PV hook up question 12v or 24 v

8n-bob8n-bob Solar Expert Posts: 35 ✭✭
Hi All:

New to this form and fairly new to solar.

I have a couple of questions that I tried to search for but just can't find what I am looking for.

I just purchased the Sunsaver MPPT controller and it is not hooked up yet, but am in the next few days.

Some Background.
4 sets of Harbor Freight 45W. = 180W
Sunsaver MPPT controller
3 sets of golf cart batts. Series/Parallel for 12V
35 feet from panels to control/distribution
2000 Watt, modified sine wave inverter
I also have a 400W wind unit through a MPPT dedicated controller for it hooked to the other end of my battery bank.


I understand that the HF pannels are not the best and don't give their rated wattage etc. I got them real cheap from a guy who bought them and never used them.

My current hook up is each set of HF panels is feeding it's own controller. The total charge I am getting out of it is 9.0 amps at 12.5V = 112.5Watts so lets just use those numbers for a starting point.


Here is the questiion,

Given that the MPPT controller will "step down" voltage, and that the MPPT controller will take the watts in and give you the watts out for a 12V system, Negating controller and wire losses, and higher voltage results in less amps and heat in the wire, What is the better option for hooking up the panels so I can size up the wire comming from the panels. The MPPT controller can handle 75V open current

All panels with in a set are in Parallel and:
4 sets parallel at 12V = 12V/9A
Series 2 sets then parallel the 2 series sets = 24V/4.5A
Series all 4 sets = 48V/2.25A

Would the MPPT controller waist less power to heat going 24V to 12V then 48V to 12V or is that not a big issue and I should run the wire for 48V/2.25A and over size a bit for less wire loss.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: PV hook up question 12v or 24 v

    Welcome to the forum.

    Your best bet for panel configuration is to wire them as a "24 Volt" array. This will give you higher Voltage between the array and controller which will reduce power loss from V-drop while at the same time not reducing the efficiency of the controller's down-converting. This assumes wire sizing is sufficient over the distance involved.

    A bit about array configurations: http://forum.solar-electric.com/showthread.php?16241-Different-Panel-Configurations-on-an-MPPT-Controller

    One concern not related: you have three parallel strings of batteries? That presents two problems, the main one being that the 9 Amps of charge current is not going to do much for that much battery capacity. Typically a golf cart battery is 220 Amp hours. Three sets in parallel would be 660 Amp hours. Against that 9 Amps (you might get 12 with the MPPT controller) would be an insignificant 1.3% charge rate; barely enough to maintain the batteries.

    For full charging one set of GC2's needs about 350 to 400 Watts of panel. 180 Watts would work best with only 100 Amp hours @ 12 Volts. Do not expect those panels to be able to recharge all that battery.
  • BB.BB. Super Moderators, Administrators Posts: 29,497 admin
    Re: PV hook up question 12v or 24 v

    Welcome to the forum Bob.

    First, the HF panels will be disapointing... But if you want to try them, connect two in series, then into parallel strings. 4 panels in series may be too high of voltage (Voc-cold, voltage open circuit cold) for the little MorningStar Controller (which is a very nice small MPPT controller).

    Next, you should focus on getting the batteries charging correctly. Too small of solar array (or other charging sources) will cause your batteries to sulftate.

    So first, charging from solar. For an operating (cycling battery bank), you should have 5% to 13% rate of charge from solar for charging the battery bank. Or:
    • 6 batteries * 7.3 volts charging * 220 AH * 1/0.77 controller+panel deratings * 0.05 rate of charge = 625 Watt array minimum
    • 6 batteries * 7.3 volts charging * 220 AH * 1/0.77 controller+panel deratings * 0.10 rate of charge = 1,251 Watt array nominal
    • 6 batteries * 7.3 volts charging * 220 AH * 1/0.77 controller+panel deratings * 0.13 rate of charge = 1,627 Watt array "cost effective maximum"

    Note that the numbers above are rough estimates--I am carrying out the digits so you can repeat my math and follow their use.

    Your 185 Watt array is just above the amount need to "float" such a large battery bank (keep batteries charged during storage). You will not be able to cycle the batteries and recharge them without the batteries sulfating.

    Next, while the MorningStar 15 amps 12/24 volt MPPT controller is one of the nicest (small) MPPT charge controllers out there--It is really too small for this battery bank. If you wanted to charge your bank as 12 volts, you would need:
    • 660 AH * 10% rate of charge = 66 Amp rated charge controller (assuming a nominal 10% rate of charge)

    Your batteries, and a 2kW AC inverter, comprise a fairly good sized off grid power system... Enough to run a very energy efficient cabin (nor refrigerator) or just an Energy Star Refrigerator + lights.

