Newbie question Re: 24v battery string and 12v inverter

S2T_Bama
S2T_Bama Registered Users Posts: 8 ✭✭
Hi everyone,
I'm a newbie to solar power. I've been tinkering around with a set of Harbor Freight panels for several months now. I upgraded to a Renogy (was Tracer) 20A MPPT charge controller which allowed me to wire my 3x15 watt panels in series. I use this to run security lights at night.
Anyway, I'm trying to expand and have just gotten a Grape Solar 160 watt panel which will replace the HF panels. If (when) I add a 2nd 160 watt panel, the charge controller limits on amperage will require me to convert to 24v. I plan to use 4 x GC2 batteries in series at that point.

My question is, can I connect a 12v inverter to the "center" 2 batteries in this string? 

Thanks in advance for your guidance!

Adam
(S2T_Bama)

Comments

  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    S2T_Bama said:
    My question is, can I connect a 12v inverter to the "center" 2 batteries in this string?
    You can, but you should not.  You need to try and keep the batteries balanced.  By drawing current from some, but not all of them you unbalance the bank and that will shorten the life of the battery bank in several ways.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    4x golf cart batteries--You are right in the area where you could go with a larger 12 volt solar charge controller, or convert over to a 24 volt battery bank (and AC inverter/24 volt loads).

    What are your load needs? If you do not grow the system (or loads) much more--Either could work out fine for you.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • S2T_Bama
    S2T_Bama Registered Users Posts: 8 ✭✭
    Thanks for the feedback, I suspected that was the case having read a few posts here. I will bite the bullet, and sell the Craigslist bargain 12v inverter to partially pay for a new 24v inverter.

      I plan to expand to 3 x 160 watt panels (=480 watts) which will max out capacity of my charge controller. My plan is to run my 18 cu ft garage refrigerator from this system with some excess capacity. Kill a watt readings on the fridge indicate it consumes 0.91 kWh/day. If I assume 4h insolation / day average that should give twice the needed power, a nice safety margin. Since I'm not off grid, I can charge batteries from the grid if we go through an extended cloudy period.

    If this is reasonably successful, I will probably replicate this "module" to run more small appliances, electronics, and lighting for emergency backup during our stormy Spring and Fall.

    Thanks again!
    Adam
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    A refrigerator is a "difficult" load for a smaller solar power system... It is enough to push you from a "small" system to a medium size system. You need a large enough inverter+battery bank to start/run the fridge, and a larger solar array to keep the battery bank "happy".

    At a minimum you need a 1,200-1,500 Watt AC inverter for a "typical" energy star fridge. And, ideally, roughly a 480 AH @ 12 volt battery bank to supply enough surge current.

    So, some quick math assuming 2x golf cart batteries (220 AH @ 6 volts) in series for 12 volts. 2x parallel strings for 440 AH @ 12 volt battery bank.

    A 440 AH @ 12 volt battery bank would be good for (assuming 2 days storage (1-3 days recommended), 50% maximum discharge):
    • 440 AH * 12 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 maximum discharge = 1,122 Watt*Hours per day (no sun, over night, etc.)
    To charge such a bank, need 5% to 13% rate of charge... 5% for an emergency/seasonal system. 10%+ for full time off grid:
    • 440 AH * 14.5 volts charging * 1/0.77 panel+charge controller deratings * 0.05 rate of charge = 414 Watt array minimum
    • 440 AH * 14.5 volts charging * 1/0.77 panel+charge controller deratings * 0.10 rate of charge = 829 Watt array nominal
    • 440 AH * 14.5 volts charging * 1/0.77 panel+charge controller deratings * 0.13 rate of charge =1,077 Watt array "cost effective" maximum
    There there is sizing the array based on where you live. Guess at Mobile AL, fixed array:
    http://solarelectricityhandbook.com/solar-irradiance.html

    Measured in kWh/m2/day onto a solar panel set at a 60° angle from vertical:
    (For best year-round performance)
    Jan Feb Mar Apr May Jun
    3.41
     
