How to rewire GT PV Array for off grid?

n1stn1st Solar Expert Posts: 77 ✭✭✭✭
Folks,

In prep for a using a GT PV array for off-grid...

I have 2 strings each of 9 SolarWorld SunModule sw175 panels. Voc: 44.4/40.2, Vmpp:35.8, Impp: 4.89

I have 2 runs of 10g wire from panels to the house.

I don't know what I'd need for wire harnesses to rewire the panels at the panels, but I'm assuming that I can. What's the best configuration to make best use of the most number of panels and the existing 10g runs to the house and connecting the power to a FM60 or 80 to charge a 24v battery bank? I know this is going to take some brain effort, so thanks in advance.

Scott
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Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How to rewire GT PV Array for off grid?

    I might be mis-reading you but ...

    How about two arrays with 9 panels each, 3 series/3 parallel giving 132 Voc & 105 Vmp @ 1575 Watts?
    On 24 Volt system, that would be a peak charge current of about 65 Amps which the FM 80 can handle (but you'll need one for each array) and the 10 AWG should be able to handle the array current as well.

    Just a rough estimate without Voltage losses (how long are your wire runs?) and I may have done the math wrong.:p
  • mike95490mike95490 Solar Expert Posts: 8,461 ✭✭✭✭✭
    Re: How to rewire GT PV Array for off grid?

    I think single panels, @ 35-44V, would put you in the MPPT good spot, but you will have lots of amps in the feed.

    But the entire array, downconverted to 26V, will be about 104Amps, which is beyond the capacity of a 60 or 80 amp controller.

    If you went to 48V, it's only be 52A, whhich woudl be sweet!
    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 ,

  • tallgirltallgirl Solar Expert Posts: 413 ✭✭
    Re: How to rewire GT PV Array for off grid?

    Check the winter record low temp for temperature adjusted Voc on the "strings of 3" configurations. A designed Voc of 132 is a bit high, and I don't see winter record low temps mentioned.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How to rewire GT PV Array for off grid?
    tallgirl wrote: »
    Check the winter record low temp for temperature adjusted Voc on the "strings of 3" configurations. A designed Voc of 132 is a bit high, and I don't see winter record low temps mentioned.

    Agreed. If you're in a really cold place, like the Cariboo :p , the panels can exceed the safe input max of the controller. But if the wire run is particularly long, it may negate that. So those two factors have to be considered as well.
  • n1stn1st Solar Expert Posts: 77 ✭✭✭✭
    Re: How to rewire GT PV Array for off grid?

    Thanks for the help.

    I'm in Connecticut. The distance between panels and house is about 100'.
    ...would a FM60 handle that (3S/3P)? I have a FM60 and I don't know if I can trade up to an 80 without taking a loss.
  • mike95490mike95490 Solar Expert Posts: 8,461 ✭✭✭✭✭
    Re: How to rewire GT PV Array for off grid?

    Are you converting 1 string, or both strings?

    1 string - either can manage the wattage, but all 18 panels, is more then 1 controller can handle at 24V.

    2 panels in series, would safely be in the MPPT range, and would help with losses in your long cable run. Ideally, you want the max power voltage to be 5-10v higher than your EQ voltage, otherwise you get a % or 3 more loss downconverting. Trade that against the copper losses in your long runs.
    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 ,

  • Solar GuppySolar Guppy Solar Expert Posts: 1,962 ✭✭✭
    Re: How to rewire GT PV Array for off grid?

    200ft round trip is a long run, your going to be looking at some serious copper costs.

    Id say your looking at 9 strings, each two panels, so your going to need a bunch more wire and run the numbers for your voltage drop as well. Connecticut gets well below 0F so you can't run 3 in series for a 150V controller.

    Since you have a 60 amp controller, put 4 strings on that and get a second 80 amp unit for the remaining 5 stings.

