A question that I've asked myself for long time about why batteries in series work as they do. I'll try to explain:

If you take two batteries, let's say A and B, and you put A's anode touching the B's cathode (only that, without closing the circuit). It turns out that not only A and B poles don't discharge each other but the resulting device sums the voltages of both batteries.

When I think of it my idea is that A and B poles should get "lost" (they have cancelled) but the poles that remain unconnected still have a different potential so it shoud work at that voltage, not the sum. After all you are connecting a + pole to a - one and it's supposed that one pole has a different charge that goes to the other until they are balanced.

So, why batteries discharge when closing the circuit but not when it's open if poles of different batteries are in touch? Why the charge from one pole doesn't flow to the other? And why the voltages sum?

I'm afraid that the explanation is quite simple but...which is?

Thanks in advance!

NOTE: I edit my own question. As Murphy states, once published I've found there can be a possible explanation here:

Why two connected 1.5 volt battery become 3 volt

I'll see if it tells me what I want. Nevertheless, feel free to give me your own answer.

NOTE 2: Ok, guys, thank you very much for your answers. Finally I think I got it!

The point is in the electrons that cause the potential difference. The accumulation of electrons in one pole makes impossible that the electrons of the other battery enter the possitive pole. They are rejected electrostatically. Allowing it you would have the two poles charged but the battery doesn't work that way.

So, when you close the circuit, both accumulations of electrons sum because one battery would help expelling the other one's charge pushing with it's electrons.

Am I right?


2 Answers 2


A battery is effectively an electron pump. Inside the battery a chemical reaction (typically a redox reaction) pumps electrons from the cathode to the anode. If the two ends of the battery aren't connected to anything there's nowhere for the electrons to go and the reaction stops. When you connect the battery to an external circuit the reaction resumes and pumps electrons round the circuit until all the reagents are used up and the battery is used up.

So connecting two batteries in series is just like connecting two pumps in series. If there's nowhere for the electrons to go the pumps halt in both batteries and the batteries just sit there doing nothing.

  • $\begingroup$ Isn't it the case that batteries of a different voltage (whether one is less charged than another or they're different types of batteries) will even each other out? Either by running down the battery of the higher voltage due to internal resistance of each battery or by charging one and depleting the other? $\endgroup$ Aug 24, 2014 at 18:52
  • $\begingroup$ @BrandonEnright: if you connect the batteries in parallel that will happen, but not when they're connected in series. $\endgroup$ Aug 24, 2014 at 18:58
  • $\begingroup$ Ah darn I read the question as parallel. I'm dumb. $\endgroup$ Aug 24, 2014 at 18:58
  • $\begingroup$ Do you know of a battery that does not have a redox reaction? $\endgroup$
    – LDC3
    Aug 24, 2014 at 19:18
  • $\begingroup$ @LDC3: I can't think of one offhand, but I'm not confident enough to swear under oath that no such batteries exist :-) $\endgroup$ Aug 24, 2014 at 19:26

You need to think of batteries as complete units. Inside, you have a chemical reaction producing the EMF. Leads are connected from the anode and cathode of the cell to the terminals on the battery. When you connect 2 batteries, all you are doing is connecting the terminals together; you are not changing the connection to the voltaic cell inside.


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