    3.95
     
    4.76
     
    5.23
     
    5.35
     
    5.07
     
    Jul Aug Sep Oct Nov Dec
    5.14
     
    5.11
     
    5.02
     
    4.85
     
    4.03
     
    3.54
     
    Say 1,122 WH per day average load and 4 hours of sun (November):
    • 1,122 WH per day * 1/0.52 typical off grid system eff * 1/4.03 hours per day of sun = 535 Watt array minimum for November "break even" month
    Your thoughts?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • S2T_Bama
    S2T_Bama Registered Users Posts: 8 ✭✭
    My order of mathematical operations was different, so I'll have to go through yours carefully to see how they compare. Thank you for including the efficiency factors which I had not included. I will be using a 24v inverter, and hoped to get by with a 1000 watt model. I take it that's not enough? I'm disappointed, since the mfg said it should be. Nevertheless, I'll go with your real world (not marketing) recommendation, thanks.

    The panel angle is quite different from what I've read and determined empirically, though. 60* from vertical would not be a good year -round angle here in Alabama (Birmingham area = 33.5* latitude). In summer, the sun is about 15* from vertical, and your suggestion would be okay. But in Winter, it is more like 55-60* from vertical, and that angle would be totally ineffective. I have been adjusting panel angle quarterly, and perversely tend to think of angle from horizontal.
    Using that construct, my summer angle is about 15* from horizontal. Fall / Spring angle is about 45*. Winter angle is about 70* from horizontal. These values give pretty close to perpendicular angle of incidence determined by shadow angle.

  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    S2T_Bama said:
    My order of mathematical operations was different, so I'll have to go through yours carefully to see how they  I will be using a 24v inverter, and hoped to get by with a 1000 watt model. I take it that's not enough? I'm disappointed, since the mfg said it should be. Nevertheless, I'll go with your real world (not marketing) recommendation, thanks.
    Your refrigerator is what I refer to as a 'defining load'.  It defines the need for a larger inverter that you otherwise wouldn't need.   A larger inverter has larger tare losses.   My outback inverter has a tare loss of 20 watts (very good for a 3500 watt inverter), but that 20 watts over 24 hours (480 watthours) uses almost as much energy as my fridge.

    When you have a defining load such as your fridge, it often makes sense to spend more on the fridge and get a model that has no surge.  There are a number of DC fridges that fill the bill, and now there are AC versions available. 

    These "non surge" fridges use variable speed compressors.  It is more efficient to run a compressor all the time at one third speed, than to run it at full speed for one third of the time.  As a bonus, these variable speed compressor fridges (in addition to being more efficient) have no startup surge.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭

    More things to think about, in what environment did you test the fridge? During winter when ambient temps were in the 70's? You'll need more run time when ambient temps are in the 100s.

    Also if you are hoping to run this on a modified sine wave inverter, it will use more energy than with a true sine wave inverter and may reduce the life of your compressor as motor run hotter on a modified sine.

    You are likely to have more sun exposure during the hot months so things might offset. I lived in Tallahassee for a long time. and after noon clouds were a daily event. so your micro climate might be a factor in setting the best panels angle.

    You might try sticking with a 12 volt system, 480 watts of panels will typically provide about 30 amps so pretty close to 13% of 2 golf cart batteries, or in the recommended charging value for a 12 volt 220 amp hour battery bank. Save the $200 on batteries and buy a small chest freezer and convert it to run at refrigeration levels.

    Good Luck.

    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.
  • S2T_Bama
    S2T_Bama Registered Users Posts: 8 ✭✭
    Defining load. I like that.

    Anyway, I planned to use a pure sine wave inverter. Regarding the starting surge, can I quantify that with the Sears clamp on multimeter that I have seen referenced elsewhere on here? I would prefer to know the load as accurately as possible.

    Continuous running load is ~125 watts. I have read startup surge can run 4-7 times this amount, and that 's the reason I said I'd use a 1000 PSW inverter. The rated surge is 2000, but I know that is meaningless for fridge startup because it is longer duration than used to arrive at that number. Recommendation for a 3500 watt inverter, though, puts the multiple at well over 20X for surge amperage.