    If you haven't bought your inverter yet, you should really consider a 48V system
  • n1stn1st Solar Expert Posts: 77 ✭✭✭✭
    Re: How to rewire GT PV Array for off grid?
    mike90045 wrote: »
    Are you converting 1 string, or both strings?

    1 string - either can manage the wattage, but all 18 panels, is more then 1 controller can handle at 24V.

    2 panels in series, would safely be in the MPPT range, and would help with losses in your long cable run. Ideally, you want the max power voltage to be 5-10v higher than your EQ voltage, otherwise you get a % or 3 more loss downconverting. Trade that against the copper losses in your long runs.

    It'd be nice to convert both stings, but funds are limited.
    Is there such a thing as too much power generation for a given battery bank? I know that nimh shouldn't be charged faster than .1C if I recall. My 4 6v Napa 8146s are rated at 226Ah @20 Amps. Is the bank too small for 2 FM controllers?
  • mike95490mike95490 Solar Expert Posts: 8,461 ✭✭✭✭✭
    Re: How to rewire GT PV Array for off grid?

    Different batteries can take different charge rates. AGM can take a lot, flooded cell, less. What battery chemistery are you thinking of?
    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 ,

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How to rewire GT PV Array for off grid?

    Yes you can put too much current to a battery bank and charge it at too fast of a rate. This will ruin the bank in short order. If your bank is 226 Amp/hrs your maximum charge current shouldn't be over ~30 Amps. Around 22 Amps would be better. To supply this mount of charging you only need about 550 Watts (24 VDC system). Again and as always, these are rough calculations. Your 3150 Watts of panels would be capable of charging a whopping 1300 Amp/hrs of battery.

    Scale back for now, and expand when you can afford the additional outlay. Or re-size your system according to your requirements and sell the surplus panels.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: How to rewire GT PV Array for off grid?

    do note that the current coming from the controller's output will be high after downconverting and that the wires will need to be much thicker so #10 will not suffice after the cc. it may not be valid for your runs either unless you go through the calculations to figure for voltage drop. if you combine strings before reaching the cc this automatically warrants larger sized wires from that point on as well.
  • n1stn1st Solar Expert Posts: 77 ✭✭✭✭
    Re: How to rewire GT PV Array for off grid?
    mike90045 wrote: »
    Are you converting 1 string, or both strings?

    1 string - either can manage the wattage, but all 18 panels, is more then 1 controller can handle at 24V.

    2 panels in series, would safely be in the MPPT range, and would help with losses in your long cable run. Ideally, you want the max power voltage to be 5-10v higher than your EQ voltage, otherwise you get a % or 3 more loss downconverting. Trade that against the copper losses in your long runs.

    I've reviewd all your posts and even read the FM60/80 manual! It looks to me with the winter temps here, that 2 panels in serial is the max. My goal is to be in a position to make use of the current GT system when the grid is unavailable - without permantently altering the current system. So here's what I'm thinking...

    -Make use of 2 panels in serial from each of the 2 current strings that I have. Connect these in parallel at the CC. Use a 36v or 48v battery bank.
    -Do not alter the 4 conductors of #10 running from panels to house - 330 ft round trip (1.78v drop @ 5amps).
    -If the grid goes down, disconnect the MC type 4 connectors leaving only 2 panels on each pole connected in serial. This should be about vmpp 72v-3.6v = 68v @ 4.8a x 2 = 670w at the controller. No loss in amps, correct?
    -Tap into the current DC disconnect and run to a 20amp 120v/240v breaker into 12g wire. This could not be connected until the MC connectors are connected to just 2 panels in series.
    -Run the 72v into cc.
    Downconversion 36v: 43 @ 16a
    Downconversion 48v: 57.6 @ 11.6a
    -Output from cc into a 20amp 120v/240v breaker into 12g wire to battery bank.

    What have I overlooked or miscalculated?

    Regarding rewiring the current config of 9 panels down to 2... The current PV array has an MC type 4 cable coming out of the conduit into a PV panel then daisy chaining through all 9 panels and back into the conduit. Can I disconnect after the first panel, connect an MC 4 extension, run it to the last panel, and have my 2 panels in serial?

