Battery sizing for 600W panels

trungsontrungson Registered Users Posts: 3
Hi,

Newbie here! I'm upgrading my existing old system from 2x100W to 6x100W panels and replacing the old battery. I just got Midnite Solar 150 Classic MPPT controller. I plan to re-use the current 12V inverter so it'll be a 12V system. I read here that the charge rate should be 5%-13% of the battery bank's Ah so that the batteries last as long as possible. Usage is small (some USB-powered wifi cameras). Would 600W panels enough to keep 4 6V (Costco golf cart) batteries (225Ah x 2) fully charged? 

Thanks for any advise.

Comments

  • PhotowhitPhotowhit Solar Expert Posts: 4,302 ✭✭✭✭
    So calculate your loads, we can't do this.

    Then figure how much charging you will get from your panels, 600 watts using a PWM charge controller, will yield differently depending on the VMP. many of today's 100 watt panels have a vmp of 18.9 volts. So 600 watts would yield about 30 amps IMP and a bit less in real world numbers. say 25 amps to be safe, should support about 250 amphour battery bank at 10%, I'd just use 2 golf cart batteries. 

    With daily cycling I like to use the 10% charging rate as a minimum. if you are in a particularly sunny area, say the Southwest US, you could support 4 6 volt batteries, but in areas with multiple cloudy days, I likely wouldn't try it.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • mcgivormcgivor Registered Users Posts: 1,352 ✭✭✭✭
    The 5-13% rule of thumb is a guideline, the lower rate being for occasional use such as a weekend cabin, the higher for full time off grid, in reality it really comes down to the loads present when there is no PV input. A calculation of the actual loads during no input is the most important figure of the equation, in order to determine what would be required to replenish what's taken out. So as an example the 450 Ah battery is discharged by 45Ah or 10%, the 600W array would produce 600W × 0.8 = 480W, 480W ÷ 14.4 V = 33.3A, now assume you have 3 hours of peak sun, there is approximately 100Ah available, enough to cover the withdrawal from the battery bank, including losses by a margin of~50% . It's always better to design with reserve capacity rather than being close to what the actual consumption is. The answer to your question, is a 600W array enough to keep 4 × 6V 225Ah batteries fully charged is, yes, as long as the loads do not exceed the array's ability to replenish withdrawals and losses by a healthy margin, to cover the days of reduced PV output. Just a non technical explanation since the question is in beginners corner. 
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • Raj174Raj174 Solar Expert Posts: 495 ✭✭✭✭
    @trungson
    Although the Classic 150 will allow more power production, ~40 amps in the summer and ~30 amps in the winter, or 9% charge rate in summer and 6% for winter with a 450 amp hour 12 volt battery bank, I would agree with Photowhit in recommending two 225 amp hour batteries. I you had higher loads, I would say go with the 450AH bank and add a couple more panels, but with the current load, I don't see any reason to risk insufficient charging during winter months. If going with a 12 volt 225AH battery bank, I would recommend setting the charging amps limit in the Classic to no more than 29 amps to protect the batteries from current above the 13% rate.

    Rick    
    12 x 300W Renogy PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 195AH HI Power LiFePO4 no BMS, 4000W gen.
  • ThomThom Solar Expert Posts: 172 ✭✭✭
    I have 4 batteries and 430w solar plus windmill which isn’t much.  . Batteries will b 8 years old in July. Inverter is only on at night and Sunny days when I’m sure of full charge of batteries. Charge phones iPads , DSL with 12v . I run a generator to pump water, vacuume every 3 days . When no sun for a few days I charge the batteries with 2 black and decker 10 amp smart chargers. Every 1st of the month perform battery maintenance. 


    Thom
    Off grid since 1984. 430w of panel, 300w suresine , 4 gc batteries 12v system, Rogue mpt3024 charge controller , air breeze windmill, Mikita 2400w generator
  • trungsontrungson Registered Users Posts: 3
    Thanks everyone for their answers! Very useful for beginner like me. Also, another question: if I only use USB-powered devices, should I skip the inverter (or only turn it on when needed) and instead use a car USB charger (12V to 5V) since it's direct DC? Thanks
  • EstragonEstragon Registered Users Posts: 1,699 ✭✭✭✭
    Depending on the size and type of inverter, using a 12v car charger could save a fair bit of power. Inverters can be pretty inefficient powering loads that are small relative to inverter capacity. Some use a lot just being on.

    The only reason I can think of to use an inverter would be if the loads are a long distance from the batteries (120vac will have less voltage drop), or if the inverter is on anyway for other loads.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • ThomThom Solar Expert Posts: 172 ✭✭✭
    trungson said:
    Thanks everyone for their answers! Very useful for beginner like me. Also, another question: if I only use USB-powered devices, should I skip the inverter (or only turn it on when needed) and instead use a car USB charger (12V to 5V) since it's direct DC? Thanks
    That’s what I do . 2 triple 12v accessory  sockets. 120v only on when needed. 

    Thom
    Off grid since 1984. 430w of panel, 300w suresine , 4 gc batteries 12v system, Rogue mpt3024 charge controller , air breeze windmill, Mikita 2400w generator
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