Check my charge rate math please

Thick8
Thick8 Registered Users Posts: 49 ✭✭
in Off Grid Solar & Battery Systems #1
8D 12v FLA battery 430 minute reserve capacity. 
430x25/60 = 179Ah battery 
179/8 = 22 amp charge rate
Discharge rate should be equal to charge rate. So with 4 batteries I have a (recommended) max amp draw of 88 amps or 1056 watts. 
Yes?
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  • 706jim
    706jim Solar Expert Posts: 523 ✭✭✭✭
    #2
    Ok but for how long?

    Island cottage solar system with appriximately 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing due south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter which has performed flawlessly since 1994. Outback Flexmax 80 MPPT charge controller four 467A-h AGM batteries. Insignia 11.5 cubic foot electric fridge 1/4hp GSW piston pump. My 31st year.
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #3
    With a reserve capacity rating, it's likely a starting battery, designed for high rate discharge for short periods (seconds).  The rate should be specified (Cranking Amps and/or Cold Cranking Amps) by the manufacturer, and is likely higher than 22a/battery.

    This type of battery can be used in a deep cycle or standby application, but with a reduced life expectancy vs batteries designed for such applications.
    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
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  • Marc Kurth
    Marc Kurth Solar Expert Posts: 1,174 ✭✭✭✭
    edited January 2020 #4
    Yes, your battery rated storage capacity is around 179 ah based on a 25 amp load for 7 hours, to reach 0%. I am unclear about your statement that the discharge rate should equal the charge rate. They are often totally different numbers.
    The maximum charge and discharge rates should always be based on the particular manufacturer's specifications in lieu of rules of thumb.
    What is the application? What is the specific battery model?
    Marc
    I always have more questions than answers. That's the nature of life.
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