    Thanks!
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How to rewire GT PV Array for off grid?
    n1st wrote: »
    My goal is to be in a position to make use of the current GT system when the grid is unavailable - without permantently altering the current system. So here's what I'm thinking...

    Can you clarify this? Do you want to be able to use the same array either for power a GT inverter or for charging batteries for back-up? I think we'd need to know the specs of your existing GT system before advising on this.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,962 ✭✭✭
    Re: How to rewire GT PV Array for off grid?

    If this is for emergency's only, its really simple

    Have your battery bank 48V and the inverter and just direct connect the HV strings to the batterys ( well fuse or breaker )

    It will do exactly what your proposing , maybe a little more and requires no extra long strings ... just keep an eye on the battery voltage and disconnect if your battery's are getting to high voltage
  • n1stn1st Solar Expert Posts: 77 ✭✭✭✭
    Re: How to rewire GT PV Array for off grid?
    Can you clarify this? Do you want to be able to use the same array either for power a GT inverter or for charging batteries for back-up? I think we'd need to know the specs of your existing GT system before advising on this.

    Yes, same array, for both GT and when grid goes down, for charging a battery bank. Inverter would likely be Outback VFX35xx - 24 or 48v. The specs of my current GT PV array are way back on my first post in this thread. Thanks.
  • n1stn1st Solar Expert Posts: 77 ✭✭✭✭
    Re: How to rewire GT PV Array for off grid?
    If this is for emergency's only, its really simple

    Have your battery bank 48V and the inverter and just direct connect the HV strings to the batterys ( well fuse or breaker )

    It will do exactly what your proposing , maybe a little more and requires no extra long strings ... just keep an eye on the battery voltage and disconnect if your battery's are getting to high voltage

    Wow, really? No charge controller? Just connect both pairs of conductors from PVs to a 48v 226ah battery bank (each pair is 324v and 4.89a)??
  • Solar GuppySolar Guppy Solar Expert Posts: 1,962 ✭✭✭
    Re: How to rewire GT PV Array for off grid?

    Yup, the magic of using PV ... The you will get the Isc current @ the battery voltage of the string.

    That's why you should go 48V ... will give you 2X the charge power going direct. Just remember you have to manually watch the battery voltage
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: How to rewire GT PV Array for off grid?
    Yup, the magic of using PV ... The you will get the Isc current @ the battery voltage of the string.

    That's why you should go 48V ... will give you 2X the charge power going direct. Just remember you have to manually watch the battery voltage

    That's the part most people would have trouble with.
  • n1stn1st Solar Expert Posts: 77 ✭✭✭✭
    Re: How to rewire GT PV Array for off grid?
    Yup, the magic of using PV ... The you will get the Isc current @ the battery voltage of the string.

    That's why you should go 48V ... will give you 2X the charge power going direct. Just remember you have to manually watch the battery voltage

    How do you get "2 X"'? Wouldn't it be 324 / 50 = about 6 times?

    So if I connect 324v to the 48v battery bank. How many volts will the inverter see at it's input, still ~48v?

    What voltage would I watch for to know when to disconnect the PV from the battery? Is there a device for this this that would trigger an alert, or disconnect a relay?

    If I was asleep at the wheel, would the voltage climb so high that it could damage my inverter (vfx3548)?
  • Solar GuppySolar Guppy Solar Expert Posts: 1,962 ✭✭✭
    Re: How to rewire GT PV Array for off grid?

    The amps are the near the same at 1 volt or 300 volts for your GT array assuming your at or below the VMP. Directly connect will provide energy to charge but there is no current boost like you would have with down converting with a Mppt charge controller.

