Please help me double check my array modification.

Hairfarm
Hairfarm Solar Expert Posts: 225 ✭✭✭
edited September 2016 in Off Grid Solar & Battery Systems #1
Greetings,

I'm looking for some input about increasing my array size to better match my battery bank's amp hours. I'm a bit rusty and would appreciate it if someone could check my math before I make any purchases.

My current array looks like this:

Three strings @ 35.4 volts @ 22.8 amps for 810 watts. (Series raises voltage, parallel raises amps).  

The current I can expect from 810 watts on an MPPT controller:
810 * 0.77 (typical efficiency) = 623 / 12-volt minimum battery Voltage = 52 amps.

My battery bank uses six 6V FLA batteries wired in series and then paralleled to make 696Ah. My 696Ah battery bank actually needs 69.6 amps to charge at a C/10 rate, (696 * .10 = 69.6). The 52 amps that my array makes now isn't enough for my 696Ah bank. This has been bugging me in the back of my mind for the last year. This is why I want to increase my array by adding two more panels: 135w Kyocera 17.7V @ 7.6A.

This will make my array like this now:

I would be reducing my array from three strings to two strings. Obviously I'll have an extra string's worth of wire just sitting idle in my conduit after the modification, but no big deal right? This was the best configuration I could come up with that can make 8 panels work.

So now it will be: 1080w * .77 = 831.6w / 12 = 69.3Ah. And my 696Ah battery bank needs 69.6 amps. Almost perfect for a C/10 rate, right? Any problems with my math or other issues?

Btw, I'm using an MPPT 80A Outback controller. Even with my array pushing almost 70A I still have a 10A buffer before my CC reaches its 80A maximum capacity.


thanks!

hairfarm











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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #2
    Your controller will be more efficient and run cooler if you add a fourth string, keeping the string voltage as it is.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Hairfarm
    Hairfarm Solar Expert Posts: 225 ✭✭✭
    #3
    My Outback CC manual states that the nominal array voltage for a 12volt system be 24volts or higher. The key word being "higher". My incoming voltage is 70V. I double checked and it states that the maximum voltage the CC can handle is 150VDC. That's over twice what is it now. Logic (and the manual) tells me that the MPPT CC can handle up to 150VDC. What am I missing here? How much increased temperature are we talking about here? 

    Besides I don't have any more room in my conduit for a set of wires for a fourth string ;)



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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    edited September 2016 #4
    You will observe a large increase of heat dissipated in the MPPT when you increase the string voltage.  At some voltage, the increase will force thermal limits.
    Put the PV combiner at the array, 4 breakers,  2s panels, 4p strings and keep the voltage the same,  Run a single pair wires from combiner to the controller in the conduit,  wire lasts forever, charge controllers burn out.
    That would be 30A 8ga or 6ga wire depending on the length
     See if you can locate the array voltage/battery voltage/efficiency chart.
     I'm not happy running 160V into my 200V classic for 48V battery.  Runs hot and fans scream all the time.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #5
    Hairfarm said:
     What am I missing here? How much increased temperature are we talking about here? 
    The flexmax reaches its maximum efficiency with an input power about 350 watts.  As power goes up from there, at any particular input voltage, the controller becomes less efficient. 

    To answer your question, let's take the input power to be 750 watts.  With two panels in series the controller will be 94.5% efficient and 41.25 watts (of the 750) will heat up the controller.  With four panels in series the controller will be 93.5% efficient and 48.75 watts (of the 750) will heat up the controller.

    Your proposed array size is at or over the limit for that controller in a 12 volt system. And as mentioned, as you approach the limits the controller becomes less efficient.
    mike95490 said:
     See if you can locate the array voltage/battery voltage/efficiency chart.
    Glad to oblige (from the manual):

    --vtMaps

    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Hairfarm
    Hairfarm Solar Expert Posts: 225 ✭✭✭
    edited September 2016 #6
    Thanks for putting that together for me VT. That is very helpful and I didn't notice that graph in my manual before on page 62.

    At 1080 watts of array it looks like my efficiency is around 93% according to that graph.

    Btw, according to the manual the Flexmax 80 can handle an array size up to 1250w for a 12 volt system, page 76.


    I'd like to go to a 24 volt system but then my battery bank would need to go from 6 batts currently to 8, (or down to four).


    thanks!
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