sizing pv panels and batteries

farmgirl
farmgirl Registered Users Posts: 5 ✭
in Solar Beginners Corner #1
How do you calculate the right balance of pv panels to battery bank?
I have 1080 watts of panels and a 48 V battery bank wired in series (?) at 312 amps.
Is that enough panels to charge my batteries?
I have had good sun the past few days here in wiscosin, and I have been using only minimal lighting in the evening (so not much draw) and still cant get my battery voltage above 51-52 V according to my charge controller. any thoughts?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: sizing pv panels and batteries

    The rule of thumb is that the panels must be able to deliver a charge rate of 5%-13% of the batteries' total Amp/hr rating (20hr rate). This means over and above supplying for any loads running.

    In your case, 5% of 312 Amp/hrs = 15.6 Amps. Absorb charge Voltage of 56 Volts @ 15.6 Amps = minimum 880 Watts averaged output. The panels peak rating is not their averaged output, which is usually 70-80%. There is some degree of flexibility here, but your panels are just about borderline low-end at probably 860 Watts averaged output - providing there isn't significant draw during charge time.

    This is why I recommend using 10% charge rate for initial calculations. Quite often it will get you close enough, even with some load, that the system will be viable. But it is important to check exact numbers (Vmp of panels, for one thing) before finalizing a design.

    So using 'nominal' numbers that would be: 312 * 10% = 31 Amps @ "48V" = 1488 Watts.

    (For the skeptics and naysayers, you'd be surprised how often that simple calculation works out to be usable. The 'up-rating' of the charge current and the 'down-rating' of the panels work together to compensate for the usual system inefficiency losses. No, it is not exact and I have not said this is how you do it: it's how you start to do it. I wish I didn't have to keep putting disclaimers on posts to try and waylay the nit-pickers! :p)
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  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
    #3
    Re: sizing pv panels and batteries

    What is your Bulk charging voltage set at? How much power (or Amps) your charge controller outputs during peak sun? Are you running any loads (inverters?) during solar charging? What voltage are your solar panels and how are they wired together? Post makes and models of your equipment.

    Issues come to mind:
    1. Your panels are wired for the voltage lower than bulk battery voltage.
    2. Charge controller Bulk voltage is set too low
    3. System not performing as it should.
    4. Too much loads
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,622 admin
    #4
    Re: sizing pv panels and batteries

    In general, a good rule of thumb is 5% to 13% of the battery bank's AH capacity... We can use volts (simply AH*volts) and convert to Watt*Hours:

    312 AH * 48 Volts * 0.05 = 749 Watts
    312 AH * 48 Volts * 0.13 = 1,947 Watts

    The above could be increased by 1/0.77 or so to account for actual solar panel output and charge controller loses... But you are probably OK on the minimum side of solar panels.

    The next issue what is the specific gravity of the batteries... You should measure all 24 cells and write down the numbers (keep track over time). Once you have the cells balanced (within 0.030 or less differnce)--you can use just one cell to take a quick reading of your battery bank capacity.

    Of course, if you have bad cells that don't recover--you are kind of stuck until you replace.

    Regarding the bank voltage... You could have a bad meter reading (meters do fail calibration sometimes).

    Also, it could be how your charge controller is set up... Can you give us a few Volt and Amp readings over the day:

    8 am, 10am, 12noon, 2pm, 4pm 8pm (or something like this).

    If we see V=52 volts and 20 amps during the middle of the day, we know the charge is trying to charge your battery bank.

    If we see V=52 volts and 2 amps during the middle of a sunny day, then the charge controller (for some reason) is not trying to charge the bank.

    If we see V=52 volts and 2-20 amps in the morning, V=57 volts and 2 amps later in the afternoon, and 48-50 volts in the evening (before you start large loads)--the everything going OK (at least with the charger).

    There could lots of issues (running the inverter during the day will reduce battery voltage and charge, etc.)...

    I assume minimal lighting is a few 10's of watts of lights?

    Also, if you have bad cells in a battery bank, 22 cells could be charged, and 2 cells are dead/shorted.... And no matter how much charging you do, those "two cells" will still keep the battery voltage low.

    Checking the Specific Gravity of each cell will help us identify any bad ones.

    If you can use a digital volt meter and measure the voltage of each cell, that would be helpful too (some batteries you can access each cell's bus bar connection to make the readings).

    As always, be careful when working around batteries. Dropping metal tools on battery banks can be very dangerous. Wearing glasses/safety glasses when reading the electrolyte specific gravity, etc. will help keep you safe.

    Having some baking soda and a couple plastic bottles of fresh water handy in case of spills and splashes will help.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • mtjag
    mtjag Registered Users Posts: 32 ✭✭✭
    #5
    Re: sizing pv panels and batteries

    Just wanted to say the answers and ideas you gave this person was very helpful. I have been trying to determine what was happening to my system and you ideas on checking throughout the day was practical and helped me determine it wasn't my charge controller.
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