    Given these widely varying rules of thumb, I think I'd better figure out a way to get real world numbers. Any suggestions are greatly appreciated, as is the wisdom shared so far!

    Adam


  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    S2T_Bama said:
    Regarding the starting surge, can I quantify that with the Sears clamp on multimeter that I have seen referenced elsewhere on here? I would prefer to know the load as accurately as possible.
    No.  That meter is a great bargain for a DC clamp meter, but you need a meter that can capture and hold shorter peak currents. 

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    S2T_Bama said:
    Defining load. I like that.

    Anyway, I planned to use a pure sine wave inverter. Regarding the starting surge, can I quantify that with the Sears clamp on multimeter that I have seen referenced elsewhere on here? I would prefer to know the load as accurately as possible.

    Continuous running load is ~125 watts. I have read startup surge can run 4-7 times this amount, and that 's the reason I said I'd use a 1000 PSW inverter. The rated surge is 2000, but I know that is meaningless for fridge startup because it is longer duration than used to arrive at that number. Recommendation for a 3500 watt inverter, though, puts the multiple at well over 20X for surge amperage.

    Given these widely varying rules of thumb, I think I'd better figure out a way to get real world numbers. Any suggestions are greatly appreciated, as is the wisdom shared so far!

    Adam



    S2T_Bama said:
    Defining load. I like that.

    Anyway, I planned to use a pure sine wave inverter. Regarding the starting surge, can I quantify that with the Sears clamp on multimeter that I have seen referenced elsewhere on here?
    No, you'd really need a scope plus a current probe for that.  The scope allows you to capture the waveform of the turn on pulse.
  • S2T_Bama
    S2T_Bama Registered Users Posts: 8 ✭✭
    I found the locked rotor amperage (I think) on a replacement compressor for my refrigerator. That number is 11.7A. So, at 120v that works out to 1404 watts. So in my case, that makes the "multiple" about 10. i.e. the surge wattage is about 10x greater than running wattage.
    It's too bad this "defining load" will change my plans pretty substantially.  A 24v 2000 watt pure sine wave inverter will cost almost as much as the fridge. I guess I'll examine some alternatives in a spreadsheet to decide my next move.
    Thanks again for your help and advice!
    Adam
  • Johann
    Johann Solar Expert Posts: 245 ✭✭✭
    S2T_Bama said:
    Defining load. I like that.

    Anyway, I planned to use a pure sine wave inverter. Regarding the starting surge, can I quantify that with the Sears clamp on multimeter that I have seen referenced elsewhere on here? I would prefer to know the load as accurately as possible.

    Continuous running load is ~125 watts. I have read startup surge can run 4-7 times this amount, and that 's the reason I said I'd use a 1000 PSW inverter. The rated surge is 2000, but I know that is meaningless for fridge startup because it is longer duration than used to arrive at that number. Recommendation for a 3500 watt inverter, though, puts the multiple at well over 20X for surge amperage.

    Given these widely varying rules of thumb, I think I'd better figure out a way to get real world numbers. Any suggestions are greatly appreciated, as is the wisdom shared so far!

    Adam



    S2T_Bama said:
    Defining load. I like that.

    Anyway, I planned to use a pure sine wave inverter. Regarding the starting surge, can I quantify that with the Sears clamp on multimeter that I have seen referenced elsewhere on here?
    No, you'd really need a scope plus a current probe for that.  The scope allows you to capture the waveform of the turn on pulse.
    Or something like that . I got to play with one today for the exact reasons,  
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    This is when you have to look closely at the inverter specs, if the required wattage is within the inverters 1 second capacity.  It's possible an 800W inverter could start the fridge, if there are no other loads at the same time.   All in the fine print of the specs
    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 ,

  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    Johann said:
    No, you'd really need a scope plus a current probe for that.  The scope allows you to capture the waveform of the turn on pulse.
    Or something like that . I got to play with one today for the exact reasons,  
    That's definitely a nice tool.  But for $3500 you can probably get a used scope and current probe - and scopes are more versatile test instruments, IMO.