Upgrading an old system and need a sanity check

TecateKid Registered Users Posts: 6 ✭✭
Hello, I have been using an old PV system on an off-grid house in Mexico. I've been burning through batteries every two years so I've decided to start over. The old system was 12v with 6, 135w panels, Xantrex sw2512 inverter, outback Flexmax80 controller and 4, L-16 Trojan batteries.
We have figured out that the batteries have been undercharging all this time. Being that the inverter is old (and 12v) we've decided to upgrade that as well. Assuming all shunts and circuit breakers are in place, wire sizes and equipment locations have been considered, I'd like to get a sanity check on this system. So here is what I've come up with...

We have calculated that we (if EVERYTHING is on at once) draw 4500wh/day. It's rare the sun doesn't shine but I like to plan on 3 days of autonomy just in case. 

So, the big question is...Does this look like it'll charge (and keep) the batteries above their 50% charge rate? Suggestions?

Thanks in advance.


  • MrM1
    MrM1 Registered Users Posts: 487 ✭✭✭✭
    edited September 2019 #2
    Just wondering,  when you say "if EVERYTHING is on at once" and they you say "we draw 4500Wh/ day".    Do you run everything on all the time 24 hrs a day?  Watt Hours per day I would think would be how many Watt hours you use in a 24 hr period on a typical day of typical loads.  That is usually calculated with something like a KIll-A-Watt meter.   So not sure if you just added up all your loads, lights and appliances, or if you did a load calculation.  Is that 4500 Wh/day or 4500 watts per hour?  Not being critical just trying to understand what you mean.
    But with your proposed bank,  you would have about 8,880 watts of storage down to 50%.  Your array would be good for at best 3,700 Watts.  You try using the Midnite solar sizing tool.  I know you are not using any midnite solar products,  but it will give you an idea as to if you are on track. 
    BTW.  I really like the TwinPeak 2 panels.
    REC TwinPeak 2 285W 3S-3P 2.6kW-STC / 1.9kW-NMOT Array / MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware / OB PSX-240 Autotransfomer for load balancing / Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system / Grid-Assist or Backup Solar Generator System Powering 3200Whs Daily / System went Online Oct 2017 / System, Pics and Discussion
  • TecateKid
    TecateKid Registered Users Posts: 6 ✭✭
    The load was calculated based on using a Kill-A-Watt meter for each device and then adding all worst-case scenarios together. In reality, we probably use 1/3rd of that. I used the Midnight Solar tool you suggested but it only suggested I use another controller.

  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    The L 16 batteries would like to see something better than ~40 amps regularly.  The 3270 watts of PV should put out ~55-60 amps as configured in rough numbers.  

    My rule of thumb for off grid goes like this, take the name plate rating of the PV, divide in halve to account for all cumulative system loses, then multiply that number by 4 to account for the AVERAGE hours of GOOD sun, per day over the course of the year.  (YMMV)

    So for a load calc, your system should put our an AVERAGE of ~7.4KWH/day average.  So, at first glance it would appear that your system is reasonably well sized given what you have given us.  I suggest a better load analysis however. If in fact your average daily use is ~1/2 of that, closer to 1.5 kwh, you could save A LOT of money, a halving the battery bank, keeping the PV at the right amount to keep that bank happy. 

    The mistake that many people make is too much battery and not enough PV.  extra PV and extra battery is generally sort of wasted.  I use ~1-1.5 kwh/day, and do fine with ~500 watts of PV, and 4 T105s, into 12 vdc, or 450 AH (and I am on year 12 with those batteries. 

  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    If I read your diagram correctly, you have to rewire your solar array. For a 48 volt battery bank on an MPPT charge controller, you want >80 volts Vmp-array (to around 100 Volts maximum Vmp-array, depends on how cold it gets for you place in Mexico).

