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  • Battery sizing help

    Hello,

    If someone could please help me, i am currently looking to deploy an 24V off-grid solar system. I have read through the guides and sized my system accordingly, the one question i have is that my system requires and 1100AH battery bank would it be OK to have 4 6V 300AH batteries wired in series in to produce 24V The issue i have is that i have read wiring in series only increases the voltage and not the Amp hours. So will 4 6V batteries wired in series to produce 24V but remaining at 300 amp hours as the amp hours don't increase when you wire in series be ok ?

    Or will i need to increase both the amp hours and voltage ? if so how will i do this as you can only increase either the voltage or the amp hours ? Any help would be greatly appreciated thank you.

  • #2
    When you wire the batteries in series it only increase voltage and not the amperes, when you wire them in parallel it increase the amperes not voltage. It is preferred to wires in series as parallel wiring may cause unbalance issues between banks. If you have to wire in parallel don't wire more than two banks.

    Comment


    • #3
      Originally posted by paulcheung View Post
      When you wire the batteries in series it only increase voltage and not the amperes, when you wire them in parallel it increase the amperes not voltage. It is preferred to wires in series as parallel wiring may cause unbalance issues between banks. If you have to wire in parallel don't wire more than two banks.
      ok thanks but I understand I parrael and series, it's just that my set up requires 1100 amp hours but I have 4 6v 300 amp hour batteries wires in series to tgive me 24v. As the amp hour domt in cream would this me sufficient to meet my needs or will I need to increase the amp hours as well ?

      Comment


      • #4
        1) try doing all the calcuations in WATT HOURS, not amp hours

        2) if you need more than 200ah storage @ 24V, adding batteries in series, to make a 48V system, reduces the need to have parallel batteries (parallel batteries are a bad thing)

        see http://www.smartgauge.co.uk/batt_con.html
        Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
        || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
        || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

        solar: http://tinyurl.com/LMR-Solar
        gen: http://tinyurl.com/LMR-Lister

        Comment


        • #5
          Originally posted by Mike90250 View Post
          1) try doing all the calcuations in WATT HOURS, not amp hours

          2) if you need more than 200ah storage @ 24V, adding batteries in series, to make a 48V system, reduces the need to have parallel batteries (parallel batteries are a bad thing)

          see http://www.smartgauge.co.uk/batt_con.html
          Thanks for the reply, I need a total if 1200 amp hours, I done think I can achieve that in 24v system with 6v batteries as I belive the highest 6v battery is 800 amp hours. So the only other alternative would be to run the batteries in series and parralell, or I could possible split the whole system into two different systems with two different charge controllers etc. What so you think is my best option ?

          Comment


          • #6
            Originally posted by ValPachino View Post
            ...I need a total if 1200 amp hours,...
            By itself that statement is meaningless. At what voltage did you figure that?

            At 12V that would be 14,400 Watthours (1200 * 12V)

            At 24V that would be 28,800 Watthhours (1200 * 24V)

            At 48V that would be 57,600 Watthours (1200 * 48V)

            That's why Mike asked for you to figure everything in watthours, then convert to amphours for the voltage of you system.

            If you really need 1200 Amphours @ 24V you would be better off finding 2V batteries for a 24V system or going with a 600AH @ 48V system.

            WWW

            Comment


            • #7
              Originally posted by Wy_White_Wolf View Post
              By itself that statement is meaningless. At what voltage did you figure that?

              At 12V that would be 14,400 Watthours (1200 * 12V)

              At 24V that would be 28,800 Watthhours (1200 * 24V)

              At 48V that would be 57,600 Watthours (1200 * 48V)

              That's why Mike asked for you to figure everything in watthours, then convert to amphours for the voltage of you system.

              If you really need 1200 Amphours @ 24V you would be better off finding 2V batteries for a 24V system or going with a 600AH @ 48V system.

              WWW
              Hello,
              Yes sorry I wanted to deploy a 24V system, as I stated I need 1200 amp hours but that amount of batteries whether it be 6V pr 2V is quite expensive. So I go reduce the load sightly and have 2 24v system as it might work out slightly cheaper rather than having 1 system with a massive battery bank. Or maybe u should just bite the bullet and just have the one 24v system.

              Comment


              • #8
                Originally posted by ValPachino View Post
                Hello,
                Yes sorry I wanted to deploy a 24V system, as I stated I need 1200 amp hours but that amount of batteries whether it be 6V pr 2V is quite expensive. So I go reduce the load sightly and have 2 24v system as it might work out slightly cheaper rather than having 1 system with a massive battery bank. Or maybe u should just bite the bullet and just have the one 24v system.

                Based on your 1200 amp hour need at 24volt you are talking about a system that will delivery almost 29 kWh. That is a huge load.

                How did you calculate the needed 1200 Ah?

                Comment


                • #9
                  Originally posted by SunEagle View Post
                  Based on your 1200 amp hour need at 24volt you are talking about a system that will delivery almost 29 kWh. That is a huge load.

                  How did you calculate the needed 1200 Ah?
                  Here is my calculations;
                  It is based on a estimate of the usage over a 24hr period according to the off grid system design in one of the other forums;

                  15 LED bulbs 47 hours @10W = 470 watt hours
                  Phone chargers 9 hours @ 10 = 90 watt hours
                  Laptop chargers 6 hours @ 60W = 390 watt hours
                  Led Television 10 hours @35W = 350 watt hours
                  Satellite 10 hours @25W =250 watt hours
                  Solar fridge 24 hours @85W =2040 watt hours
                  Total 3590 watt hours

                  I have revised these calculations in order to bring down the load and i now have it at about 750 amo hours. Some of the hours seem somewhat long like the led bulbs I just based that on average as lights are switched on and off and there are quote a few lights in the place.