    So lets say your array is 5 amps, at a charge of 50V you get 250 watts vs 25 * 5 if you went with a 24V battery bank

    With the array direct connected, over time you can over charger the batterys, that why you would have to manually connect/disconnect but we are talking about the once in a blue moon power outage, not a 365 day/year situation. Also, for maintaining the battery's when not being used in the loss of power time, the inverter will keep them topped of so basically you have a big UPS that when called for you can get a few hundred watts of PV charger, not allot different that your idea of tapping some panels to get a lower voltage

    So things to keep in mind:

    You must disconnect the PV lines from the GT inverter
    You must have a fuse to protect the wiring and PV from the Battery's
    You must manually watch the battery voltage and disconnect if it exceeds the Bulk Charger voltage
    The Plus is, no messing with the array wiring
    No Charge controllers to buy

    As a practical matter, if you have a load on the inverter when you lose power, its unlikely that you direct GT array connect will have enough energy to over charge the battery's, but you have to watch this to be safe regardless
  • mike95490mike95490 Solar Expert Posts: 8,461 ✭✭✭✭✭
    Re: How to rewire GT PV Array for off grid?

    And, on a sunny day, with the grid off, and the ice cream melting in the freezer, when you disconnect the array, you are dealing with Open Circuit Panels, which won't be at their nominal 327V, they will be closer to 400V DC. You should close the circuit with a switch rated at that power, or else you will have a nice arc-welded conncection where you hook the PV to the battery. After it's been connected, your batteries drag the array harmlessly down to 48VDC, and when you disconnect, the arc will be a 400V dc arc again, as the array looses the battery load.
    Wear Sunglasses !
    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 ,

  • n1stn1st Solar Expert Posts: 77 ✭✭✭✭
    Re: How to rewire GT PV Array for off grid?
    mike90045 wrote: »
    and, on a sunny day, with the grid off, and the ice cream melting in the freezer, when you disconnect the array, you are dealing with open circuit panels, which won't be at their nominal 327v, they will be closer to 400v dc. You should close the circuit with a switch rated at that power, or else you will have a nice arc-welded conncection where you hook the pv to the battery. After it's been connected, your batteries drag the array harmlessly down to 48vdc, and when you disconnect, the arc will be a 400v dc arc again, as the array looses the battery load.
    Wear sunglasses !

    . .
  • n1stn1st Solar Expert Posts: 77 ✭✭✭✭
    Re: How to rewire GT PV Array for off grid?

    You guys have been very helpful. A few more questions if I may...

    Please enlighten me. I don't know much about the role voltage plays in charging wet batteries. Say I have 2 different pv arrarys. Both generate 700w. One is 24v out @30a. The other is 350v out @2a.

    Charging at 350v and 2a into a 200ah 24v battery bank, it'd take ~100 hours to charge from about 0 to 100%.

    Charging at 24v and 30a into the same battery config, it'd take less than 10 hrs.

    ...in both cases I'm using 700w, but one situation takes 10 times as long as the other... how could this be?

    Secondly, You could charge 12v with 1000v as long as the current is low (.1C) without harming the batteries?
  • BB.BB. Super Moderators, Administrators Posts: 29,696 admin
    Re: How to rewire GT PV Array for off grid?
    n1st wrote: »
    Please enlighten me. I don't know much about the role voltage plays in charging wet batteries. Say I have 2 different pv arrarys. Both generate 700w. One is 24v out @30a. The other is 350v out @2a.

    Charging at 350v and 2a into a 200ah 24v battery bank, it'd take ~100 hours to charge from about 0 to 100%.

    Charging at 24v and 30a into the same battery config, it'd take less than 10 hrs.

    ...in both cases I'm using 700w, but one situation takes 10 times as long as the other... how could this be?
    A solar panel is a "current source" (term from electronics). Basically, it is a constant current output (based on the amount of sunlight hitting the panel). The panel's output voltage is set by device that is attached to it (roughly, as long as Vpanel < Vmp).

    The battery is a "voltage source"--within reason, no matter how much current you put in the battery, it will hold around 28 volts.

    Power is equal to Voltage * Current... So in this case, you have a set voltage (by the battery) and a set current (by the solar panel).

