How many Panels in Series/Parallel?

OffGridRoryOffGridRory Solar Expert Posts: 26
I've accumulated lots of solar supplies over the past year, wanting to gather the supplies as cheap as possible by buying mostly secondhand materiels. I've been reading on this forum for about a year now and thought I had a pretty good grasp on how to set everything up now that I have it all (I think I do anyway)

Here is the list of components I plan on hooking up in the upcoming weeks

1 12v Samlex 1500w pure sinewave inverter
2 Evergreen ES-A-205-fa3 solar panels
1 Morningstar Tristar 45A MPPT charge controller with monitor screen
4 12v 110Ah deep cycle AGM batteries

The advice I need is two fold...

1) How many panels should I run in series/parallel (I noticed that the panels themselves actually have a maximum amperage that can be put through them) Unless I'm looking at this wrong, my panels only have a 20A fuse? If this is true than doesn't this mean that with about a 12A output from each panel that I cannot wire them together without using a combiner box? I've searched and searched and cannot seem to find the answer to this.

Here are the specs of the solar panel which are Evergreen ES-A-205-fa3 (I currently have 2 of them, but plan on continuing to accumulate them as I see deals)

Power (Watts) 205
Maximum Power - Pp,max 209.99
Minimum Power - Pp,min 205
Voltage - Vmp (Volts) 18.20
Current - Imp (Amps) 11.27
Open Circuit Voltage -Voc (Volts) 22.70
Short Circuit Current - Isc (Amps) 11.93
Cells (polycrystalline string-ribbon) 114
Max Series Fuse (Amps) 20

The second thing I would like some advice on is on the wiring of the system. Due to space constraints and the fact I have a 12v inverter, I need to wire the 4 12v 110Ah batteries in parallel (Making 440Ah at 12v) but should I wire/can I wire the Evergreen panels in series so I can make 24v going to the MPPT charge controller? Would this bennefit me in anyway? I've heard some people talk about higher voltage working better for feeding the battery bank in partially shaded conditions etc. And others say this is of no concern and 12v vs 24v is more a question of choice and future expansion than anything... According to Morningstar's MPPT calculator I can either wire 3 panels in series, or 3 in parallel when charging a 12v battery bank....

I'm confused.... Please help :)

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How many Panels in Series/Parallel?

    Welcome to the forum.
    The advice I need is two fold...

    1) How many panels should I run in series/parallel (I noticed that the panels themselves actually have a maximum amperage that can be put through them) Unless I'm looking at this wrong, my panels only have a 20A fuse? If this is true than doesn't this mean that with about a 12A output from each panel that I cannot wire them together without using a combiner box? I've searched and searched and cannot seem to find the answer to this.

    Okay, you're a little bit confused about panel operation. Understandable because they're confusing things. You don't put current through panels; they generate current. In the case of these panels Imp is 11.27 Amps. No matter how many you put in series they will still only generate 11.27 Amps, but the Voltage will go up. The Isc is 11.93 (Impedance short circuit). Even if you have twenty in series the Isc and Imp remain the same. The largest fuse you'd put on a string would be 20 Amps, although we see from the basic formula (Isc * 1.25 = fuse) that 15 is more likely. With only one string you technically do not need any fuse. Two in parallel will not required fusing either, as even if one panel or string is a dead short the remaining can not generate enough current to do any harm to the short. Three in parallel and you need fuses, as two good panels will generate 2X the Isc which will cause fireworks if the 3rd is shorted.

    The second thing I would like some advice on is on the wiring of the system. Due to space constraints and the fact I have a 12v inverter, I need to wire the 4 12v 110Ah batteries in parallel (Making 440Ah at 12v) but should I wire/can I wire the Evergreen panels in series so I can make 24v going to the MPPT charge controller? Would this bennefit me in anyway? I've heard some people talk about higher voltage working better for feeding the battery bank in partially shaded conditions etc. And others say this is of no concern and 12v vs 24v is more a question of choice and future expansion than anything... According to Morningstar's MPPT calculator I can either wire 3 panels in series, or 3 in parallel when charging a 12v battery bank....

    I'm confused.... Please help :)

    First problem: you have 410 Watts of panel, which is on the low side for charging 440 Amp hours of battery: 410 @ 77% = 315 / 14.2 charge Volts = 22 Amps current = just barely 5% of the Amp hour capacity.

    Second problem: wiring four batteries in parallel. Without bus bars it's practically impossible to get an even current flow in and out of all four.