    Purely from battery bank, assuming 2 days of storage and 50% maximum discharge (for long battery life):
    • 660 AH * 12 volt * 0.85 inverter eff * 1/2 days storage * 0.50 max discharge = 1,683 Watt*Hours per day

    Assuming you live somewhere around Norfolk VA, using PV Watts with a fixed array titled to latitude:
    Month    Solar Radiation (kWh/m 2/day)
    1      3.84     
    2      4.53     
    3      4.69     
    4      5.47     
    5      5.40     
    6      5.69     
    7      5.44     
    8      5.61     
    9      5.02     
    10      4.95     
    11      4.04     
    12      3.37     
    Year      4.84
    

    4 hours of "noon time equivalent sun" for a balanced system design:

    • 1,683 Watt*Hours per day * 1/0.52 system efficiency * 1/4 hours of sun per day = 809 Watt array minimum (break even month)

    So, to meet your system's balanced capabilities for November (break even month), you would need around 809 Watt to 1,627 Watt array, with 1,251 Watt array being a "healthy nominal" size.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 8n-bob8n-bob Solar Expert Posts: 35 ✭✭
    Re: PV hook up question 12v or 24 v

    Thanks Caribocoot and BB for the reply and I am digesting all this VERY GREAT information.

    I am going to ask a question with about Batts.

    I understand the 660amp/hrs on my bank and only charging them with lets say 10 amps for a nice number. IT would take 66 hrs to charge them back up. I don't use nearly 2000 watts out of them right now. I run a little high efficiency wine cooler fridge, some lights and charge some mobile devices.

    If say my batt voltage is 13.2V at the evening and in the AM it's down to 12.6 and with out shutting every thing off it's back up to 13.2 the next evening. Am I doing harm to the cells via sulfating?

    So I guess my question is would it be better to Pull off batt's and run one set for 12V and work them harder then trying to charge the whole bank.

    Remember I have a wind turbine rated at 400Watts. but when the wind is blowing I usually get about 200 watts from it. Not factoring this into the equation at this point

    I Know I will have more questions when I finish digesting this information, but I have to start some where.

    I'll wire the panels up for 24V that seems to be the way to fly.
    Thanks for the input and your time.

    PS don't feel bad for my like I bought all this stuff and am out a bunch of $$$$ except for the charge controller (which) I have very little in the system. Bought it PV, Wind, Batts, and (2) @KW inverters, from a guy who bought all this and never hooked it up.

    My "BIG" plan is to put some batts and those panels in the barn for lights etc with the MPPT controler and buy big PV's adn a ample charge controler for the house eventually.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: PV hook up question 12v or 24 v
    8n-bob wrote: »

    I understand the 660amp/hrs on my bank and only charging them with lets say 10 amps for a nice number. IT would take 66 hrs to charge them back up.

    Ah, no. Battery charging is not a simple linear thing. For one, as the battery Voltage goes up the current from the (solar) charger will go down meaning you don't get a constant 10 Amps. For another it always takes more power to replenish the battery than it will have available as stored capacity. Typically 20% more. If you have a system running, it is possible that the actual charge rate will be zero or less even though power is coming from the panels because of the power being used by any concurrent loads.
    If say my batt voltage is 13.2V at the evening and in the AM it's down to 12.6 and with out shutting every thing off it's back up to 13.2 the next evening. Am I doing harm to the cells via sulfating?

    Sulphation happens no matter what. The thing is the longer the battery spends at a lower state of charge the faster it happens. As a rule it is recommended to not let them be below 70% SOC for more than a couple of days. Again, not a linear function; once it starts happening it speeds up because the sulphation effectively reduces capacity causing the sulphating action to accelerate.

    Voltage is not a great indicator of state of charge. If all the panels manage to do is push the Voltage from 12.6 to 13.2 in a day you aren't getting the kind of charging needed to prevent premature damage. Batteries need to be taken up above 14 Volts and held there to achieve full charging.
    So I guess my question is would it be better to Pull off batt's and run one set for 12V and work them harder then trying to charge the whole bank.

    I have saved several systems by having the owners remove batteries. Most of them functioned better after storage capacity was reduced. This is why it is so important to know how much power capacity you actually need. Adding more and more batteries is quite often done to 'improve' systems - and it never, ever works if there isn't enough charging capacity.
    Remember I have a wind turbine rated at 400Watts. but when the wind is blowing I usually get about 200 watts from it. Not factoring this into the equation at this point

    Don't get your hopes up. These turbines are notorious for not producing their rated power, if they produce any at all.

    Feel free to ask questions: it's why we're here. :D
  • BB.BB. Super Moderators, Administrators Posts: 29,497 admin
    Re: PV hook up question 12v or 24 v
    8n-bob wrote: »
    I understand the 660amp/hrs on my bank and only charging them with lets say 10 amps for a nice number. IT would take 66 hrs to charge them back up. I don't use nearly 2000 watts out of them right now. I run a little high efficiency wine cooler fridge, some lights and charge some mobile devices.

    The minimum "useful" charging current is really ~5% to get decent life from the battery bank:

    0.05 * 660 AH = 33 amps minimum recommended charging capacity

    Flooded cell batteries, as they get old, start having self discharge issues too--Which can be over >1% per day near the battery end of life (if >2% per day, you should replace the bank for safety reasons).

    5% is the minimum recommended by several battery mfg. for charging current (others recommend 10%). You need at least 2.5-5% rate of charge to properly equalize a battery bank (basically "over charging" the whole battery string to bring up "weak cells").

