Series versus Parallel

[Ravi]

I just bought a Morningstar MPPT 60A controller and have 7- 100w solar panels designed for a 12volt battery system. My question is that since solar panels are a current source, wouldn't wiring them in series (7 panels in series) mean that the panel producing the least amperage wins -- thus the battery would get charged with least amount of current. Conversely, if I put all panels in parallel, then wouldn't the entire array suffer by the single panel that produces the least voltage?

I have not bought the inverter yet because I need to know if I should wire the panels in series or parallel or a combination there of.

[Sunking]

7 IS A PRIME number. you only have one option. All 7 in parallel. If you wired 7 in series would be an over voltage for the controller. Now if you had 6 ot 8 panels then you have options. But with 7 only 1 option.

No for the other thing you do not understand. MPPT is a power converter. Series is always better than all in parallel because you are running less current between the panels between the panels and controller which means much less line losses. The Output Current = Panel Wattage / Battery Voltage.

[PNjunction]

Ravi - I think you got it! Generally that is why when choosing panels, especially a replacement, one tries to stay within about 10% or less variance of either voltage or current depending on how you wire it as you've noted.

[Ravi]

7 IS A PRIME number. you only have one option. All 7 in parallel. If you wired 7 in series would be an over voltage for the controller. Now if you had 6 ot 8 panels then you have options. But with 7 only 1 option.

No for the other thing you do not understand. MPPT is a power converter. Series is always better than all in parallel because you are running less current between the panels between the panels and controller which means much less line losses. The Output Current = Panel Wattage / Battery Voltage.

Thanks for the reply. The MPPT controller says it works with 12,24,36,48 volt systems and can handle a max incoming voltage of 150. So, My thought was that if I wire the 7 in series, then max mppt input voltage is approximately 140 (My panel specs are shown below). However, from my earlier school days, I remember that when you put current sources in series, the source that produces the least current determines the line current. Similarly for voltage in parallel.

One of my many confusions is that if the MPPT controller says 150vdc max incoming voltage, then why are they saying it works with 12,24,36,48 systems and further goes to say, it works with 12,24,36,48 battery systems.

my pv specs: PV (Pmax=100W, Vmpp=18v, Impp=5.56A, Voc=22.10v, Isc=5.91v)

[Ravi]

Ravi - I think you got it! Generally that is why when choosing panels, especially a replacement, one tries to stay within about 10% or less variance of either voltage or current depending on how you wire it as you've noted.

Thanks for the reply. It appears that with my morningstar MPPT 60A controller, the max input voltage can be as high as 150vdc. But if the system voltage is limited to 12,24,36,48, how would anyone possibly get a system voltage that high -- assuming i understand system voltage to mean panel voltage and not battery bank voltage. Based on this assumption, it appears that MPPT controllers do current boost so they can charge the battery at the correct voltage, but higher current. But it also means, that the array can only be 12,24,36,48 volt, so wiring more than 4 of my panels in series would be out of the question??

[Wy_White_Wolf]

....my pv specs: PV (Pmax=100W, Vmpp=18v, Impp=5.56A, Voc=22.10v, Isc=5.91v)

7 * 22.10V = 154.7V

Seven in series will not work. You also need to derate the controller based on the coldest temperature you would expect for your location.

WWW

[Wy_White_Wolf]

Thanks for the reply. It appears that with my morningstar MPPT 60A controller, the max input voltage can be as high as 150vdc. But if the system voltage is limited to 12,24,36,48, how would anyone possibly get a system voltage that high -- assuming i understand system voltage to mean panel voltage and not battery bank voltage. Based on this assumption, it appears that MPPT controllers do current boost so they can charge the battery at the correct voltage, but higher current. But it also means, that the array can only be 12,24,36,48 volt, so wiring more than 4 of my panels in series would be out of the question??

Because MPPT controllers work like a DC-DC converter.

If you wired 6 of your panels in series you would have about 5.56A@108V working power into the charge controller. When operating at 12V it would have about 41.5A@14.5 output (vary depending on batty charge) going to the batteries.

WWW

[Ravi]

Thanks for the reply. So, if i understand this correctly, the best option for me would be to wire the 7 panels in parallel to the MPPT controller (I already have enough 14 gauge wire so line loss should be minimal considering we are talking about 5.56A per panel and a 14 gauge wire is rated for up to 20 amps), have a 12v battery bank (i have 16 - 12v batteries that i will wire in parallel). Then, in the future, if I ever buy 7 more panels, I should rewire all panels in strings of 2 (thus 7 parallel strings of 2 panels in series going to the charge controller). This way, weather I have single panel strings or double panel strings wired in parallel, I can still charge the batteries since Vmp would always be above my battery voltage.

