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Battery bank configuration for powering all Loads at the same time.

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  • Battery bank configuration for powering all Loads at the same time.

    I am designing my battery bank but I'm kinda confused :'( so I need you guys to help me. I did some research online on how to calculate sizing the correct battery bank for your system and almost every website had similar steps/instructions, but then I stumbled upon a pdf file from xantrex saying that whatever amp hour is required for your daily load after you add inverter efficiency which is normally 85% and discharge limit on battery which is a recommended 50% max depth of discharge you should also multiply the daily amp hour requirement by 2 to compensate for the fact that in case you have a in-rush load(s) on the system, meaning that instead of running some loads at the preset time you normally run them this time you decide to run all your required loads at the same time, which normally is not the case in a average household. Doing this would put more stress on the system and when you multiply the daily amp hour requirement by 2, the battery bank will be able to can sustain all the loads at once running each of them at the required time that they normally runs at.

    Is this theory correct?
    If the theory is correct does that mean that if I decide to have a 3 day or 5 day backup(days of autonomy), after I sized my battery bank would I still have to multiply the total batteries by 2 to compensate for the fact that in case I decide to run all loads at once running them at their required time daily and doing this method for each of the backup days?

    Or this theory just applies to a one day backup? because when you configure the battery bank for a 3 day or more backup it would be quite large so it can withstand the in rush loads(stress) on the systems with each load running at the same time for each of the backup days?

  • #2
    Xantrex is wrong. You multiply by 5 or 5 day autonomy. If you discharge every day to 50% DOD you will severely shorten battery life and just one cloudy day you are shut down until you get recharged. A 5 day autonomy gives your about 3 days before having to shut down.

    Secondly you should not be working in Amp Hours, work with Watt Hours as it is easier and less prone to errors.
    MSEE, PE

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    • #3
      Originally posted by Sunking View Post
      Xantrex is wrong. You multiply by 5 or 5 day autonomy. If you discharge every day to 50% DOD you will severely shorten battery life and just one cloudy day you are shut down until you get recharged. A 5 day autonomy gives your about 3 days before having to shut down.

      Secondly you should not be working in Amp Hours, work with Watt Hours as it is easier and less prone to errors.
      If I understand you correctly, having a bigger bank allows one to discharge 20% on day1, another 20% on day2 and another 10-20% on day three before charging. The Concorde user guide defines a cycle as a discharge and charge, so charging after 2.5 or 3 days would be one cycle. Is that correct?

      If that is correct, then how does one automate this behavior?

      ### Edit ###
      I found this old post of yours, so I think I did not understand you correctly -> http://www.solarpaneltalk.com/showth...ll=1#post57402

      However, if I may present my own line of reasoning If you take the # of cycles from the chart in that post at each level, say 20% DOD * # cycles, you get the same amount of electricity being discharged as at the 50% level (0.5 * #cycles). My reasoning was that Concorde says a cycle is anytime one recharges. So if one could discharge for three days and then charge that would be one cycle, correct?

      So, if one discharged to 50% and then re-charged, that would give the cycles listed in your chart X 2.5 or three. If one simply discharged to 20% every day and then re-charged, the amount of electricity given by the battery bank would be less than what I outlined.

      Of course, I am a neophyte with no real experience, so I am probably barking up the wrong tree, but I would like to inform my view also from the battery manufacture's documentation as a guide, and that is how I read it.


      ### Second Edit ###
      Sigh
      I am finding some sites define a cycle as a full-charge discharge. Which is it?
      #####################
      Last edited by lkruper; 06-11-2015, 10:45 AM. Reason: More research

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      • #4
        Originally posted by Sunking View Post
        Xantrex is wrong. You multiply by 5 or 5 day autonomy. If you discharge every day to 50% DOD you will severely shorten battery life and just one cloudy day you are shut down until you get recharged. A 5 day autonomy gives your about 3 days before having to shut down.

        Secondly you should not be working in Amp Hours, work with Watt Hours as it is easier and less prone to errors.
        I think you misunderstood what I was talking about. Lets convert the amps to watt hours as you suggested. Say for example your total connected loads(appliances,lites etc.)comes up to be 2500watt by adding the wattage of each device in your house. Suppose you decide to run every devices at the same time running all of them for the allotted time that you give each devices daily would you pull more juice from the battery compare to if you just ran some of them at once and the rest at a later time and still adhering to the same allotted time that you give each device?

