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Thread: Ram from India- Can a 24V panel charge a 12V battery

  1. #1
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    Default Ram from India- Can a 24V panel charge a 12V battery

    Hello,

    My name is Ramachandran and I live in Chennai, India. I want to purchase about 1 KW of PV panels and connect them to my current UPS ( un interrupted power supply). The UPS charges a 12V 125Ah battery when we get 250V A/C supply and when we have a power cut it takes 12V DC and inverts it into 250V A/C and supplies to my house.

    The UPS is rated for 1KVA . This UPS runs all the critical load in my house, like LED lamps, ceiling fans, TV, laptop computer, Modems, Wifi router etc. Actually I can survive with out power for almost 3 days with the existing set up as my load is very less.

    I want to purchase PV panels worth 1000 W and charge my 12V 125 Ampere Hour Battery and get off the grid for atleast my day to day needs. Only high powered equipment like the fridge and A/C will be connected to the grid.

    24V panels are availabe at nominal rates. 250 W @ 24 V cost 65 Rs/Watt for Mono.

    But I have an invertor that runs on a 12V battery. I do not want to throw away my invertor and my battery and go for a 24V system.

    How can I charge my 12V battery with a 24V solar panel.

    Regards

    Ramachandran.

  2. #2
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    You can if you use a 80 amp MPPT charge controller, but you are in for a little surprise when you hit a 125 AH lead acid battery with 80 amps of charge current. BOOM! Minimum size battery with 80 amps of charge current:

    Flooded Lead Acid = 640 AH
    AGM = 320 AH
    MSEE, PE

  3. #3
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    So is there any way around the problem or should I go for a 24V system to be safe?.

    Sir, I am a beginner and may be wrong but if I go in for 1000W @ 24V then I should be making only 1000/24 = 41.66 Amps during peak power, correct?

    The max they offer in a 12V panel is 150 W , which means I have to buy more panels and more cost.

  4. #4
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    You are going to have to figure out the solution for yourself. 1000 watt solar panel screams for a 24 or 48 volt system. Whatever solution you use needs to be based on the limit that the battery max charge rate is C/8 for FLA, and C/4 for AGM where C = the 20 hour Amp Hour rating of the battery. If you exceed that limit you will damage or destroy the battery.

    FWIW a good 80 amp MPPT charge controller will cost about $600 to $800 USD, and when dealing with off-grid battery systems cost is no object because you are volunteering to pay many times more for electricity over buying it commercially for the rest of your life.
    MSEE, PE

  5. #5
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    What does FLA and AGM mean? and how did you assign a value of 20 for the variable C?

    OK, FLA = Flooded lead acid . Just googled it

    Charging rate should be C/10 for FLA = 125Ah/10 = 12.5 Amps . So if I hit my FLA with 42 Amps , then I will kill it.

    Only way is to go for even higher batter or lower my PV capacity to say 500 W ?? Correct???

    Just three posts in and I am already learning

  6. #6
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    Quote Originally Posted by rrnsss View Post
    What does FLA and AGM mean? and how did you assign a value of 20 for the variable C?

    OK, FLA = Flooded lead acid . Just googled it

    Charging rate should be C/10 for FLA = 125Ah/10 = 12.5 Amps . So if I hit my FLA with 42 Amps , then I will kill it.

    Only way is to go for even higher batter or lower my PV capacity to say 500 W ?? Correct???

    Just three posts in and I am already learning
    You are on the right track. AGM = Absorbed Glass Mat

    Charger current is completely dependent on what type of charge controller you use, and things get tricky if you do not understand what is going on. There are two basic types of charge controllers being PWM and MPPT.

    To determine the charge current with MPPT is real easy Current = Panel Wattage / Nominal Battery Voltage. For PWM Input Current = Output Current

    So if you have 100 panel watts and a 12 volt battery using a MPPT CC 1000 watts / 12 volts = 83.3 amps

    For PWM comes the shock. If you use your 24 volt panels, wire all of them in parallel for a total wattage of 1000 watts, the input current to the CC from the panels is 27.7 amps. Which means the output current is 27.7 amps at 12 volts. I hope that catches your attention because what you have effectively done is turned your 1000 watt panels into 12 volts x 27.7 amps = 332 watts.
    MSEE, PE

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