Please help me double check my array modification.
Hairfarm
Solar Expert Posts: 225 ✭✭✭
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 / 12volt 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
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 / 12volt 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
Comments

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 
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

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 SO59 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/LMRSolar
gen: http://tinyurl.com/LMRLister , 
Hairfarm said:What am I missing here? How much increased temperature are we talking about here?
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.
vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i 
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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