                  Get back to me and let me know your advice thank you.

                  Comment


                  • #10
                    Originally posted by ValPachino View Post
                    Here is my calculations;
                    It is based on a estimate of the usage over a 24hr period according to the off grid system design in one of the other forums;

                    15 LED bulbs 47 hours @10W = 470 watt hours ??? Not sure how you get 47 hours in one day
                    Phone chargers 9 hours @ 10 = 90 watt hours
                    Laptop chargers 6 hours @ 60W = 390 watt hours this comes to 360 watt hours
                    Led Television 10 hours @35W = 350 watt hours
                    Satellite 10 hours @25W =250 watt hours
                    Solar fridge 24 hours @85W =2040 watt hours
                    Total 3590 watt hours

                    I have revised these calculations in order to bring down the load and i now have it at about 750 amo hours. Some of the hours seem somewhat long like the led bulbs I just based that on average as lights are switched on and off and there are quote a few lights in the place.

                    Get back to me and let me know your advice thank you.
                    Ok. Some of your math is wrong like the Laptop chargers and the LED lights as I show in Bold type above. But for simplicity lets say you use 3600 watt hours in a 24 hour period.

                    For a 24volt system that comes to 3600 wh / 24v = 150 Ah. A proper sized battery system should be 5 times your daily usage so 150Ah x 5 = 750Ah which is what you have calculated.

                    To get your 24volt battery system you would need to find a 800Ah battery at 2, 4 or 6 volts and then wire enough in series (2v = 12, 4v = 6 and 6v = 4) to get to 24volts.

                    From there you can calculate the amount of solar panel wattage which is based on where you live and how many "quality sun hours" you get during the worst month of the year to generate that 3600 watt hours. You then size your charge controller based on the panel wattage and battery Ah.

                    Your system is pretty big and will be expensive. If you have incorrectly determined your watt hour usage you could easily drain them and shorten their life. That is why the first and most important item you need to determine for an "off grid" system is what your daily watt hour load truly is or what you can live with.

                    Comment


                    • #11
                      Originally posted by SunEagle View Post
                      Ok. Some of your math is wrong like the Laptop chargers and the LED lights as I show in Bold type above. But for simplicity lets say you use 3600 watt hours in a 24 hour period.

                      For a 24volt system that comes to 3600 wh / 24v = 150 Ah. A proper sized battery system should be 5 times your daily usage so 150Ah x 5 = 750Ah which is what you have calculated.

                      To get your 24volt battery system you would need to find a 800Ah battery at 2, 4 or 6 volts and then wire enough in series (2v = 12, 4v = 6 and 6v = 4) to get to 24volts.

                      From there you can calculate the amount of solar panel wattage which is based on where you live and how many "quality sun hours" you get during the worst month of the year to generate that 3600 watt hours. You then size your charge controller based on the panel wattage and battery Ah.

                      Your system is pretty big and will be expensive. If you have incorrectly determined your watt hour usage you could easily drain them and shorten their life. That is why the first and most important item you need to determine for an "off grid" system is what your daily watt hour load truly is or what you can live with.
                      Ok I fully understand and appreciate your help I will take another look into everything thankyou.

                      Comment


                      • #12
                        Originally posted by SunEagle View Post
                        Your system is pretty big and will be expensive. If you have incorrectly determined your watt hour usage you could easily drain them and shorten their life. That is why the first and most important item you need to determine for an "off grid" system is what your daily watt hour load truly is or what you can live with.
                        Actually this system is not big, for 3600 watts hours daily he can get away with 1000 watts panels if he is in tropical area, if on the north part he can use 1500 watts panels and a bank of 24 volts 450amps for two days and a cheap 1000watts generator for rainy days.

                        If it were me, I just get 1000 watts panels 4x250watts and 4 6volts Rolls S-550 deep cycle batteries and a cheap 1000 watts gas generator and a 24 volt battery charger. If funds is limited on initial setup, I could go to the big store and buy 8 of those 225 amps 6volt golf cart batteries wired 4 in series then parallel two banks until can afford the Rolls or Trojan batteries.

                        Comment


                        • #13
                          Originally posted by paulcheung View Post
                          Actually this system is not big, for 3600 watts hours daily he can get away with 1000 watts panels if he is in tropical area, if on the north part he can use 1500 watts panels and a bank of 24 volts 450amps for two days and a cheap 1000watts generator for rainy days.

                          If it were me, I just get 1000 watts panels 4x250watts and 4 6volts Rolls S-550 deep cycle batteries and a cheap 1000 watts gas generator and a 24 volt battery charger. If funds is limited on initial setup, I could go to the big store and buy 8 of those 225 amps 6volt golf cart batteries wired 4 in series then parallel two banks until can afford the Rolls or Trojan batteries.
                          Yes funds are kind of tight initially and I want to try to be as economical as possible without being to cheap. I plan to use 1750 watt panels at 48V and 8 6V 400 amp batteries. In relation to generator which you mentioned do you recommend I just have it separate from the solar system along with a battery charger with can be connected if needed. Get back to me and let me know thanks.

                          Comment


                          • #14
                            If you are going for 48 volts system, you better get one of those good quality inverter which has a battery charger as well has generator input to charge the batteries while also supply the house load.

                            For 400 amps 48 volts battery bank you need at least 2000 watts generator to charge the batteries properly. Also the 1750watts PV panels is bit low, you need 2200 watts minus line lost to properly charge the battery bank. If you go for 2200watts PV and 400amps at 48 volts battery you will get much more power to use than you plan for from the beginning, it also cost more.

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