Battery Charge Rate ? Check my Math Please.

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verdigo59
verdigo59 Registered Users Posts: 2 ✭
in Solar Beginners Corner #1
MY Old Interstate GC2 Battery Bank bit the dust after around 6 years, and because of limited funds I bought some cheap Rual King GC2s. System is 48 volts using 8 180 ah GC2 batteries to replace the 16 interstate batteries. If I remember correctly my charge rate should be 10-15% of the amp hour rate. I have a Midnight Classic 150 charge controller and too many panels in full sun for the amount of batteries at present. So at 180ah x .15 = 27 amps. I can limit the Classic to that but I like air conditioning. Can I safely go to C20 On sunny days. With my two window units going at the same time my Outback Mate says I am using a total of 2400 watts at times Is 36 amps too much for this battery bank and have them live a couple years? 

Thanks
Dennis
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  • softdown
    softdown Solar Expert Posts: 3,822 ✭✭✭✭
    #2
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    I can't answer the math equation but I'll make a different point. I think our golf cart batteries are made by either Johnson Controls or East Penn. I'm not aware of a present difference in the quality of their GC2s. Personally - I prefer East Penn batteries.

    Guessing you could charge at roughly 40 amps - going from rusty memories here.
    First Bank:16 180 watt Grape Solar with  FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    edited September 2020 #3
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    Do you have a Whiz Bang Jr and external battery shunt?

    https://www.solar-electric.com/midnite-solar-whiz-bang-jr-current-sense-module.html
    https://www.solar-electric.com/search/?q=current+shunt

    You can program your Classic to limit charging current--But it will still output full current (load+max battery charge current) very nice.

    Watch the battery bank temperature--You can charge upwards of 20% rate of charge if needed.

    You are using  a remote battery temperature sensor?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #4
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    The battery charging will largely take care of it's self, If you see C20 displayed at the charge controller, understand that much of that is NOT going to the battery bank, but is going to the inverter for air conditioning.

    If you can read the wattage or amps your inverter is using, you can deduct them from the charging amps going to the battery bank.

    So at a system voltage of 58 volts you see the charge controller indicating 36 amps with the 2 window units going, I'll assume a minimum of 440 watts each that's 880 watts or (880/58=) 15 amps the battery bank isn't seeing. So assuming nothing else is running, that's (36-15=) 21 amps going to the battery bank well with in the 10-15% range you are desiring.
    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.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #5
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    So long as you have a temperature sensor from your charge controller to your battery bank, you should be fine. The charge controller basically limits voltage, the flooded lead acid battery bank, limits the current flow. Once the system hits absorb voltage the charge controller keeps the voltage there. The batteries have reached about 80% full and they slowly get topped of limiting the amount of current they will accept. If the batteries get hot the charge controller will reduce the voltage.
    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.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #6
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    BTW - You shop at Rural King, you are likely near me, I'm in Central Missouri. We've had an ugly year for air conditioning, with it clouding up and staying hot in the afternoons. I've got minimal reserve capacity, though 50% more than you have (roughly) and have run my battery pretty low by morning because it's been draining the battery after 2 or 3 and sometimes not getting fully charged before that. Bad weather for my system. The brutal heat coming next week will likely be better, since I'll be running off the array until 7:30-8.
    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.
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  • verdigo59
    verdigo59 Registered Users Posts: 2 ✭
    #7
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    Thanks for all the input. I am kind of late getting back, and was out of the state for a bit. My inverter has died. It is an Outback 3648 Interactive 48 volt unit. No breakers have tripped and the battery bank and charge controller are fine. Guess its time to crack it open.
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #8
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    Hi vertigo,   sorry to hear of the issue with your inverter.

    To follow up on BB Bill's comment about using the WbJr/shunt to Limit the battery charge current,   here is how to do the setup:

    http://www.midniteftp.com/support/kb/faq.php?id=37

    Good luck,   Vic 
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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