    Third: given the small size of your array, the MPPT charge controller probably won't help much. There's nothing wrong with feeding it "24 Volts" though; it will down-convert that to proper charge Voltage for the 12 Volt system. Morningstar's panel recommendations are based on this:
    45 Amps maximum output @ 14.2 Volts = 639 Watts.
    Divide those Watts by 205 per panel and you get three panels max. Whether they're in series or parallel doesn't matter in that respect. Higher input Voltage from series wiring, however, can help overcome Voltage drop through the wire and will keep the current below 15 Amps so you will not need larger wires. Depending on the length of run you're looking at 14 or 12 gauge.
  • SlappySlappy Solar Expert Posts: 251 ✭✭✭✭✭✭
    Re: How many Panels in Series/Parallel?

    " According to Morningstar's MPPT calculator I can either wire 3 panels in series, or 3 in parallel when charging a 12v battery bank" the controller will take up to 150 volts max before being damaged. But it can take more than the max current (amps), but anything above the max current will be wasted. So for an example; if you have X amount of panels wired in parallel that will produce 60 amps at max on a sunny day, 15 of those amps will be lost, watch this video from morningstar, it is a training video, it has a lot of good info in it on the tristar MPPT controllers, it is about an hour long, but well worth the watch. http://www.morningstarcorp.com/en/support/item.cfm?ItemId=444
  • OffGridRoryOffGridRory Solar Expert Posts: 26
    Re: How many Panels in Series/Parallel?

    Thank both of you! So I guess my final question would be what would you do if you were in my situation since I have not hooked anything up yet (Like I said before the one restriction would be that I have the 12v inverter and do not want to purchase another one to make 24v)... I plan on getting a 3rd panel in the next month or two as well...

    A little background info: I purchased the 45A MPPT because I plan on building a house in the next few years when I'm done with school so I will expand to the point where one day I will have a 48v system (Assuming I will have about a 2KW system eventually) I also have 4 of the 110AH batteries because it was buy 3 get 1 free :)... I'd like to still have all 4 wired since I don't plan on using more than 20-30Ah a day as is and having the "emergency" power in the batteries would be good... I of course do not want to damage the batteries though so they will last about 10 years (Assuming I will not drain below 80% more than a few times in those years)

    Any advice on what you would do if you already have the equipment I have and want to be able to easily expand a few panels in the near future?

    Oh yeah: Secondary question that I cannot seem to find a straight-forward answer to... Can I add different spec. panels to this system? The reason I ask is because I shop around for used equipment a lot, and there are quite a few 100W panels with slightly different open voltage than the ones I have...

    Thanks again for the help guys! I've visited this forum LOTS in the past years and am pleasantly suprised at the speed and accuracy of your responses to my first post :)
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: How many Panels in Series/Parallel?

    I would wire the two to three panels in series... That will keep your current lower and wire gauge/complexity low too...

    The basic rule of thumb for adding parallel panels to a MPPT charge controller is that the Vmp match voltage within ~10% or better. And for series connections, the Imp should match within ~10% or better.

    The farther apart the Vmp/Imp match, the less efficient the system will be (also MPPT charge controllers are not designed to find the "true" Pmp (maximum power) peak when there are several with mismatched panels. The controller could easily "track" the a lower power peak.

    With different panels, sometimes you can match two panels in series with a higher Vmp in parallel with three panels in series that have a lower Vmp:
    • 3x18.2 volt Evergreen = 44.6 volt Vmp array
    • 44.6 v / 2 = two x 22.3 volt panels in series (+/-10%)
    But, in general, adding more panels with mismatched Vmp/Imp will usually be done with a second charge controller setup to support the different Vmp/Imp array (connected in parallel to the same battery bank).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How many Panels in Series/Parallel?

    I pretty much agree with Bill & Slappy; add a third panel in series with the other two. Preferably the same panel, otherwise you're trying to match up Imp. It's just easier to use the exact same make/model.

    I'd prefer to use four 6 Volt 225 Amp our batteries instead of four 12 Volt 110 Amp hours, but if you've already got the batteries there's no point in changing now. The 6 Volt configuration would give you the same power potential with easier, less problematic wiring (current sharing problems with multiple parallel batteries).

    Slappy's got it dead-on for expansion. Don't expect to retain much from the original system. The charge controller, the panels. But by then the batteries will probably need replacing anyway.

    If you are interested in future expansion, try to plan out your "ideal" system as well as your "what I've got now" system and see if/where any components are shared. Sometimes it's best to spend the money up front on an "over sized" MPPT controller which will be adaptable to future needs. Sometimes it's best to add on additional controllers to match needs. Sometimes it's best to flog the whole thing and start over.
  • OffGridRoryOffGridRory Solar Expert Posts: 26
    Re: How many Panels in Series/Parallel?

