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I know you all say NO to parallel batteries, but

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  • I know you all say NO to parallel batteries, but

    I was trying to explain the WHY to a buddy, I understand BIR & cable IR, cable length, etc. and understand that, but can't answer his question of why that is not also issues on batteries in series. What is it specifically about being in series that then resolves the problems the issues with parallel, cables, connections, etc. thanks

  • #2
    Series and parallel circuit laws.

    In series current is equal where I1 = I2 = I3 = I4
    In parallel the current splits and are not equal.
    MSEE, PE

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    • #3
      Originally posted by littleharbor
      current will take the path of least resistance .
      That is pure Rubbish. Current takes all paths available.

      By that idiotic logic if you were to put a 1 ohm resistor in parallel with a 10 ohm resistor only the 1 ohm resistor would have current flowing which is pure rubbish. Assuming 1 volt is applied, you would have 1 amp flowing through the 1 ohm resistor, and .1 amp in the 10 ohm resistor for a total current of 1.1 amps.
      Last edited by Sunking; 04-01-2018, 07:59 PM.
      MSEE, PE

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      • #4
        Originally posted by Sunking View Post

        That is pure Rubbish. Current takes all paths available.
        OK, but not equally
        2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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        • #5
          ok, clear as mud......so you made me go look up what "I" is which is current. so the current through each of the batteries in series by [U][B]must [/B][/U]be equal, but in parallel they could be different.

          But physically why is that so? is there a flow of water analogy, like in parallel each battery is a reservoir that can have slightly different sized inlets and outlets that impacts how easily water can flow in or out of each reservoir on its way the end user, but in series what flows into the first battery must flow through each subsequent battery and out the end, thus no one battery can be less filled than the rest. I'm quessing this isn't right, but can you give a similiar sort of analogy?

          And while cabling/connections can impact how freely current flows, thus different IR of cabling/connections can impinge the flow in either parallel or series, the negative consequences of such impingment in parallel connections is the different speeds of filling and emptying of each battery, and thus they can have different levels of "fill" or charge, whereas in series all the batteries must be at an equal level???

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          • #6
            Originally posted by Sunking View Post

            That is pure Rubbish. Current takes all paths available.

            By that idiotic logic if you were to put a 1 ohm resistor in parallel with a 10 ohm resistor only the 1 ohm resistor would have current flowing which is pure rubbish. Assuming 1 volt is applied, you would have 1 amp flowing through the 1 ohm resistor, and .1 amp in the 10 ohm resistor for a total current of 1.1 amps.
            Yea, it bugs me too every time someone mentions the shortest path. First mentioned it Sat. Bruce Roe

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            • #7
              Originally posted by dorff View Post
              ok, clear as mud......so you made me go look up what "I" is which is current. so the current through each of the batteries in series by [U][B]must [/B][/U]be equal, but in parallel they could be different.

              But physically why is that so? is there a flow of water analogy, like in parallel each battery is a reservoir that can have slightly different sized inlets and outlets that impacts how easily water can flow in or out of each reservoir on its way the end user, but in series what flows into the first battery must flow through each subsequent battery and out the end, thus no one battery can be less filled than the rest. I'm quessing this isn't right, but can you give a similiar sort of analogy?

              And while cabling/connections can impact how freely current flows, thus different IR of cabling/connections can impinge the flow in either parallel or series, the negative consequences of such impingment in parallel connections is the different speeds of filling and emptying of each battery, and thus they can have different levels of "fill" or charge, whereas in series all the batteries must be at an equal level???
              Each parallel path has a slightly different resistance. That will cause the current amount to also be slightly different. The path with the lowest resistance will receive the highest current.

              That is based on Ohm's law where E = I x R or (E= voltage, I = current, R = resistance). If E is the same in each "path" then lowering or raising the R value will change the I value in the opposite direction.

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              • #8
                Originally posted by littleharbor View Post

                OK, but not equally
                Now that is more correct.

                MSEE, PE

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                • #9
                  Originally posted by Sunking View Post
                  Now that is more correct.
                  Ya, my bad. Short answer, needed a little more info. I can see where it could have been misinterpreted as ALL current taking the path of least resistance. Thanks for the clarification.
                  2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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                  • #10
                    Originally posted by littleharbor View Post

                    Ya, my bad. Short answer, needed a little more info.
                    NP and my following comments are directed to the OP.

                    In theory if I take two 0.1 Ohm resistors, place then in parallel, apply 1 volt, each resistor will have 10 amps flowing. Ask any teacher or instructor and that is what they will tell you because that is all they know and never done it. In practice it does not work that way.

                    Now the question should be: WHY NOT?

