Best configuration for 2x 24 volt panels and 12 volt battery bank?? (Help a Newbie!!)

Hi everyone, I am new to this site (and to off-grid living), and I had a couple of questions concerning the system I will be installing at my week-end retreat.

I bought 2 Kyocera KD185GX-LPU (24volts) solar panels and an Outback FM80 MPPT charge controller in order to charge my battery bank wich consists of (2) 6 volt, 225ah batteries wired for 12 volt usage. (I know the FM80 is overkill for such a small system but I plan on expanding as time goes by as I plan on eventually moving there one day... )

My main question is how I should wire the panels in order to get the best results... Should I stay 24volts to the charge controller or wire to get 48volts?? I am obviously looking for MAX amps in the battery!! For now, I would like to stay with a 12 volt battery bank, as the house is already wired for it and I don't think I will need more batteries. I'm getting a 25-30% DOD for the 2 days I spend there on the week-ends with NO charging whatsoever during my stay.

My loads consist of CFL lighting (dc 12volts, 7 watts), a shurflo 9300 @ 100ft depth in well (about 25 watts/hour) hand/shower use only, no dishwasher or clothewashing machine, and a 19" TV and satellite (105watt/hour, 8-10 hours use during week-end.) I"ll be buying a 19" LCD as they are about 2 times more efficient than the 15 year old one I have right now! :roll:
I have no A/C loads except for TV/SAT plugged in a small 300 watt inverter that I shut off when not in use.

Any help you guys could give me would be greatly appreciated BUT PLEASE try to go easy on me and not be too technical... I am mechanically inclined (I'm a machinist) but the really technical stuff is sometimes too much.... Although some wiring diagrams showing combiner box (if needed??), lightning arrestors, D/C disconnect and such would be GREAT!!! Basically, I'm just trying to get the most out of this particular setup.

Thank you in advance!!

Comments

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Best configuration for 2x 24 volt panels and 12 volt battery bank?? (Help a Newbi

    it should work in either configuration. if you have a long wire run from the pvs to the controller or if the wires are too small to pass without a significant voltage drop then wire for 48v otherwise wire it for 24v as the controller will run very efficiently there. how long of a wire run are you thinking for it and with what wire gauge? if you'd like you can try to figure it for voltage drop yourself in the FAQ's, Links, & Information Sources section of the forum.
  • BB.BB. Super Moderators, Administrators Posts: 30,520 admin
    Re: Best configuration for 2x 24 volt panels and 12 volt battery bank?? (Help a Newbi

    Outback has a nice set of sample wiring diagrams for various sizes and combination of equipment.

    Regarding a combiner box with fuses/circuit breakers--You don't need one for two strings or less. You only need one if you have 3 strings or more in parallel where the fuses/breakers protect one string from current supplied by the other panels--if that one panel string gets a short.

    The MPPT controllers tend to be a little bit more efficient at lower voltages, but the high panel voltage configuration allows you to use less thick wire to save money. And, you can save money by avoiding a combiner box (i.e., 3 panels in parallel needs a combiner box vs running 6-8 panels of 3-4 panels in series x 2 parallel strings and no combiner).

    Also, you need more/larger wire to run 3 parallel panels vs 6-8 series/parallel panels. If your panel wiring run is more than a few 10's of feet, the high voltage panel configuration can be a much easier/cheaper solution. 12 volt configurations do not handle voltage drop issues very well at all.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Mr.CockburnMr.Cockburn Solar Expert Posts: 34
    Re: Best configuration for 2x 24 volt panels and 12 volt battery bank?? (Help a Newbi

    Hi guys, thank you both for replying to my post, your help is very much appreciated!

    The max distance I'm thinking of installing the array is about 75 feet from the house... I just need to do some "shade tests" to determine the exact spot. ( It's in the woods, but luckily the southern part of the land is mine so I can cut some trees that cause shading.)

    I checked for voltage drops and this is what I came up with, assuming I stick with 24 volts (Please correct me if I'm wrong):

    *75 foot run, 24volts, 15.4 amp draw, 6 AWG wire = 0.938v drop or 3.91%
    *75 foot run, 24volts, 15.4 amp draw, 4 AWG wire = 0.59v drop or 2.46%

    I wasn't sure if I needed to take into account voltage drops from the panel itself when, let's say it's cloudy outside or something ??? But I don't think so, since I'm assuming the output "wattage" of the panel also diminishes with the voltage...
    (Like I said, the technical stuff gets to me!!!)

