How low can i go?

yanyan Registered Users Posts: 2
Hi everyone
             First of all, i want to thank you for your  answers in advance and also want to apologize for my bad english, wish you still can understand my questionsss!  I've build this little set up for the cabin; (4x) 100w pannels, (6x) 6v 232ah batteries mounted for 12v, a cheap 30a coleman regulator and a 3000w inverter. The general use is kindda low, not really more than 100w at once for the cpap machine (for 8 hours)or the 50a needed for the satellite internet. During day time i've no issue in my opinion, everything's just fine, the batteries keep charging and i have plenty of power to run a lot of things (yes i've tested) there's my questions;

     #1- when the sun goes down and the battery indicator shows 13v, with a little charge (60w) , it quickly go down to 12.6/12.5 and freeze there for a couple hours and then drop to 12.4 for a little while...and so on..How low can i let it go without a premature damage to the batteries?

   #2- when i'm running the generator on a rainy day, what can i use and how should i connect some kind of a charger to fill them up. Can i use a 12v charger and charge the batt bank at once?

 Right now i use a manually switched jumper whether i use the batteries or the gen.

I've more questions but those two are the most concerning right now.

Thanks again and enjoy your lockdown!

Comments

  • 706jim706jim Solar Expert Posts: 448 ✭✭✭✭
    Lead acid batteries will quickly drop to about 12.4 volts when the charging source is removed. The voltage above this I think of as "surface charge" which looks impressive, but disappears with even the most modest load.

    So don't worry about that.

    And yes you can use a standard 12 volt charger to help restore charge to the batteries as they wear down although generator charging gets less efficient over time as the battery voltage slowly increases.

    The law of diminishing returns.
    Island cottage solar system with 2400 watts of panels, 1kw facing southeast 1kw facing southwest 400watt ancient Arco's facing south.Trace DR1524 MSW inverter, Outback Flexmax 80 MPPT charge controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 28th year.
  • BB.BB. Super Moderators, Administrators Posts: 32,204 admin
    Welcome to the forum Yan,

    I guess you are in the "Great White North"? New Brunswick or Quebec?

    Anyway, to make some clarifications. Your battery bank is 6x 6 volt @ 232 AH batteries... Probably "Golf Cart" type and flooded cell. Made into 2x 6 volt in series (for 12 volts) and 3x parallel strings (3x 232 AH) for 696 AH @ 12 volt battery bank.

    With flooded cell batteries, you should have a Hydrometer of some sort:

    https://www.solar-electric.com/search/?q=hydrometer

    A hydrometer is the "Gold Standard" for measuring your battery capacity, and general maintenace (checking each cell for specific gravity once a month, checking electrolyte levels, adding distilled water when needed--Do not overfill, do not let plates get exposed, do not fill until batteries are "warm" and "bubbling" near full charge--I.e., avoid 100% fill, especially with cold batteries, they can pump electrolyte out the caps when warmed up and bubbling).

    We like to design a system based on your daily loads... But in this case, since you have the battery bank, we can "define" the ideal daily loads--Typically discharging to 75% (25% cycle depth) per day, for 2 days, to 50% max discharge (for longer battery life).

    Ideally, use the specific gravity to check a "pilot cell" to estimate bank state of charge (rinse out hydrometer with a couple shots of distilled water before putting away--And glass hydrometers tend to roll off tables and break).

    For your bank, I would suggest the daily (over night/1-2 days of cloudy weather) available energy as:
    • 12 volts * 696 AH * 1/2 days of storage * 0.50 max discharge * 0.85 AC inverter eff = 1,775 Watt*Hours per day
    • OR 696 AH * 0.25 discharge per day = 174 AH @ 12 volts of daily usage
    Next, charging from solar... Typically 5%, 10-13%+ rates of charge are suggested. 5% good for weekend/sunny non-winter days. 10%+ suggested for full time off grid usage:
    • 696 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 655 Watt array minimum
    • 696 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,311 Watt array nominal
    • 696 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,704 Watt array typical "cost effective" maximum
    And then there is sizing the system for the amount of energy you use per day... Say:
    • 100 Watts * 8 hours per day = 800 WH per day CPAP machine
    • 50 AH * 12 volts = 600 WH per day Stat Internet/Tablet/etc.
    • Total = 1,400 WH per day
    So, lets go ahead and use the 1,775 WH per day for 25% discharge of bank... And guess you are around Fredericton, NB. Fixed array facing south:
    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Fredericton
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 45° angle:
    (For best year-round performance)