                    Real simple, in real practice the resistors will not be equal. Now compound that with the fact cable resistance and contact resistance will not be equal. So now the teacher has to ask a tech, engineer, or sparky how to make it work? KISS (Keep It Simple Stupid) teacher, put the batteries in series.
                    Last edited by Sunking; 04-02-2018, 12:24 PM.
                    MSEE, PE

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                    • #11
                      Originally posted by dorff View Post
                      ok, clear as mud......so you made me go look up what "I" is which is current. so the current through each of the batteries in series by [U][B]must [/B][/U]be equal, but in parallel they could be different.

                      But physically why is that so?
                      Because what really sets the current flow is the equivalent resistances of each path. You will get more current down a lower resistance path due to Ohm's Law. And weaker batteries tend to have higher equivalent resistances. So the better batteries get most of the charge current.

                      is there a flow of water analogy, like in parallel each battery is a reservoir that can have slightly different sized inlets and outlets that impacts how easily water can flow in or out of each reservoir on its way the end user, but in series what flows into the first battery must flow through each subsequent battery and out the end, thus no one battery can be less filled than the rest. I'm quessing this isn't right, but can you give a similiar sort of analogy?
                      You can only take the water analogy so far.
                      And while cabling/connections can impact how freely current flows, thus different IR of cabling/connections can impinge the flow in either parallel or series, the negative consequences of such impingment in parallel connections is the different speeds of filling and emptying of each battery, and thus they can have different levels of "fill" or charge, whereas in series all the batteries must be at an equal level?
                      Lead acid batteries are different than many other batteries in that they have a built in balancing mechanism. If you overcharge them a little they gas out a bit but stay at close to 100% charge. So a series string will eventually balance itself out if you keep charging it (albeit with some loss of electrolyte.) And lead acid batteries want to be kept at a full state of charge.

                      If they are in parallel you can't force that to happen as easily, because most of the charge current will be going to the "better" cell strings that don't need to be balanced in the first place. The weaker strings will be neglected and will not get full balancing charges.

                      Note that this is unique to lead acid, which is why chemistries like LiFePO4 have very different requirements for balancing, monitoring and charging.

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                      • #12
                        great, thanks guys, I'll see if I can explain to my buddy to convince him.

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                        • #13
                          Originally posted by dorff View Post
                          ok, clear as mud......so you made me go look up what "I" is which is current. so the current through each of the batteries in series by [U][B]must [/B][/U]be equal, but in parallel they could be different. But physically why is that so?
                          Not could be, they will be different. Read my reply in post 10

                          Originally posted by dorff View Post
                          But physically why is that so? is there a flow of water analogy, like in parallel each battery is a reservoir that can have slightly different sized inlets and outlets that impacts how easily water can flow in or out of each reservoir on its way the end user, but in series what flows into the first battery must flow through each subsequent battery and out the end, thus no one battery can be less filled than the rest. I'm quessing this isn't right, but can you give a similiar sort of analogy?
                          You are making the same mistake an academic makes. You assume everything is equal and fair. That is not real. You have assumed every cell in a series string is at the same SOC and capacities are equal. They are not equal. Most batteries, there are some exceptions like lithium, are self equalizing with respect to SOC. Lead Acid batteries are self equalizing. When a cell reaches 100% SOC will still pass charge current and hold 100% SOC. What it does is goes into gassing voltage and you start electrolysis of the water. Result is you start using water and heating the cell up as waste products because the battery is now consuming power rather than storing power. It is a good thing assuming the other cells are healthy.

                          Originally posted by dorff View Post
                          And while cabling/connections can impact how freely current flows, thus different IR of cabling/connections can impinge the flow in either parallel or series, the negative consequences of such impingment in parallel connections is the different speeds of filling and emptying of each battery, and thus they can have different levels of "fill" or charge, whereas in series all the batteries must be at an equal level???
                          Again you are not grasping Series and Parallel circuit Laws. Think of series circuit as a 1-lane highway. Everyone must go the same speed and follow the same path. Think of parallel as a multi-lane highway where weak ole grandpa and slower drivers keep right. Or better yet study [B][I][U]Series/Parallel circuits[/U][/I][/B].

                          In a [B]series circuit[/B], the current through each of the components is the same, and the voltage across the [B]circuit[/B] is the sum of the voltages across each component. In a [B]parallel circuit[/B], the voltage across each of the components is the same, and the total current is the sum of the currents through each component.
                          Just do not make the mistake academics and teacher make assuming the resistances are equal. That only happens on paper, not the real world.
                          Last edited by Sunking; 04-03-2018, 11:26 AM.
                          MSEE, PE

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