    What would be a reasonable voltage drop ? I'm thinking no more than 5% MAX, But I'm the type of maniac that likes to do "twice as good" so I was thinking of going with the 4 AWG.... I don't really mind spending the extra dough, as long as it's done right. I also think it would make things even better to go with the 4 AWG because when I eventually expand my array, I'll probably be running 48 watts at that time making my wiring even more efficient.. (But then again, maybe I'm just TOO MUCH of a maniac!!!) :roll:

    I was wondering how much more efficient the charge controller would be in 24 volts compared to 48 volts... Is this something I should really consider or is the difference negligeable ??

    Also, let's say I wanted to add more batteries (but still keep a 12 volt battery bank) , How many more 6 volt batteries could I add for this system? I'm asking because I don't live in an area where it's particularly sunny all the time so maybe more batteries would prevent too much draw down.

    As always, your opinions and ideas will be very much appreciated and thank you for your time!!!
  • AntronXAntronX Solar Expert Posts: 462 ✭✭
    Re: Best configuration for 2x 24 volt panels and 12 volt battery bank?? (Help a Newbi

    On page 62 of FM-60 manual there are controller efficiency vs. input power graphs. I assume FM-80 manual is identical. Looks like you are looking at 1% difference between 24V and 48V panel configuration. At 24V input FM-80 output = 283W, at 48V = 280W. Run panels in series and save copper. 10 AWG wire should be enough.
  • BB.BB. Super Moderators, Administrators Posts: 30,520 admin
    Re: Best configuration for 2x 24 volt panels and 12 volt battery bank?? (Help a Newbi

    The solar panels will reach Vmp when weak sunlight is on panel (Vmp is also temperature dependent, Vmp falls as temperature rises). Basically the output current is proportional to sunlight intensity.

    Since you are already running panels at 24 volts on a 12 volt bank--basically, you have lots of "volts" to burn (obviously, it would be an energy waste).

    Personally, I would crank the array up to 96 volts or so and then you could run 45 amps with less than 3% wiring drop on the same 6 awg wire run...

    96 volts * 45 amps = 4,320 watts of solar array (at high voltage)
    24 volts * 15.4 amps = 370 watts of solar array (at lower voltage)

    Your FM 80 controller could only control ~1,200 watts of panels (see manual for specifications) on a 12 volt battery bank.

    If you are planning on an array >~1,200 watts--you should look at a 24 or 48 VDC battery bank (unless you need the 12 VDC for your loads). With a 48 VDC battery bank, you can run ~3,800 watts of solar panels through your single FM80 and the one 6 awg cable.

    It is getting late--I hope I am both clear and accurate.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Mr.CockburnMr.Cockburn Solar Expert Posts: 34
    Re: Best configuration for 2x 24 volt panels and 12 volt battery bank?? (Help a Newbi

    Thanks, I WILL be reading the whole manual, it's just that I don't have it yet because the panels and controller are on their way from Florida and should be arriving shortly. ;)

    It's also nice to get input from people who have done this before, that's why I'm asking before hand.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Best configuration for 2x 24 volt panels and 12 volt battery bank?? (Help a Newbi

    if you went with #10 wire for a 75ft run and the pvs in series it would give a bit over 3% for the voltage drop %. some say it should be 3% minimally and some say 2%, but you say you want to add on to the batteries and that means adding more pvs to charge them and the added current on that same wire run will cause an increase of the voltage drop %, which is undesirable.
    i am not to sure where it is you got 15.4a as paralleling 2 of those pvs would yield 16.7a as each is 8.35a at 23.6v.

    as to your calcs,
    *75 foot run, 24volts, 15.4 amp draw, 6 AWG wire = 0.938v drop or 3.91%
    *75 foot run, 24volts, 15.4 amp draw, 4 AWG wire = 0.59v drop or 2.46%

    1st example i get 4.4% and the 2nd 2.77%. i used 140 degrees f (60 degrees c) and *24v array voltage with 12v battery voltage and remember a 75ft run represents 150ft of wire as there are 2 wires to go that run. i also used your 15.4a figure just for verifying the calculations you did and as i stated i get a different result. it may even be good to use 90 degrees c for that as the nec requires wiring to withstand 90 degrees c (194 degrees f) in conduit if memory serves.