    JanFebMarAprMayJun
    2.67
     
    3.59
     
    4.52
     
    4.69
     
    4.56
     
    4.75
     
    JulAugSepOctNovDec
    4.68
     
    4.73
     
    4.32
     
    3.37
     
    2.39
     
    2.15
     

    And let's assume that we drop the bottom three months (not there, use genset when needed, etc.):
    • 1,775 WH per day * 1/0.52 off grid end to end AC efficiency * 1/3.37 hours of sun (October break-even) = 1,013 Watt array (Oct break even)
    Your present 400 Watt panels is too small for your size of battery bank (that is a good sized 12 volt battery bank). An array of 655 to 1,704 Watts would make sense (depending on your needs/my guesses).

    I would suggest that you get a DMM (digital multimeter)--And a DC Current Clamp meter is really nice to have (you can measure AC/DC current just by putting a "clamp" on the cable--very easy and safe--Makes it easier to understand how your system works and debugging):

    https://www.amazon.com/gp/product/B019CY4FB4 (mid-price meter)
    https://www.amazon.com/UNI-T-Digital-Current-Capacitance-Multimeter/dp/B0772FYF5M (low cost/good enough for our needs)

    And another suggestion... Your 3,000 Watt AC inverter is just too large for this battery bank. Based on battery bank AH capacity, the suggested largest inverter would be:
    • 696 AH * 0.85 AC inverter eff * 250 Watt inverter * 1/100 AH [12 volt fudge factor] = 1,479 Watt inverter suggested maximum
    • 1,479 Watt inverter / 2 = 740 Watt AC inverter "good midrange" choice
    A problem with very large AC inverters (relative to AC battery bank)--First they take a lot of current at full load:
    • 3,000 Watt inverter * 1/0.85 AC inverter eff * 1/10.5 battery cutoff voltage = 336 Amps
    Or, that inverter would drain your bank in less than 2 hours at full rated load (plus, 336 Amps needs very heavy/short inverter DC power cables).

    The other is that very large inverters can use 20-40 Watts "just turned on" (tare losses)... If your inverter uses 30 Watts, that is energy wasted from the battery bank (100 Watt CPAP load + 30 Watt inverter Tare = 130 Watt "battery load")... Not even using the 85% inverter efficiency losses.

    Your better bet would be a much smaller AC inverter--A couple to look at (our host, but you are welcome to buy elsewhere):

    https://www.solar-electric.com/morningstar-si-300-115v-ul-inverter.html (nice, rugged, has remote on/off, and "low power search mode")
    https://www.solar-electric.com/sa300wa12vos.html (this one also charged to 16.5 VDC--Good for Lead Acid batteries in sub freezing weather).

    Just some starting suggestions. Lots of choices out there.

    For discharge voltage limit... Voltage is not an accurate method for measuring FLA battery Capacity... But a good place to start is 12.0 volts max discharge (resting/no loads for 3+ hours). And to avoid running below 11.5 volts under load (you may get to 10.5 volts during surge conditions--That won't hurt). If you discharge below ~75% state of charge--You do want to get back to 80-90%+ state of charge the next day or too... FLA batteries sitting at below 75% state of charge will start sulfating (permanent loss of capacity)... Days/weeks/months at low state of charge can kill your batteries.

    And regarding charging, you can parallel two or more battery chargers. The AC battery charger--You need to make sure that you don't over charge the battery (at ~25C, you want around 14.75 volts for an FLA battery charging... And once fully charged turn off the charger/drop to float charging of ~13.2-13.6 volts).

