote: »
    You need 100ah per kw that your selling ... so for a 6kw system you should have 600ah @48v

    Doesn't matter the make, whether is an Outback or Xantrex unit

    What drives that (i.e. How can I calculate that) and does it vary by battery type? If I'm selling, I really shouldn't be going through the batteries, so it seems to me, the batteries need to be sized such that they can handle the worst case charging rate. Some AGM's (The SunXtender in particular) are speced with some very high charging rate. Nevertheless, with 6Kw, there is obviously some significant charging current available.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #10
    Re: Sunextender batteries in a minimum battery configuration

    I believe part of the issue is the Ripple Current from the single phase 6kW inverters... Basically, the battery bank has to act like a giant capacitor bank to supply the 120 Hz ripple current of the DC input to the GT inverter (remember, sine wave output, a series of zero to max power Voltage*Current sine wave).

    AGM's should be able to handle the current better... But, I do not know if that is good enough for battery based Grid Tied inverter system (Solar Guppy may have tested AGM batteries in his setup--I don't know).

    The other issue I have seen is that MPPT solar charge controllers do dump maximum current into the battery bank while estimating the Vmp*Imp=Pmp equation... It appears with a "small battery battery bank" and a large solar array that it is possible for the Charge Controller to take a 48 VDC AGM battery bank to over 72 VDC while running the MPPT search algorithm.

    It appears that a fully charged AGM when subjected to a period of max charging current will see its "absorb" voltage shoot way up. While it may not damage the battery bank (Concorde AGM batteries appear to tolerate higher charging voltages), it still can cause issues with inverters and other attached "48 volt" electronics/loads.

    You can argue that the MPPT controllers should never exceed some Vbatt-max number--But the over voltage has been seen on one or two systems here with MPPT controllers and with battery banks at about 1/3rd the recommended capacity.

    From Solar Guppy's experience--He has used the rule of thumb of 100 AH of 48 volt bank capacity for every 1kW of Inverter (or solar panel?) power/rating. (note that this would be 200 AH/1kW for 24 volt bank; and 400 AH/1kW for 12 volt battery bank).

    If you assume that a "good inverter" will supply 6kW continuous and 12 kW surge--Then it turns out that 12 kW surge on a 48 volt battery bank (with 600 AH capacity) works out to ~C/2.5 or C*0.40 -- Which is the surge numbers that we normally recommend for flooded cell battery banks anyway. So, the numbers are consistent with other expectations based on battery physics.

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