    Thanks so much for the info. I guess what I'll do now is wire the panels in series, and when I get a 3rd panel I will do the same with it making it 48v. I'm sure in the future I will need to get a much bigger inverter that is also 48v to match a future battery bank, but I would like to continue purchasing the same panels as I can get ahold of them and use the same charge controller. I think what I will do is continue to aquire other panels if they are cheap and just purchase cheap controllers to hook them up as well, this should simplify the wiring etc.

    Again, thank you for all your help!

    Rory
  • PhotowhitPhotowhit Solar Expert Posts: 5,655 ✭✭✭✭✭
    Re: How many Panels in Series/Parallel?

    Gosh I hope your still around... Buy 2! or none. 3 will leave you looking for a 4th of the same V and Amps and design, for your 48 volt system!
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • OffGridRoryOffGridRory Solar Expert Posts: 26
    Re: How many Panels in Series/Parallel?

    Oh Yes... I'll continue to snatch these panels up when I can get them... There is a 3rd one for sale right now for $250 bucks!! So as soon as I get paid this week I'll be grabbing that one if its still for sale! I've found there are HUGE savings by buying single panels from those who buy pallets of B Grade panels and doing a little shopping around... I think given my energy needs (When I go off grid and no longer have my electric water heater YIKES!) will be taken care of with about 2KW of panel, a 400W or so wind generator (Provided where I build will be good for wind) and a honda EU2000i generator... In the mean time, I'm doing what I can in regards to purchasing panels here and there since I do not have the money to purchase more than a couple of units a month right now... The long term plan is to have roughly all the components (or as many as I can get) before I build the house, this way I have all my power investment taken care of...

    Everyone has told me to get the generator first then add components to the solar system, but hey.... Gas generators are simply not as fun :)

    So yes, I'll have a 3rd panel in the next few weeks (making the system 36v temporarily) but I will continue to grab panels and modify the system with the exception of the battery bank since its not really possible to add new batteries to the old batteries from what I understand... I guess I'll just keep my eyes open for 48v components since that seems to be what would make the most sense for a big off grid system eventually... For now I'll plug away with 24-36v input from the panels and 12v battery bank.
  • Peter_VPeter_V Solar Expert Posts: 226 ✭✭✭
    Re: How many Panels in Series/Parallel?
    BB. wrote: »
    The basic rule of thumb for adding panels to a MPPT charge controller is that the Vmp match voltage within ~10% or better. And for parallel connections, the Imp should match within ~10% or better.

    I think you have that backwards.

    In a series string the current is the same through all the panels so you'd want the Imp to match, otherwise the panel with the lowest Imp sets the whole strings Imp.

    With parallel panels the voltages have to match, so you'd want the Vmp to be as close to the same as possible, otherwise the array voltage will end up being a compromise between the panels and you'll lose some power.
  • Peter_VPeter_V Solar Expert Posts: 226 ✭✭✭
    Re: How many Panels in Series/Parallel?
    First problem: you have 410 Watts of panel, which is on the low side for charging 440 Amp hours of battery: 410 @ 77% = 315 / 14.2 charge Volts = 22 Amps current = just barely 5% of the Amp hour capacity.

    I'm confused, what does the battery voltage have to do minimum with charge current?
    Minimum charge current is based on AH capacity and it doesn't matter what voltage you battery bank is.

    I've always figured that the charge current should be at least 10% of the batteries AH rating unless you're using AGMs which typically like it a bit higher than that.

    I might be wrong, but I think you're trying to mix PV output power in with the AHs here.
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: How many Panels in Series/Parallel?

    Thank you Peter, I have fixed the post...

    I thought I had fixed that when I originally posted it. :blush:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How many Panels in Series/Parallel?

    Yes, Bill meant serial connections need the match-up on Imp.
    I fixed it in the post.

    It's so easy to get the two confused - even without getting them confused! :p
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How many Panels in Series/Parallel?

    Sorry, Bill; didn't notice you were on-line. :blush:
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How many Panels in Series/Parallel?
    Peter_V wrote: »
    I'm confused, what does the battery voltage have to do minimum with charge current?
    Minimum charge current is based on AH capacity and it doesn't matter what voltage you battery bank is.

    I've always figured that the charge current should be at least 10% of the batteries AH rating unless you're using AGMs which typically like it a bit higher than that.

    I might be wrong, but I think you're trying to mix PV output power in with the AHs here.