        If you pull more wattage running them all at once instead of some at a time would it be that xantrex suggestion of multiplying the 2500watt by 2 and sizing the battery bank to match that wattage would that suggestion correct? As I said before this move that they suggest would allow the battery to handle that constant pull/stress.

        Also in regards to what you said earlier, "If you discharge every day to 50% DOD you will severely shorten battery life and just one cloudy day you are shut down until you get recharged". I know that is the case if you discharge to 50% every day. I am just asking if you size a battery bank for say 3 day backup supply with you reaching 50% DOD at the end or close to the 3 day cycle. Running all loads at the same time pulling the 2500watt with each device running for their allotted time within that 2500watt so that mean that after a while the wattage would drop when one of the device time is up Would it really give you 3 day supply compare to if you just decide to run some of the devices in the 2500watt list at interval time and still adhering to the same allotted time associated for each device?

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        • #5
          Originally posted by fabieville View Post
          I think you misunderstood what I was talking about. Lets convert the amps to watt hours as you suggested. Say for example your total connected loads(appliances,lites etc.)comes up to be 2500watt by adding the wattage of each device in your house. Suppose you decide to run every devices at the same time running all of them for the allotted time that you give each devices daily would you pull more juice from the battery compare to if you just ran some of them at once and the rest at a later time and still adhering to the same allotted time that you give each device?
          No sir. You calculate your daily 24 hour watt hour usage. Lets say it is 5000 watt hours, and you are using a 48 volt battery. Then you need a [5000 wh x 5 day reserve] / 48 volts = 520 AH battery.

          For panel wattage with a minimum 4 Sun hour day using MPPT you would need [5000 wh x 1.5cf] / 4 Sun Hours = 1875 watts, so you would want a 1800 to 2000 watt panel because 200 watt panels are great building blocks. Avoid Prime Number of panels.

          Last is determine charge controller again assuming MPPT is 2000 watts / 48 volts = 42 amps. So you are now looking for a 45 amp or larger controller.

          Amp Hours and Amps are end results based on Watts, Watt Hours, and Battery voltage selected.
          MSEE, PE

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          • #6
            Imagine this. You size your battery for 50% DOD per day. You know you do not want to go below 50%. So you have a nice Sunny day, and by next morning you are down to 50% DOD and it rains for 3 days or cloud cover. WTF are you going to do?

            Without a genny you set in the dark for 3 or 4 days. 3 days of rain, plus another 1/2 to full day to recharge. Have a 5 day reserve, and you conserve for 3 days with limited power, and day four you start recovering. If your system is designed correct to CYA for short winter days, you have a lot of extra panel wattage to recover quickly in Spring, Summer, and Fall. You can take a Concorde AGM down to 80% DOD, just do not make a habit out of it. Save it for a rainy/cloudy spell.
            MSEE, PE

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            • #7
              Originally posted by Sunking View Post
              Imagine this. You size your battery for 50% DOD per day. You know you do not want to go below 50%. So you have a nice Sunny day, and by next morning you are down to 50% DOD and it rains for 3 days or cloud cover. WTF are you going to do?

              Without a genny you set in the dark for 3 or 4 days. 3 days of rain, plus another 1/2 to full day to recharge. Have a 5 day reserve, and you conserve for 3 days with limited power, and day four you start recovering. If your system is designed correct to CYA for short winter days, you have a lot of extra panel wattage to recover quickly in Spring, Summer, and Fall. You can take a Concorde AGM down to 80% DOD, just do not make a habit out of it. Save it for a rainy/cloudy spell.
              If you go down to 50% SOC over the course of three days, it is usually unrealistic to expect to get back up to 100% SOC in anything less than two days of full sun.
              You will get to a high enough SOC to start drawing on the bank within a half day or so, but you will not get to full charge.
              You need to plan on getting back up to 80% SOC as soon as possible to prevent damaging sulfation.
              SunnyBoy 3000 US, 18 BP Solar 175B panels.

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