    * note that technically the voltage is lower than what a standard 24v pv would be, but for this i rounded it to 24v. you could plug in the exact voltages if you would like as it would reveal a slightly better v drop %, but going to 90 degrees c would make the v drop worse too. the 140 degrees f i used may be more typical of what one may see, but then again it depends on where you are and the time of year with summer representing the worst case scenarios for wire temperatures. higher wire temperatures cause more resistance in the wires.

    when laying out wiring try to go with the largest current you may plan on using into the calculator, for now and in the future, as you don't want to have to buy more wires later just because you added some pvs to power extra batteries that were added.
  • Mr.CockburnMr.Cockburn Solar Expert Posts: 34
    Re: Best configuration for 2x 24 volt panels and 12 volt battery bank?? (Help a Newbi
    niel wrote: »
    if you went with #10 wire for a 75ft run and the pvs in series it would give a bit over 3% for the voltage drop %. some say it should be 3% minimally and some say 2%, but you say you want to add on to the batteries and that means adding more pvs to charge them and the added current on that same wire run will cause an increase of the voltage drop %, which is undesirable.


    when laying out wiring try to go with the largest current you may plan on using into the calculator, for now and in the future, as you don't want to have to buy more wires later just because you added some pvs to power extra batteries that were added.

    My thoughts exactly on the wiring aspect... (future expansion, etc...)

    -So right now I'm thinking I'll wire the panels in series for 48volt (47.2v) @ 8.35amps.

    -I'll keep my battery bank at 12 volts (2x6volts in series) 220Ah.

    -For wire, I have not decided YET.... What would you go for??? I checked for voltage drops and this is what I got: (note that my v.drop calculator does not account for temp....)

    * 75 foot run , 48 volts, 8.35 amp draw, 4 AWG wire = 0.67% drop
    * 75 foot run , 48 volts, 8.35 amp draw, 6 AWG wire = 1.06% drop
    * 75 foot run , 48 volts, 8.35 amp draw, 8 AWG wire = 1.69% drop .....

    -My head is saying "Go with the 4 AWG and you'll have it forever" plus, it opens up some options for ramping up the amps eventually when I expand the system...

    Am I right of thinking this way ?? :confused: Or will I need to add more wire ANYWAY when I do add more panels??? I don't think I'll have a 5000 watt array any time soon but I don't want to redo everything each time I add 2 panels to the system either..

    What are your thoughts, guys??

    #1: on my chosen operating voltage
    #2: on the wire size I'm thinking of using (4AWG) vs. what you would/I should use
    #3: on my battery bank.. Could I add two more 220Ah (6v./series) and make my bank 12volts, 440Ah or would that be too much for only 2 panels?? Would it be better just to make my battery bank 24 volts, 220Ah? If so, what would be the advantage of doing so? ( Obviously, my house wire will be twice as efficient at 24v. but are there any more advantages... )

    Your input is truly appreciated!!!! ;)
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Best configuration for 2x 24 volt panels and 12 volt battery bank?? (Help a Newbi

    what v drop calculator are you using? using mine it shows the following at 140 degrees f,
    example with #4 = .75%, with #6 = 1.19%, and with #8 = 1.90%.
    you can see what temperature does to a wire's ability to handle the current and that causes an increase in heat losses that further escalates the power losses.

    you have to remember the mppt controller has a maximum current (by nec rules) that it can handle on the output. an fm80 is .8 x 80a = 64a. if your batteries are at 12v then the power handled is 64a x 12v = 768w. if your battery bank is at 24v then the power handled is now doubled with 64a x 24v = 1536w. it will also be double the power at a 48v battery voltage over a 24v b bank for 3072w. going beyond these output power limits does mean another charge controller to handle the excess power so keep this in mind for your expansion purposes. remember that you also don't want to have to change inverters to handle future battery bank changes and i say changes as years down the road you should not add new batteries to the old batteries in the b bank as the new ones will be downgraded to the level that the old ones are at.

    this is allot to take into consideration, but you can plan for things on paper to allow expansion to go without much of a hitch. try designing it again with requirements now and in the future. when you think you have a workable configuration then run it past us for our thoughts on it.
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