    It makes a difference when you're calculating array size. 'X' watts divided by 'Y' Volts = 'Z' Amps.
    Change the 'Y' Volts and you don't get the 'Z' Amps out of the 'X' Watts, so you don't get the charge current you're looking for. In basic math:

    120 Watts / 12 Volts = 10 Amps
    120 Watts / 24 Volts = 5 Amps

    So the same size array (in Watts) may supply sufficient current for the 12 Volt system's 100 Amp hour battery bank but not for the 24 Volt system's 100 Amp hour battery bank.
  • Peter_VPeter_V Solar Expert Posts: 226 ✭✭✭
    Re: How many Panels in Series/Parallel?
    It makes a difference when you're calculating array size. 'X' watts divided by 'Y' Volts = 'Z' Amps.
    Change the 'Y' Volts and you don't get the 'Z' Amps out of the 'X' Watts, so you don't get the charge current you're looking for. In basic math:

    Yup, I was confused. The 440AH and 410 watts got me.

    You started talking about the battery capacity (440AH) and then went on to calculate the Amps produced by the 410 watt array and I missed the change.

    I totally spaced on that, sorry :blush:
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: How many Panels in Series/Parallel?

    Marc,

    Not a problem.
    Peter_V wrote: »
    I'm confused, what does the battery voltage have to do minimum with charge current?
    Minimum charge current is based on AH capacity and it doesn't matter what voltage you battery bank is.
    With a PWM solar charge controller, the maximum current is related to Imp and sun conditions.

    With a MPPT charge controller, the output current is based on:
    • Power = Voltage * Current
    • Current = solar panel power * derating * 1/voltage when charging
    Since the battery charging voltage varies between ~11.5 to 15.0 volts ("dead battery" to equalization voltage), you can get quite a range in output current...

    So, for me any way, I like to be conservative, so I use the 14.2-14.5 volts or so as the maximum voltage a battery bank will reach in bulk. The battery voltage will be less than the absorb voltage if the battery bank is less than ~90-80% state of charge.

    We throw a lot of rules of thumbs around here instead of asking the details/making confusing answers such as; is this PWM or MPPT charge controller, is this AGM or Flooded Cell, etc., when trying to first size a battery bank/solar array.

    The other thing to not forget is that if the solar/battery/load numbers are withing 10-20%, that is pretty much "dead on"/close enough with all of the variables out there.
    I've always figured that the charge current should be at least 10% of the batteries AH rating unless you're using AGMs which typically like it a bit higher than that.

    I might be wrong, but I think you're trying to mix PV output power in with the AHs here.
    We have been using the 5% to 13% range as a good compromise for battery life, system design, and costs.

    AGM's, because they have lower self discharge and don't need "equalization" to mix electrolyte--Can work better below 5% than flooded cell... But that still can take a long time to recharge a battery bank (spending more time below ~75% state of charge increases sulphation and reduces life of battery bank).

    For flooded cell batteries, above ~13% or C/8 charging current for long periods of time can overheat batteries. AGM can certainly take more charging current because they have lower internal resistance (Concorde can take upwards of C*4).

    AGM's are used in UPS's where the battery may be subjected to C*2 discharge rates (30 minutes at full power to take the UPS to dead).

    However, battery wiring must be much heavier gauge wiring to support C*4 loads vs C/2.5 max surge for typical flooded cell batteries. And for solar arrays, that is a lot of solar panels that will quickly recharge a battery bank and then sit, unused, for the rest of the day. So, we tend to ignore >13% rate of charge for solar arrays unless there is something different going on (like most power is taken during daylight hours and little at night--such as a home with large irrigation needs--pump during day, few lights and electronics at night).

    And regarding mixing Array Wattage vs AH for battery bank... We sort of do have to use both... With an MPPT charge controller (and 12 volt/120 volt/240 volt loads, AC inverters, etc.), it is usually easier to use the Power/Energy numbers for consistent calculations (Watts is Watts at 12 volts or 240 volts).

    However, for battery banks, they are more nearly 100% efficient when looked at as Amp*Hour devices... More or less, from a pure AH number, if you discharge 100 AH, you need to replace 100 AH (for flooded cell batteries as they approach full charge and start gassing, efficiency does drop a lot).