    I will stop here... A long post with lots of information. Questions/Corrections?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • yanyan Registered Users Posts: 2
    Wow! Thanks a lot sir! I'll read it all again trying to assimilate all of this but be sure that this is really kind and well appreciated. BTW i'm a french Quebecers 😜 but solar coef for NB is good enough. It's the most valuable, and personnalized infos i was able to get so far.  People around here are more into selling kits than explanations.  
  • mcgivormcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    yan said:
    Hi everyone
                 First of all, i want to thank you for your  answers in advance and also want to apologize for my bad english, wish you still can understand my questionsss!  I've build this little set up for the cabin; (4x) 100w pannels, (6x) 6v 232ah batteries mounted for 12v, a cheap 30a coleman regulator and a 3000w inverter. The general use is kindda low, not really more than 100w at once for the cpap machine (for 8 hours)or the 50a needed for the satellite internet. During day time i've no issue in my opinion, everything's just fine, the batteries keep charging and i have plenty of power to run a lot of things (yes i've tested) there's my questions;

         #1- when the sun goes down and the battery indicator shows 13v, with a little charge (60w) , it quickly go down to 12.6/12.5 and freeze there for a couple hours and then drop to 12.4 for a little while...and so on..How low can i let it go without a premature damage to the batteries?

       #2- when i'm running the generator on a rainy day, what can i use and how should i connect some kind of a charger to fill them up. Can i use a 12v charger and charge the batt bank at once?

     Right now i use a manually switched jumper whether i use the batteries or the gen.

    I've more questions but those two are the most concerning right now.

    Thanks again and enjoy your lockdown!


    An observation, if you are using this system daily, the array may not be able to sufficiently charge the batteries. Although this may not be obvious at first because the battery  capacity is large thus masking undercharging. One  needs to ensure the voltage rises to somewhere in the 14.4V region  (follow the manufacturer recomendations ) , holding that value for around 3 hours or more to ensure absorption is completed. When designing a system it's best to do a load calculation first as reverse engineering is more difficult.

    Other things to consider, the inverter simply being on is also a load, the larger and cheaper it is generally the greater the self consumption, some as high as 75W or more. Then there is inverter efficiency, at low loads say 100W on a 3000W unit the efficiency may be as low as 40%, so the 100W load becomes 160W as an example.

    Solar modules, panels, do not produce their rated output but usually around 75-85% depending on temperature and location, this needs to be considered in design as well. For full time a minimum  10% of the battery capacity charging so for 696Ah is 69A, part time 5% is often used or 35A, currently your array, I would estimate, produces around 23A max on a good day or 3.3%.

    As far as charging with a generator using a small automotive charger is a poor choice, a high current charger, around 60A in your case will make more efficient useage of fuel.

    Having said all this, if this system is used for one or two days a week it may be fine, more details would be helpful such as equipment  specs. all loads including inverter run hours.

    Hope this is useful to you 

      



    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • BB.BB. Super Moderators, Administrators Posts: 32,204 admin
    You are very welcome Yan.

    Around here we ask questions and try to model (some math) the system first... Before getting into details--And long before suggesting buying any equipment. Paper designs are much cheaper to change vs the "real hardware".

    Also, suggest you look at battery based/portable CPAP machines. They tend to be designed to use much less power--And conservation is usually the first step when going off grid. It is usually cheaper to conserve power than to generate it.

    If you need humidification, you may be stuck with those that have heating for humidity and using a (heated) hose the air from cooling and condensing out the moisture.

    Anyway, lots of information and jumping in with both feet. Please feel free to ask questions and adjust my guesses. We try to be relative conservative here--It is better to have a somewhat more expensive system that meets your needs, vs a cheap system that does not.

    Also, you may already have one, but a Kill-a-Watt meter is handy for measuring your AC power (Watts) and energy (Watt*Hour) usage.

    https://www.amazon.com/s?k=kill-a-watt+meter&crid=2HCTCXRE1TA9D

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