    P=V*I
    P=28v*2a= 56 Watts
    P=28v*30a= 840 Watts

    Your confusion is that the panels at Vmp=350 volts and 2 amps:

    P=350v*2a= 700 Watts

    When charging a 24 volt battery is not outputting 700 watts, it is outputting only 56 Watts... Again it gets back to the formula for power and the fact that a solar panel's power output is variable:

    P=0v*2a= 0 Watts
    P=28v*2a= 56 Watts
    P=100v*2a= 200 watts
    P=200v*2a= 400 watts
    P=300v*2a= 600 watts
    P=350v*2a= 700 Watts (Vmp?)
    P=400v*0a= 0 watts (assume Voc--open circuit voltage, no current flow)

    The MPPT Solar Charger uses the Pmax=Vmax*Imax=Vmp*Imp equation to use its internal power converter to figure out the Pmax/Pmp operating point of the solar panel every so often, then use the converter to down convert the high V/I of the panel down to the lower V/I of the battery:

    P=350v*2a= 700 Watts = Vbatt*Ibatt = 28v*25a (less some 5-10% losses)
    Secondly, You could charge 12v with 1000v as long as the current is low (.1C) without harming the batteries?

    The old engineering answer is yes and no...

    Yes, in terms of current you can charge 1x12 volt battery or even ~71x 12 volt batteries (in series) which such a charge/current source (0.1C forever is not a great charging algorithm either).

    The No part of the answer is that it is not safe. If there is a break in the wiring/battery or you have 71 batteries in series for 1,000 Vbatt, the batteries/wiring is not rated for safe operation at 1,000 volts--you could have insulation breakdown and sustained arcing creating fire/explosion hazard.

    I know that this was a "what if/why not" question--But I always want to make sure everyone is safe when discussing the details.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • n1stn1st Solar Expert Posts: 77 ✭✭✭✭
    Re: How to rewire GT PV Array for off grid?

    Excellant explanation:D Now I understand why without a cc, the higher the battery bank voltage, the more power you can make use of from the array.

    I would like to understand more about what happens as the battery bank becomes fully charged, absent a cc. As the battery bank becomes fully charged...
    The current from the array remains constant (10a)? Assuming little or no load on the batteries, how quickly will this all happen? In other words, do I have only a few minutes or do I have an hour or 2 to disconnect PV from batteries without causing damage to the batteries?

    Since the array is 10a max, I would think a 20amp fuse/disconnect and 12g between PV entering the house and the batteries would be sufficient, yes?
  • BB.BB. Super Moderators, Administrators Posts: 29,696 admin
    Re: How to rewire GT PV Array for off grid?

    By leaving the battery bank connected at 0.1C--You are performing an "equalization" charge.

    The energy of the charging has to go somewhere... Either as heat, or in this case, a lot of the energy is going into electrolysis. Breaking down the water of the electrolyte into Hydrogen and Oxygen (plus heat too).

    Hot batteries tend not to last as long as cool batteries. However, this is a long term effect. Roughly, for every 10C (18F) increase in battery temperature, the "life" of the battery is cut by 1/2. So, 8 hours of "hot battery" means 16 hours of aging--instead of 8 hours for that part of the day--not a problem for a rare event.

    The other issues, long term equalization, if I recall correctly, can damage the surface of the plates (scrubbing of a soft surface). Hydrogen Gas is flamable--so ventilation/no sparks are important.

    Oxygen gas from electrolysis tends to recombine at the positive plates and create corrosion (on some batteries, may notice case expansion and/or positive post being pushed up out of the case).

    Lastly, if these were AGM/Sealed batteries. The catalyst inside the battery recombines the Hydrogen and Oxygen back together (more heat) and the catalyst itself ages (when catalyst fails, AGMs/Sealed batteries begin to vent gas/vapor and fail), and if the amount of energy/heat is enough, the battery will vent from excessive pressures (again, losing electrolyte/water/gas).

