Why can batteries be wired in series?

  1. I have a small background with AC power and am trying to understand DC.
    I recently considered paralleling/serializing some batteries and realized how different a DC circuit is.
    Since a complete circuit of a battery is + to -, thus creating a short when connecting the two.
    Why then is it possible to connect the + and - of two separate batteries?
    Why doesn't this cause a short circuit?
    What little I know is that the battery wants to send electrons across to the side that has less, right?
    Why doesn't this happen then when the two batteries are connected in this way?
  2. jcsd
  3. There was a thread about this within the last year. Do a search.
  4. I posted this link in another thread but this question is so entry level that I think I will share it here as well.

    The answer to your current question can be found here:
  5. NascentOxygen

    Staff: Mentor

    Electrons need a continuous conducting path (such as through wires or other conducting substances) to travel from the base of a battery back to the top of that very same battery. If there exists no such path, then no current, hence no circuit.
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  6. Why does it have to be the same battery, that's the part I am confused by.
    The electrons have that conducting path from one terminal to an opposite, just like being a battery.

    I mean, sure, no; but why?
  7. NascentOxygen

    Staff: Mentor

    Because the EMF (e.g., the 1.5V push) from any cell is directed from one of its terminals towards the other. It's just like any other force, it's all relative, one end relative to the other.
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  8. Something is seeming to surface about understanding this.
    In AC, when the hot leg goes to ground, that power will find a way to get back into the system so that it can get back to the generator at the power station.
    It doesn't care whether it can find the neutral return back to the main circuit breaker that both the hot and neutral are connected to. It will gladly fly along any ground it can find.

    Why does a battery care which terminal it completes the circuit with?
  9. Because 'ground' in our AC electrical system is connected neutral.
  10. NascentOxygen

    Staff: Mentor

    Likewise, current from one terminal of a battery will follow any path, so long as it leads back to the other end of that same battery. It will even go through another battery (itself just a conducting path with an added boost or buck) if that leads back to the battery of origin.

    In a generator, the electrons must make their way back taking any path or short-cut that leads to the very piece of wire they originated from!
  11. I think this is a really good question. Couldn't it be possible to design a pair of batteries where it WAS the case that + of one to - of the other made a short? I mean, on the ionic level if you could control the amounts of charge to be precisely complementary, why would it matter which battery's lead was used?

    I think the idea that electrons somehow know their terminal of origin is incorrect.

    edit: I mean, a practical way to accomplish that would just be to connect the two - ends of 2 batteries. In that case, obviously current would flow to either (both) - terminals from either +. Now the thought experiment is to disconnect the 2 - ends while holding the potential constant. Would it not still be the case that the same current would flow from either + to either -, given a conducting path?
    Last edited: Aug 26, 2014
  12. NascentOxygen

    Staff: Mentor

    No. They would not be individual batteries if such a thing could happen.

    Have you ever stopped one and asked it? :tongue:

  13. No, I guess I'm just thinking of situations outside of practical circuits where there is a movement of electrons. A photon incident on a particular hydrogen atom with more than 13.6 eV for example will knock the electron off. Are you saying the electron has a higher chance to be retaken by the original hydrogen atom than some other nearby proton? How is that different than the electrochemical reaction that separates charge? Where in the electron is the information about its "parent" stored?

    edit: Where in Coulombs law/Gauss' Law is that information about origins? I think it only states opposite charges attract, not that the positive charge that used to hold the electron attracts.
  14. NascentOxygen

    Staff: Mentor

    Individual electrons are influenced by any electric field they fall under, of course. Current is a macroscopic migration, and electric current due to a single voltage source traces a closed loop path. Individual electrons may never make it all the way around the loop, though with one being indistinguishable from another, how could we ever know?

    The EMF from one unconnected cell has negligible influence on another because almost all its potential appears across the high impedance air path forming the interconnect, leaving negligible electric field across other elements.
  15. I'm in total agreement with you there, but in this situation:


    Isn't there a very brief transient current in the wire? I understand that under a definition of circuit this would not qualify, but certainly there is a movement of charge from high potential to low until equilibrium is established?