    Your loads can easily be in the 1.0 to 1.5 kWH per day (just the wine cooler can be over 0.8 kWH per day). Get a Kill-a-Watt type meter and plug it into your loads. Very handy for overall conservation efforts in your home. If you don't know how much each appliance is drawing, you cannot really do much in the way of conservation.

    And, as long as I am spending your money (:roll:), look at getting a relatively inexpensive DC Current Clamp Meter (like this AC/DC clamp DMM from Sears). Very handy for debugging a DC power system. Being able to clamp around a cable and measure current is a really nice diagnostic tool (parallel battery strings, parallel solar panels, heavy loads, etc.).

    Longer term, a Battery Monitor is also nice too. Can help you better understand your battery bank and how loads/charging currents are affecting the system.
    If say my batt voltage is 13.2V at the evening and in the AM it's down to 12.6 and with out shutting every thing off it's back up to 13.2 the next evening. Am I doing harm to the cells via sulfating?

    Hard to tell, but it sounds like you are not discharging the battery very much (~12.7 volts after 3+ hours of setting without charging/discharging is ~100% state of charge).

    You should get a glass hydrometer and thermometer so you can log each cell's temperature corrected specific gravity (typically at least once per month when you check electrolyte levels).
    So I guess my question is would it be better to Pull off batt's and run one set for 12V and work them harder then trying to charge the whole bank.

    You still have the others that need to be float charged too (or recharged with a manual charger about once per month).
    Remember I have a wind turbine rated at 400Watts. but when the wind is blowing I usually get about 200 watts from it. Not factoring this into the equation at this point

    That can help--but remember that many people do not get daily wind power to recharge their battery banks... Seasonal at best.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 8n-bob8n-bob Solar Expert Posts: 35 ✭✭
    Re: PV hook up question 12v or 24 v

    Thanks for more information. Now it's really starting to make sense.

    So I am hoping for ~10 to 12 amps maybe a little more out of the MPPT charger. With an occasional kick from the wind turbine (more about that later).

    Now assuming some things about my pannels.
    at 12.5v charging the batt. ON the panel side not the batt side of the controller, the voltage was over 13 with 9 amps so that's 117 watts. When the batt were at 14.1 right at the threshold Voltage where the controller (harbor freight) the PV's were putting out 17V at about 7.5 amps so 127 watts.
    I am going to say that the pannels can produce 127 watts, much less then the advertised 180 for the amount I have.

    Now with the MPPT controller, not counting losses and keeping PV at 12V for this math.
    127Watts in at 17 volts, through the MPPT gets 127W/12V = 10.5 amps out.

    stepped up to 24 V system should give me what about 35V in or there about since 17V for 12V.
    SO
    127W/35V = 3.6 Amps on the PV line coming into the MPPT.



    One "set" of GC batts is 220AH
    5% charge rate for the batts is .05 * 220 = 11Amp

    So the Panels at 10 amps should be good to keep things charged.

    I think I am on the right track.

    The wind has a watt meeter on the charge controller. I have an amp meter on the output side and in a 15mph wind it is making about 7 amps and I live on top of the one of the highest hills in the county with my turbine on a 25' tower with no obstructions so I get a good amount of wind.

    But for now I'm focused on just the Solar end of things.

    I hope I'm not a bother Just trying to get the facts strait.

    Thanks Guys
    BobO.
  • BB.BB. Super Moderators, Administrators Posts: 29,497 admin
    Re: PV hook up question 12v or 24 v

    Bob,

    Not a problem... If we only answered questions once--I would be in double digits for my post numbers. :blush::p

    More or less, your batteries (how ever many you connect), ideally you want the solar panels+charge controller to hold around 14.4 to 14.7 volts (flooded cell batteries at ~77F) for around 2-4 hours or so (absorb). That gets the batteries from ~80% state of charge to >90% state of charge (around several times a week).

    Running off grid solar is really all about battery and charge/load management. Here is some light reading:

    http://www.windsun.com/Batteries/Battery_FAQ.htm
    http://www.batteryfaq.org/
    http://batteryuniversity.com/

    There are a lot of ways to "murder" your battery bank. For the most part, the rest of the solar equipment is pretty reliable (keep it clean, dry, free of corrosion. Don't connect wires backwards).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 8n-bob8n-bob Solar Expert Posts: 35 ✭✭
    Re: PV hook up question 12v or 24 v

    Update:

    Got the new controller, took 2 banks out of batt's off line from the solar but still hooked up to the wind, bringing the batt's on the solar side to 220A/H. Let's see how it charges.
    But it's going to be a cloudy day so I'll not get a good idea. LOL.

    The wind is blowing and when I checked this AM my wind turbine was making 13 amps into the Batt's that are removed from the solar side. I know it's not continuous but it's power 8). This is the 440A/H that was removed.

    My G/F will go down and read volts and amps on the PV side of the controller and the Batt side of the controler and log them for me.

    Just getting some base line stuff down.
    Thanks again for all the help, I learned so much yesterday.

    BobO.
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