    So, the short answer is that you have hours to turn of the charging current and there will be no issues -- Big thing to watch is how much "boiling" (electrolysis) takes place and how hot the batteries get.

    If you needed to set up for days/weeks/months of alternative charging--get a "cheap" Xantrex/Morning Star PWM controller that support "dump" mode. Put the charge controller across the battery bank and connect the appropriate load bank--and you are set. No over charging issues at all (assuming everything is programmed correctly).

    A suggestion, look at setting the Dump controller to a higher voltage than your normal charge controller(s) so that it does not "fight" with your other charge methods (i.e., generator trying to quick charge the battery bank and your dump controller trying to dump the charge current).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • n1stn1st Solar Expert Posts: 77 ✭✭✭✭
    Re: How to rewire GT PV Array for off grid?
    BB. wrote: »
    By leaving the battery bank connected at 0.1C--You are performing an "equalization" charge.
    -Bill

    Bill, one quick question about your last answer. I'm favoring going with 3s,2P into a FM60 CC for about 1000watts into a 48v bank. The 3s is close to the edge with VOC at -4F, but I'm I'm betting on global warming ;) . But I haven't given up on going CC-less. My question is about the quote above. Did I misinterpret you, or are you saying that direct connect (no cc, 322v @ 10a into 24v or 48v 225ah bank) will decrease itself down to 0.1C as the bank fills?

    The more I learn, the more questions I have.
  • BB.BB. Super Moderators, Administrators Posts: 29,696 admin
    Re: How to rewire GT PV Array for off grid?
    n1st wrote: »
    Bill, one quick question about your last answer. I'm favoring going with 3s,2P into a FM60 CC for about 1000watts into a 48v bank. The 3s is close to the edge with VOC at -4F, but I'm I'm betting on global warming ;) .
    Others here can better address the issue of Voc at -4F... Using this data (from Xantrex XW calculator which is not exactly the same as yours):
    SolarWorld SW175

    Pmax @ STC 175 W
    Pmax @ PTC 163 W
    Vmp at Pmax 35.4 V
    Imp at Pmax 4.95 A
    Voc @ STC 44.6 V
    Voltage change -145 mV/C

    14F is already at 149 Voc... So, -4F would even be worse. If you have those cold of temperatures--then you really are running the risk of an over-voltage/warranty voiding event. Obviously, running close with even a few 1/10ths of volts difference can make a difference. Will 149.9 be OK and 150.1 blow up the charge controller--No. If the controller logs 150.1 volts and will the mfg. decline any warranty repair--they would have the right.

    And this is the problem with "high voltage" (around 48 VDC Voc) solar panels with the current crop of Xantrex/Outback and other Solar MPPT Charge controllers with 150 Voc max ratings... Three panels in series produces a too high Voc in cold weather.

    And two panels in series does not produce a high enough Vmp for use in hot weather (at 95F, Vmp for two SW175 panels is ~58 VDC... At this voltage, you may not be able to equalize your batteries on a hot day--and your charge rate may be reduced because you cannot get the batteries to ~58 Vbatt after you take into account controller and wiring voltage drops...

    It seems that 2 panels in series is required to keep Voc "safe" on very cold days. And that you will have to accept some possible power / battery charging limitations in hot weather (remember the Xantrex panels add in fact for being over ambient temperature).

    The normal answer is to not use High Voltage solar panels with 48 volt battery banks, but back down to the "24 VDC" or lower panels.

    If your battery bank is 24 VDC or less--you don't have this High Voltage / 48 VDC bank issue.
    But I haven't given up on going CC-less. My question is about the quote above. Did I misinterpret you, or are you saying that direct connect (no cc, 322v @ 10a into 24v or 48v 225ah bank) will decrease itself down to 0.1C as the bank fills?

    Not quite sure exactly what you are asking on the 0.1C charge with high voltage panels (no CC). The panel output will be current limited, no matter the battery voltage (up to several hundred volts). You have a 48 volt bank, it will be "10 amps". If it was a 12 volt bank with the same panels, it will still be "10 amps".

