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Thread: How Many 12v Solar Panels Needed To Charge 24v Battery?

  1. #1

    Question How Many 12v Solar Panels Needed To Charge 24v Battery?

    Part of me doesn't even know why I'm asking this question, but just need to confirm this with a few people.

    I have 2x 12v batteries hooked up in series to produce 24v, and I need to confirm that I only need 2x 12v solar panels in order to properly charge them? Of course I'll have a charge controller and etc., but I'm only focused on the question above.

    I remember when I asked someone how many panels I needed to charge a 36v battery, I originally thought I needed only 3x 12v batteries, but according to them that would not be enough, so that's why I'm double checking about this.

    Don't know why this wasn't a popular Google question
    Me loves some solar

  2. #2

    Default Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    Nominal Voltage-wise, 2 times 12 = 24.
    Vmp-wise, 2 times 17.5 = 35.
    Providing they are standard "12 Volt" panels two in series will produce sufficient Voltage to charge 24 Volts of battery.

    The only missing bit: how big are the batteries and how big are the panels?
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  3. #3

    Default Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    Quote Originally Posted by Cariboocoot View Post

    The only missing bit: how big are the batteries and how big are the panels?
    I'm thinking of getting 2 of these 30W solar panels since they are on sale: http://www.ul-solar.com/30_Watt_12_V..._p/stp030p.htm

    As for the batteries, they are 12v, 35Ahm batteries.

    Does the battery size change something, as if so, maybe that's what they were referring to?
    Me loves some solar

  4. #4

    Default Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    The Amp hour capacity of the battery determines how much you need in Watts for proper recharging:
    V * A = W
    So a 12 Volt, 35 Amp hour battery would be recharged at (minimally) 14.2 Volts * 1.75 Amps = 24.85 Watts. Now, panels don't actually put out their "nameplate" rating all the time; they put out an average that's usually 80% or so of the rated Wattage. A 30 Watt panel would average 24 Watts - so it is just about the minimum for recharging that battery.

    Put two of those batteries in series and you get 35 Amp hours @ 24 Volts. Put two of those panels in series and you have the minimum for recharging the 24 Volt battery bank.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  5. #5

    Default Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    Quote Originally Posted by Cariboocoot View Post
    The Amp hour capacity of the battery determines how much you need in Watts for proper recharging:
    V * A = W
    So a 12 Volt, 35 Amp hour battery would be recharged at (minimally) 14.2 Volts * 1.75 Amps = 24.85 Watts. Now, panels don't actually put out their "nameplate" rating all the time; they put out an average that's usually 80% or so of the rated Wattage. A 30 Watt panel would average 24 Watts - so it is just about the minimum for recharging that battery.

    Put two of those batteries in series and you get 35 Amp hours @ 24 Volts. Put two of those panels in series and you have the minimum for recharging the 24 Volt battery bank.
    Okay, so where did you get the 1.75Amps from? Like is there some type of table you go by that tells you the minimum value?

    Also, would it be better to just get in two of these 20W 24v panels so that I'm not so close to the minimum value: http://www.ul-solar.com/product_p/stp020p-wp.htm

    Or should I go the 12v 30W route. This is something I'm a little fuzzy on as well. As the 20W 24v panels can only deliver .61A which is going to increase the charge time, where as the 30W 12v panels can deliver 1.77A.
    Me loves some solar

  6. #6

    Default Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    Ah, the magic formula! The recommended charge rate of 5%-13% of the battery's Amp hour rate. You can read about this and many other fun facts in the Battery FAQ's here: http://www.windsun.com/Batteries/Battery_FAQ.htm

    What you'd really like to have is twice that 30 Watts for the 12 Volt, or four times for the 24 Volt:

    3.5 Amps (10%) * 14.2 Volts = 49.7 Watts, less derating = 62.125 Watts.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  7. #7

    Default Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    Quote Originally Posted by Cariboocoot View Post
    Ah, the magic formula! The recommended charge rate of 5%-13% of the battery's Amp hour rate. You can read about this and many other fun facts in the Battery FAQ's here: http://www.windsun.com/Batteries/Battery_FAQ.htm

    What you'd really like to have is twice that 30 Watts for the 12 Volt, or four times for the 24 Volt:

    3.5 Amps (10%) * 14.2 Volts = 49.7 Watts, less derating = 62.125 Watts.
    Well if space is limited, and I mean, very limited, which route would you go with if you had to pick one, as both will work, but as you've just calculated, it will be charging at the very minimum...but it will charge the batteries, which is the important thing for me. I just want to check to see which one is faster, since we have an Amp difference.
    Me loves some solar

  8. #8

    Default Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    More Amps = faster charging.
    The other thing to remember is net charge rate is the gross charge rate less any current going out to supply loads. So the 5% rate works for a system that doesn't have large or continuous loads during recharge time.
    My rule-of-thumb is to "shoot for" 10% and you'll get a workable rate between 5 and 10 percent.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  9. #9
    wwwindsock6 Guest

    Default Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    I have a 48v solar system, can I mix a 36v 280 watt panel with a 12v 15 watt panel to get 48v ? I am new at this trying to learn, Thanks

  10. #10
    Join Date
    Mar 2006
    Location
    SF Bay Area (California)
    Posts
    21,369

    Default Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

    No, not really--It is like connecting a 12 volt car battery with a set of "D" cell flashlight batteries in series to get 24 volts.

    Yes, you will get 24 volt's -- but only at the current level of the battery cells--not the big car battery.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

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