    The same thing would happen if you connected the wire as in the image above vs if you connected the outside terminals. But if you had both connections at the same time, in a complete circuit, you would get double. Right?

    Lets say both batteries had only 1 single charge different between their terminals. With only one wire connected, when the electric field acts on that single charge, the battery is exhausted. A change in the electric field will propagate through space and affect distant charges (very briefly). Wouldn't that be the same as shorting a single battery?
  16. First, please note that the physics governing both AC and DC are fundamentally the same. The physical laws (represented by us as equations and natural language expressions) that govern electromagnetism are the same when concerning immediate voltages and currents. Each of AC and DC is a special case in these equations. For instance, the defining characteristic of DC is that the immediate voltages and currents are constants and therefore inductances and capacitances can be substituted for a closed and open circuit, respectively. By AC normally it's meant an electrical system with currents and voltages having a common period. Fourier analysis can be readily applied so that the quantities expressed as a function of time can now be expressed as a function of frequency. The phasors used in analysis of AC circuits are the Fourier component of the quantities in question. For each of voltage and current, if that quantity varies sinusoidally with time, it has a single frequency and so a single phasor is enough to describe it. If a quantity varies periodically and non sinusoidally with time there will be more than one frequency component (those are the harmonics in power systems) and one phasor is needed to describe each of them.

    AC systems can be connected in series just like DC systems. You probably have practical AC systems in mind where a conductor is grounded and so connecting conductors at a different (phasor) potential would make a short circuit because ground is already a common connection point and by making a second one, you make a complete circuit. The same thing wold happen with a grounded DC system. Grounding is NOT a property of DC or AC but of specific systems. What is called a “ground” is also a matter of convention. You can also, for instance, have 2 isolation transformers (akin to 2 batteries) powered by a single power source, and connect one terminal from one transformer to the other (whether of same or opposite polarity) and no short circuit will arise.

    Supposing that a battery has no net charge (in the sense of used in self capacitance, not in the usual sense of energy stored electrochemically) and has plane symmetry then both terminals will have an electric potential equal in magnitude and opposite in sign. There will be a electric field between them. When you connect the positive terminal of one battery with the negative of another, the electric field will create a current which redistributes the charges so that the terminals connected together will be at the same potential, and the other 2 unconnected terminals will have a difference of potential equal to the sum of the voltage of both batteries. This current is very short and of negligible magnitude; thus it's a transient in the broadest sense of the word but it's negligible for practical purposes.

    Note that the electric field is different with the batteries alone than with them connected. When alone, every batteri will have one terminal at negative potential and the other at positive potential, but when connected, the connected terminals will be a the same potential.

    The electrons contain no information of where they originated from. It's the electric fields generated by the power source what makes them return to the power source they came from (in a steady state operation).

    If you connect 2 power sources in series and ground the mid point, then you have a split phase power system.

    It's hard to analyze circuits from a field viewpoint. Circuits are a very useful simplification. I don't have the knowledge necessary to do so, so I can only give you a rough qualitative explanation. I recommend you to check the book The Fields of Electronics by Ralph Morrison. It's about circuit behavior explained in terms of electromagnetic fields. Beware, it contains some errors. For instance, I have seen a problem asking the reader to find a force in kilograms (even if kilograms-force are meant, the corresponding clarification and conversion factor is missing). Test charges are described as making or being made a work in units of volts (if it's work, it can't be in volts, electromotive force is probably what is meant). A capacitor in vacuum with square parallel plates of 1 m2 at a distance of 1 m is once implied to have 1 F of capacitance. I wouldn't be surprised if there was many other errors like this as well, but it's an useful book and I don't know of another similar to it.

    Your question becomes trivial to answer with circuit theory: There's no short circuit because there's no complete circuit when connecting opposite terminals of different batteries.

    This message is Copyright © 2014 Mario Castelán Castro and it is licensed under the Creative Commons Attribution-ShareAlike 4.0 International license.
  17. NascentOxygen

    Staff: Mentor

    Any length of wire has capacitance, so connecting a piece of wire to one terminal of a source will result in a brief charge movement as a new equilibrium is established.
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