Did i get it??

[SunEagle]

Thanks for the reply. So, if i understand this correctly, the best option for me would be to wire the 7 panels in parallel to the MPPT controller (I already have enough 14 gauge wire so line loss should be minimal considering we are talking about 5.56A per panel and a 14 gauge wire is rated for up to 20 amps), have a 12v battery bank (i have 16 - 12v batteries that i will wire in parallel). Then, in the future, if I ever buy 7 more panels, I should rewire all panels in strings of 2 (thus 7 parallel strings of 2 panels in series going to the charge controller). This way, weather I have single panel strings or double panel strings wired in parallel, I can still charge the batteries since Vmp would always be above my battery voltage.

Did i get it??

With all 7 panels wired in parallel your total amperage going to the controller will be (7 * 5.56 = 39 amps) so your wire size needs to be at least a #8 awg. You will also need a combiner box with individual fuses for each of the 7 panels.

A 16 - 12v battery bank is pretty big. The wiring for those batteries to your inverter needs to be sized greater than the amperage calculated by dividing your inverter wattage by the battery voltage. So for a 1000watt inverter the wires will need to handle more than 83 amps ( 1000w / 12v = 83.3 amps). A #2 awg would do it for you. If your inverter uses more wattage than the wire will need to handle more amps.

[Wy_White_Wolf]

Thanks for the reply. So, if i understand this correctly, the best option for me would be to wire the 7 panels in parallel to the MPPT controller (I already have enough 14 gauge wire so line loss should be minimal considering we are talking about 5.56A per panel and a 14 gauge wire is rated for up to 20 amps), have a 12v battery bank (i have 16 - 12v batteries that i will wire in parallel). Then, in the future, if I ever buy 7 more panels, I should rewire all panels in strings of 2 (thus 7 parallel strings of 2 panels in series going to the charge controller). This way, weather I have single panel strings or double panel strings wired in parallel, I can still charge the batteries since Vmp would always be above my battery voltage.

Did i get it??

Being how the controller is MPPT, I think you might get more power using 6 in series than having 7 in parrallel. The higher voltage system will be that much more efficient. But that's just an opinion and would have to be tested out to know.

WWW

[Ravi]

Being how the controller is MPPT, I think you might get more power using 6 in series than having 7 in parrallel. The higher voltage system will be that much more efficient. But that's just an opinion and would have to be tested out to know.

WWW

My only concern with going in series is that a single failure will bring down everything. Additionally, when going in series, the power produced would be determined by the single panel producing the least amperage if I remember my circuit analysis design days from school.

[Sunking]

Did i get it??

Sorry but no you did not.

First let's visit Voc limit. The controller in question has 150 Voc max limit. That part you got right. Howver you are not taking the temperature coefficient of the panels into account. Your spec on your panels say Voc = 22.10. But that is at a panel temp of 77 degrees. When it gets colder Voc rises higher. Long story short easiest and safe way to deal with it is take the Controller Voc / 1.25. So 150 / 1.25 = 120 volts Voc. So now with that in mind with your panels, you can only use up to 5 in series 5 x 22.10 = 110.5 volts. At 6 would take you over 120 limit.

Next issue is you have 7 panels and 7 is a Prime number. When you have a Prime number there are only two options of all in series, or all in parallel. You already eliminated the all in series option. That only leaves you all 7 in parallel. You also built another trap for yourself with 7 panels in parallel. Your only battery option is 12 volts. With 7 12 volt battery panels in parallel eliminates 24, 36, and 48 volt battery system. It also means being trapped into 12 volt a lot of other things like low efficiency, and a input power limit of 800 watts. At 48 volt battery you could go up to 3200 watts. But you are trapped at 12 volts with 7 panels.

Now here is the part you got dead wrong. With all you panels in parallel means the highest input current, not lowest. Using 14 AWG is out of the question. Currents add in parallel circuits Your Isc current = 7 amps x 7 panels = 49 amps. The bare minimum wire size you can use is # 8 AWG and if the 1-way distance between the panels and Charge Controller is more than 3 feet will require even larger wire to keep voltage loss to less than 2%.

You really built some nasty traps with 7 panels. I suggest you loose 1 panel, or gain 1 more panel. I would also suggest moving up to 24 volts. 12 volts is for boys and toys.

[Ravi]

Thanks for the explanation. That really helped and I am learning a lot from this forum. As far as wiring the panels in parallel, I currently have a separate 20 ft (14 guage) run from each panel to a bus bar. Then I have a 0 guage wire from the bus bar to the charge controller. So, no single wire could carry too much current. Is this still a problem?