    Of course, the reality is that it will be "10 amps maximum current"... The actual amount of current will be based on the amount of sunlight hitting the panel at any time of the day (weather, visibility, clouds, angle of sun, shading, etc.).

    0.1 * 225 AH battery bank will equal 22.5 Amps for 0.1C charge rate.

    The energy into the battery bank will still be based on Pbatt=Ibatt*Vbatt

    I = 10 Amps maximum. V=What ever battery bank voltage you have at the time.

    Remember that the storage capacity of a battery bank is really Voltage*AH=Watt*Hours storage.

    So a 48 volt 225 AH bank will store twice as much energy as a 24 volt 225 AH bank... The 48V 225 AH bank will also have 2x the amount of "Lead" too... Nothing is free.

    Is this along the lines you are asking about?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • n1stn1st Solar Expert Posts: 77 ✭✭✭✭
    Re: How to rewire GT PV Array for off grid?

    Bill,

    Regarding 3 vs 2 panels in serial. My sheet says 44.4 voc, so assuming that, if I multiply that by 1.17 (FM manual page 10 for down to -4F) = 52. 52 - 2v (wire voltage drop of 330' round trip with 4.8a #10 wire) = 50 x 3 panels = 150v. Yeesh that is too close! Question - This is only an issue if I turn a disconnect on when the sun is up, correct? Couldn't I wait until the sun is down to throw the connect to avoid this VOC issue, or even use my DMV to measure the voltage before throwing the connect? After the switch is thrown to begin using the CC instead of GT, from then forward, as the sun rises and sets, there is no VOC issue, correct?

    I understand the 0.1c now. I was wondering if the current from CC would decrease as the battery became full, but I see that's now what you were saying.
  • BB.BB. Super Moderators, Administrators Posts: 29,696 admin
    Re: How to rewire GT PV Array for off grid?
    n1st wrote: »
    ........Couldn't I wait until the sun is down to throw the connect to avoid this VOC issue, or even use my DMV to measure the voltage before throwing the connect? After the switch is thrown to begin using the CC instead of GT, from then forward, as the sun rises and sets, there is no VOC issue, correct?

    Remember that Voc is an "open circuit" panel... You probably could, on cold days, add a xx Ohm resistor on the panel leads and load the panels (obviously with a loss of power) and keep the "Vpanel" somewhere between Voc>Vpanel>Vmp (in cold conditions).

    When the charge controller is working hard (MPPT) and moving maximum current is not the problem. It is when the batteries are charged (controller will cut way back on Ipanel) or possibly in low light conditions, the controller will undervolt (not enough current) cut off and reconnect and during that cycle hit Voc>Vmagic-smoke.

    There are various tricks that could be used to reduce that Voc peak (i.e,, switch that when Temp <xxF, throw relay to short last panels in string) and possible controller damage. I do not have any detailed controller knowledge to know how close to push that Voc number...

    Getting "too cute" makes for a painful to operate system and/or leaves open for mistakes (i.e., forgot to throw cold weather switch on last trip to town)...
    I understand the 0.1c now. I was wondering if the current from CC would decrease as the battery became full, but I see that's now what you were saying.

    For a PWM controller (the less expensive/older tech type), there maximum output current is basically the maximum Ipanel.

    For a MPPT type charge controller... You get the equation:

    Imp*Vmp=Pmp=Vbatt*Ibatt + 5-10% losses

    Notice that MPPT adjusts the Imp*Vmp=Pmax --- And just normal operation of the MPPT controller drives Pbatt=Vbatt*Ibatt ...

    And MPPT really only matters when the battery is around 90-80% charged, or less. When the battery is nearing 100% charge, the solar charge controller is going to cut back on Ibatt -- and the tracking of the MPPT does not matter any more (less than 100% of the panel power is being taken to charge/maintain the battery).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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