    For a Vmp~33.5 volt panel, you would want to connect in series for a the Outback (note, these are quick estimates, you need to calculate for your panels and minimum temperature for your area):
    • 80 volts array minimum Vmp / 33.5 Vmp panel = 2.3 panels > 3 panels series minimum
    • ~100 volts array max Vmp (cold) / 33.5 Vmp = 2.99 > 3 panels maximum series 
    • 12 panels / 3 panels per string = 4 parallel strings (of 3 panels each)
    • 80 amps * 59 volts battery charging * 1/0.77 panel+controller derating = 6,130 Watt array "typical cost effective maximum"
    • 6,130 Watt max array / 310 Watt panels = 19.8 panels => ~20 panels cost effective maximum
    • 18 panels / 3 panels per string = 6 parallel (3x series) strings array or 18 panels (sort of optimum--21 panels and 7 strings of 3 could work too).
    Midnite should have similar numbers for their 150 Volt Classic controllers with 3 series x 6 parallel panels (18 total for a ~1.2x "over panelling"):


    PV Array
    Rated PV Array Power:5580Watts
    Anticipated Array Power @ 104F:5203Watts
    Rated PV Array Current:55.56Amps
    Battery Charging Current @ 57.6 V:96.9Amps
    VMP (Maximum Power Point Voltage) :100.5Volts
    VOC (Open Circuit Voltage):120Volts
    VMP @ 20 F°:114.9Volts
    VOC @ 20 F°:132.6Volts
    Classic, Classic SL & Classic Lite Charge Controller Selection
    Max Operating Voltage150200250
    Max Non operating VOC (HyperVOC) @ 48V Nominal Battery Voltage198248298
    Maximum Number Of Modules In Series345
    Max Number Of Modules In Series (Using HyperVOC)456
    Max Allowable Output Current Per Classic
    Based On This Current Configuration
    Max Allowable Wattage Per Classic
    Based On This Current Configuration
    Present PV Array Wattage Of This Configuration558055805580
    Design Check
    Temperature The Classic Will
    Enter HyperVOC
    -59.8 F°-286.6 F°-513.4 F°
    Classics Required1.21.41.8
    NOTE: MidNite Solar recommends a second controller be added after 1.2
    NOTE: Generally speaking you'll want to use the Classic 150 or 200 as they are less expensive and will handle more power. With MPPT controllers the higher the input voltage the less efficient they are. This is not a large value but it will add up to a little more heat in the controller and a point or two less in efficiency. BUT you also have to be careful not to have the input voltage to low. Most all MPPT controllers will want to see a minimum of 130% of the actual high battery voltage. So if we have a 48v battery and it has an Equalize voltage if 62.3 volts than we would multiply that by 130% and we would need a minimum of 81 volts on the input on the hottest day of the year in order to have enough headroom for the MPPT to work.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    If your batteries are short lived, it's very likely you are undercharging them and need to add more solar.

    As to the size of the battery bank, if you size for 50% draw down nightly, you will be buying new batteries every 3 years.
     If you increase the bank to be only using 20% a night, you will buy a larger bank of batteries every 5 years.  
      Which scenario fits your budget ?  A fresh bank every 2 years ?   Or a larger bank, for 5 years & more $$ ?

    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 ,

  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,044 ✭✭✭✭✭
    edited September 2019 #7
    Tecate,  What are your plans for the SW 2512?  I'll be driving to San Felipe on Saturday, Heading east on the 8.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

  • mvas
    mvas Registered Users Posts: 395 ✭✭✭
    TecateKid said:
    Hello, I have been using an old PV system on an off-grid house in Mexico.
    I've been burning through batteries every two years so I've decided to start over.
    We have figured out that the batteries have been undercharging all this time.
    Thanks in advance.
    How did you not know that your battery bank was being undercharged, in like a week or a month?
    Are you using AGM or Flooded batteries?

    Was your Charge Controller getting past Bulk and Absorb Mode and into Float Mode, on a daily basis?

    At what battery voltage did the Charge Controller change from Bulk Mode, to Absorb Mode?
    How many amps were flowing when the Charge Controller changed from Absorb Mode, to Float Mode?
    How long was the Charge Controller in Float Mode, before sunset?
    Do you have a Voltage DROP between the Charge Controller output terminals and the Batterer Bank terminals?

    PV Panel Vmp = 33.5 volts
    67 volts Vmp per string = 33.5 volts x 2 panels in series 
    53.6 Volts ( real world ) = 67 Volts Vmp x 80% ( it gets hot in Mexico ! )
    53.6 volts is barely enough voltage to FLOAT the battery bank.

    What exactly was wrong with the old system ? 
    Was the PV Voltage too low ? 
    Best not to repeat the same mistake, again ...