I dont mind giving up a panel and just wire the 6 in series as you suggest (then buy 5 panels in the future and add a new string of 6 total, but wire the string in parallel -- so 2 strings of 6 panels each going to the controller), and wire my battery bank to be 24 volts and then buy a 24 volt inverter instead of staying with 12 volts battery and inverter.

I ran separate wires from each panel to a bus bar in my basement that I can easily get to without going on the roof. This way I can try out various wiring scenarios since this is my first solar experiment. So weather I do series or parallel wiring, its all simple to do since the wires are all run into the basement - a set of wires for each panel. I like the morningstar MPPT60A controller as it offers a lot of diagnostics and real time monitoring over ethernet, rs232 etc. I am a software engineer and can write my own app to do additional stats.

[Sunking]

Thanks for the explanation. That really helped and I am learning a lot from this forum. As far as wiring the panels in parallel, I currently have a separate 20 ft (14 guage) run from each panel to a bus bar. Then I have a 0 guage wire from the bus bar to the charge controller. So, no single wire could carry too much current. Is this still a problem?

You tell me. You will loose 5.1% of your power. With # 10 AWG gets you down to 2% power loss.

I dont mind giving up a panel and just wire the 6 in series as you suggest

I never said that, go back and read what I said. Most you can put in series is 5, and 5 is another Prime number with only two options of all in parallel another 12 volt trap, or all in series and can use any battery voltage. You need to understand the numbers. For example take a look at maximum input power vs battery voltage with a 60 amp MPPT controller

800 watts @ 12 volt
1600 watts @ 24 volt
3200 watts @ 48 volt.

If you are using 100 watt panels with a Voc = 22 volts what is the common denominator of series panels that will work for all battery voltages. Here is your hints. It will not be a Prime number of 1, 3, or 5. It will be greater than 1 and less than 5. The first letter of the number is "f" and the last letter is "r". Figure it out and all you will need is 1 single run of 14 AWG wire down to the controller and no fuses required.

You need to be thinking ahead, and working in building blocks. Prime numbers do not work very well. Prime numbers do not work very well. Get it right and save a ton of money and frustration. Get it wrong spend a lot of money and life long frustration fixing it.

[Ravi]

Sorry for misunderstanding, but no need to be so harsh. Another user suggested I string 6 together in series.

I am a newbie and am asking questions from the experts so I dont make mistakes. I will order 1 more panel and go with 2 strings of 4 panels each. I will wire the 2 strings in parallel to the bus bar and then bus bar to charge controller. I will go ahead and wire the 4 panels in series at the roof, then use the existing 14 gage wire from the roof to my basemenet where I have the bus bar.

Can I still use my 14 guage wire from each string (set of 4 in series tied by 14 guage wire) to bus bar, is that going to be a problem as far as power loss or do I need to change my wiring to a thicker wire going to my bus bar.

Also, I will go with 24 volt battery setup where each connection is exactly the same length to equalize wire resistance so each battery is equally charged and discharged. (2-12 volt batteries in series and 8 such connections in parallel to a bus bar since I have 16 batteries total. I am using 0 guage stranded wire for all battery and battery to inverter connection) and an AIMS 4000W pure sine wave inverter (industrial grade since its surge is 12000W for 30 seconds. I will use a longer cable from the battery bank to the inverter to avoid any possible sparks. I tried a harbor freight 5000w (10000 peek) PWM inverter that only lasted 1 month and caused my fans and other motors to buzz while running and my LED tv to buzz as well). Going with the 24 volt battery system, I can add future panels in sets of 4 in series, wired in parallel to my bus bar until I get a max of 1600w (4 strings where each string is 4 panels in series and each string is wired in parallel to the bus bar). Is that ok or should I consider 48volt battery system for future expansion. I just cant find a good 48volt inverter that is pure sine wave, has high wattage and can take sustained surges for a good duration like the AIMS 24 volt industrial grade does.

AIMS does have a 48v 7000 watt modified sine wave inverter (http://www.aimscorp.net/7000_Watt_Po...ial_Grade.html) industrial grade inverter that can surge to 14000w for up to 9 seconds, but it is not pure sine wave. I was told to stay with pure sine wave due to sensitive equipment I am running. I dont mind getting that if if works well with my TV and fans etc. Also, per your suggestion, it appears that if I go with 48volt system, I have max benefits in the future.

Thus far, all I have done is bought 7 panels and put them on the roof. I have a set of 14 guage wires from each panel going to my basement (25 ft.). I have bought 16 deep cycle 105Ah batteries. I made 4ft cables for battery connections (0 guage wire).

It is not too late for me to do things correctly as you suggest.


Your input is much appreciated!