    But, since you discharge around 12 volts and recharge around 14.5 volts (just round numbers for example; 12 volts may be low, and charging finishes at 14.5 volts), then that 100 AH looks different in Watt Hours:
    • 100 AH * 12 volts = 1,200 WH discharging energy
    • 100 AH * 14.5 volts = 1,450 WH charging energy
    • 1,200 WH / 1,450 WH = 0.83 = 83% energy efficient (more less worst case)
    Lastly, I choose the more conservative approach because:
    1. Never plan on using 100% of available solar power every day--Just too many variables in available sun, losses, variations in day to day loads, etc..
    2. Panels get dirty, batteries become less efficient as they age, so a system that works well new just does not work as well with a few years on it.
    3. Electric loads tend to grow over time and it is very difficult to up size (or down size) a system by more than a factor of ~2 without some major component/battery bank changes.
    4. The person designing the system and spending the money tends to be pretty careful about power usage / battery bank state of charge. The spouse, kids, and visitors--not so much.
    People here who have much more experience with batteries than I (such as Dave Sparks who installs off grid system for a living in the California Sierras) prefer long / gentle charging currents for long battery life (smaller array + trackers vs a large fixed array).

    How much difference does it make? I don't know--but it may be the difference between one person getting 8 years vs another getting 15 years of life on a good quality battery bank.

    Anyway, sorry for the lots of hand waving. That is how I have been justifying my first pass sizing estimates.

    -Bill

    Peter, or just ignore my whole post. :blush:
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: How many Panels in Series/Parallel?
    You don't put current through panels; they generate current. In the case of these panels Imp is 11.27 Amps. No matter how many you put in series they will still only generate 11.27 Amps, but the Voltage will go up. The Isc is 11.93 (Impedance short circuit). Even if you have twenty in series the Isc and Imp remain the same. The largest fuse you'd put on a string would be 20 Amps, although we see from the basic formula (Isc * 1.25 = fuse) that 15 is more likely.

    Not to pick nits, but the code says to calculate the fuse size from Isc*1.25*1.25 and round up to the next fuse size. In his case that's 18.64A, so a 20A string fuse is correct. And Isc has nothing to do with impedance; I is the symbol for current, hence Isc is short circuit current.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How many Panels in Series/Parallel?

    Ever notice how people say "not to pick nits" and then go ahead a do it?

    15 Amps is the likely fuse size. NEC isn't an absolute.
    Impedance should be Intensity.
    In future I shall try not to post answers before I'm fully awake.

    Happy now?
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: How many Panels in Series/Parallel?
    Ever notice how people say "not to pick nits" and then go ahead a do it?

    15 Amps is the likely fuse size. NEC isn't an absolute.
    Impedance should be Intensity.
    In future I shall try not to post answers before I'm fully awake.

    Happy now?

    Ecstatic.



    :D
  • OffGridRoryOffGridRory Solar Expert Posts: 26
    Re: How many Panels in Series/Parallel?

    So I guess I'm back to my original simplified question (Reading recent posts and reading the manual for the Tristar MPPT 45 I've become confused again)

    The Controller has the ability to either auto-sense (or recommended set the controller) incoming voltage and appropriately downgrade it to the lower battery bank voltage, which is going to be 12v.

    What should I do in this case? Should I just wire the 2 panels I have now in parallel or should I wire them in series (making them 24v)? I DO plan on getting a 3rd and 4th panel in the upcoming months, so I'd like to wire the panels in a way that makes it easiest to expand those panels onto the system when I do so

    Should I 1) wire the panels in series for panels 1 2 and 3, then when I get the 4th wire it in parallel with one of the other 3? Or 2) should I just wire the panels in parallel for now and then switch over to series wiring for the 3rd panel?

    Sorry for the new guy question guys... I just never really researched much into wiring the panels... I just thought I could wire as many panels as the controller would handle in parallel, but now see that I don't want to go over the 20A limit (which is 2 panels in parallel if I have my math correct)...
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How many Panels in Series/Parallel?

    It's time to fall back on one of our many rules-of-thumb! :D For an MPPT controller, the array Voltage should not be more than 2X the system Voltage. So 12 Volt system = 24 Volt array.

    Then when you add two more panels you can parallel that string; no array fuses needed.
  • OffGridRoryOffGridRory Solar Expert Posts: 26
    Re: How many Panels in Series/Parallel?

    Awesome! Sounds like a plan to me! I'll wire the 2 I have in series making 24v, then when I get 2 more I'll wire them in parallel with the original 2 so I keep the system 24v... Thanks guys!
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: How many Panels in Series/Parallel?

    Should I 1) wire the panels in series for panels 1 2 and 3, then when I get the 4th wire it in parallel with one of the other 3?
    If you do that, the 4th module will not add anything to the array output. The other series wired modules will restrict the current to what a single module can produce, and the voltage will not change.

    I realize you've made the decision to do something else, but I looked at it as an exercise. An exercise in futility, in this case. ;^)
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