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A couple of novice questions please

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  • A couple of novice questions please

    Hello, sorry if this first question is annoyingly basic, let's say I have a 48V 100ah LiFePO4. Is that battery's energy capacity just as good as a 24V 200ah LiFePO4 and a 12V 400ah...3V 1,600ah, etc?

    Now I've been reading thru some of this forum and I forget where I read it but someone said it is very recommended to wire 24 3V 1,000ah batteries in series for a 48V set up, yet not recommended to wire a bunch of batteries in parallel. Why? As far as I knew that was the beauty of wiring batteries together, that you could mix and match any combo, like for instance put 2 sets of 6 in series and then put those 2 sets in parallel, etc. Where does the danger in over paralleling lie?

    Is there a certain rule of thumb as to when you are using too many parallel connections for a given number of batteries? Like let's say you have 32 LiFePO4s for instance, obviously 32 series is ok and 32 in parallel is looked down on...what about 2 sets of 16 in series and then those 2 sets in parallel? Or how about 4 sets of 4 in series and then those 4 sets wired in parallel is that ok or is that bad? I'm just trying to figure out my configuration based on the best deals I can find on different LiFePO4 batteries, it's not that I'm being argumentative.


    EDIT...I am actually right now looking at these 3V 180ah Calbs. I'm wondering if it is ok if I bought 16 and wired 4 sets of 4 in series, and then wired each of the 4 sets in parallel, for 12V and 720ah

  • #2
    Originally posted by LiFePO4_Only View Post
    Hello, sorry if this first question is annoyingly basic, let's say I have a 48V 100ah LiFePO4. Is that battery's energy capacity just as good as a 24V 200ah LiFePO4 and a 12V 400ah...3V 1,600ah, etc?
    Yes it is the same. Battery Energy Capacity in Wh = Battery Voltage x battery capacity in ah

    Now I've been reading thru some of this forum and I forget where I read it but someone said it is very recommended to wire 24 3V 1,000ah batteries in series for a 48V set up, yet not recommended to wire a bunch of batteries in parallel. Why? As far as I knew that was the beauty of wiring batteries together, that you could mix and match any combo, like for instance put 2 sets of 6 in series and then put those 2 sets in parallel, etc. Where does the danger in over paralleling lie?

    Is there a certain rule of thumb as to when you are using too many parallel connections for a given number of batteries? Like let's say you have 32 LiFePO4s for instance, obviously 32 series is ok and 32 in parallel is looked down on...what about 2 sets of 16 in series and then those 2 sets in parallel? Or how about 4 sets of 4 in series and then those 4 sets wired in parallel is that ok or is that bad? I'm just trying to figure out my configuration based on the best deals I can find on different LiFePO4 batteries, it's not that I'm being argumentative.


    EDIT...I am actually right now looking at these 3V 180ah Calbs. I'm wondering if it is ok if I bought 16 and wired 4 sets of 4 in series, and then wired each of the 4 sets in parallel, for 12V and 720ah
    Do you mean 16 x 3V for 48 V?

    The main problem with having batteries in parallel is that you have to be very careful that the current is shared equally between the batteries otherwise some will end up working harder that others and will not last as long. The first configuration of batteries you came up with is known as 6S2P and is not ideal for LiFePO4 (LFP) batteries if you are going to have individual cell monitoring which I would recommend, as you will have to monitor twice as many cells. The other arrangement, 2 cells in parallel then connected in series 2P6S is ok. The more cells you have in parallel the bigger the current sharing problem becomes. See this article for more information http://www.smartgauge.co.uk/batt_con.html. Also, the more cells you have the larger the number of battery interconnections you have to buy and the more chance for a bad connection.

    Was there any particular reason why you are looking at a 12 volt system or LFP batteries?

    Simon
    Off-Grid LFP(LiFePO4) system since April 2013

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    • #3
      The rules that apply to Lead Acid Batteries do not necessarily apply to Lithium. Lithium batteries are in fact paralleled. However it is done differently in a Ladder Configuration. Click here and look at Alternate Configuration.

      Having said that you do want to minimize parallel batteries.

      As to capacities what you need to know is Watt Hours. Amp; Hours is a coulombmetric measurment, not power. For capacity [B]Watt Hours = Battery Voltage x Amp Hours [/B]

      So knowing that then you know all 4 statements are true:

      48 volts x 250 AH = 12 Kwh
      24 volts x 500 AH = 12 Kwh
      12 volts x 1000 AH = 12 Kwh

      Question is which one do you use if you only have 250 AH batteries. You always choose higher voltages and would use 48 volts @ 250 AH. The issue is people get stcuk in a 12 volt box. That is all they know. If they need 1000 AH, the largest 12 volt battery you can find and actually work with is about 200 AH. That weighs around 150 pounds. A 12 volt 1000 AH battery if it existed would weigh 750 pounds. It never occurs to them to buy 2 volt 1000 AH cells that weigh 120 pounds.
      MSEE, PE

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      • #4
        This is great information! I'm on the go right now so i'll read these links later, but real quick yes I messed up, the forum poster said '24 2V batteries in series' not 24 3V, I had 3 volts in my head because i had been looking at 3 volt battery sales

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        • #5
          Sorry forgot the link. Ladder Connection
          MSEE, PE

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          • #6
            Originally posted by LiFePO4_Only View Post
            EDIT...I am actually right now looking at these 3V 180ah Calbs. I'm wondering if it is ok if I bought 16 and wired 4 sets of 4 in series, and then wired each of the 4 sets in parallel, for 12V and 720ah
            Generally the best idea with individual cells is to build your capacity first, and then series connect the groups of paralleled cells to get your voltage. Easier to stay in balance that way.

            For example, using your example of 16 cells of 180ah each, and you wanted a nominal 12v battery, then placing those 16 cells on a bench, I'd do this:

            Wire 4 of the cells in parallel. That makes a 3.2v, 180ah battery. Set it aside.
            Wire another 4 cells in parallel to make yet another 3.2v 180ah battery. Set it aside.
            Do this two more times.

            Now, wire each of those groups of 3.2v batteries in SERIES, to get your desired final voltage.

            WARNING - when constructing these things, all the cells should be at NEARLY THE SAME STATE OF CHARGE, otherwise when you connect them, huge currents can flow from one cell to the other initially when you attach the terminal lugs. That might mean using some sort of charger, like a "single-cell" charger, or high quality battery-capable adjustable bench supply.

            The best solution of course is to minimize parallelling, and use larger capacity cells to start with.

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            • #7
              Thank you PNjunction, but over the past couple of days in here I have become a lot more attracted to series connections, and high amp hours. I'm now trying to figure out how much money I want to spend on a 24V or 48V set up...i'm pretty much finding out that it comes down to how many AHs are you willing to pay for lol...the price really jumps when you reach 100, 180, 400, etc

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              • #8
                We have 4 batteries of 4 CALB Cells (180 amp-hr) in series for a 48 V nominal (54.4 V float) battery suite which could be considered a 180 amp-hr at 48 V . Much easier and less confusing to say it has a rating of 8.64 kW-hr (48 V nominal). Batteries and BMS were fabricated by Manzanita Micro and were thus designed for EV applications but works well in an RV.

                System is in a 5th wheel where weight is critical. A lead acid suite with equivalent useful capacity of approximately 7 kW-hrs would stress frame and we would be over pin weight. Most RVs utilize 12 V panels and 12 V battery suites since this is what RV'ers are used to.
                Reed and Elaine

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