When you connect two batteries in series, why doesn't the middle short?

Question

When you connect two batteries in series, why doesn't the middle short? The connection between the positive terminal on one battery and the negative terminal on the other battery has no load, so shouldn't it be shorted? But I don't notice heat or explosions or anything like that when I connect batteries in series.

Answer 1

Your problem is there are two definitions for the phrase a short:

A) A short is high current flowing through a wire unintentionally.

B) A short is two places connected by a low-resistance interconnection (as a wire).

Both are correct but as much as A implies B, this does not mean B implies A. Using the same term a short for both makes you think they are equal, which they are not.

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When you connect the plus from one battery to the minus of the other, you have a short of the second kind. However, there is no current flowing, as this requires a circuit —a closed loop— so obviously, B does not imply A.

As soon you connect the plus from the other battery to the minus of the first also, there is a closed loop, and your short of the second kind grows into a short of the first.

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EDIT: There was some confusion about the heat production as well, which I explained in the comments before.

Where the batteries are connected bumper-to-bumper, you have a very low resistance \$R_b\$ of maybe 0.01 Ω. The outer wire resistance \$R_w\$ be 0.1Ω instead because the wire is a longer and thinner conductor than the bumper-to-bumper contact.

Because \$P=I^2\cdot R\$ for each individual connection, and \$I\$ being identical in both "resistors" as they are part of the same loop, this means there is ten times more heat produced in the wire than in the bumper-to-bumper connection.

In addition, the batteries are lumps of metal and able to take a lot of heat without a significant temperature rise.

Both is why the bumper-to-bumper connection stays considerably cool during an outer short circuit.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

When you connect two batteries in series, why doesn't the middle short? The connection between the positive terminal on one battery and the negative terminal on the other battery has no load, so shouldn't it be shorted?

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. A short-circuit is usually defined as an unintended bypass of the intended circuit. Here the intended circuit is the loop consisting of the battery and lamp. The short-circuit is bypassing the lamp and providing a "short-cut" between the terminals of the battery. If the resistance is low then high currents will flow.

^{simulate this circuit}

Figure 2. (a) No short-circuit occurs when the batteries are properly connected in series. (b) Attempted series-connection of two grounded batteries would result in a short-circuit as the current could flow through the ground connection as indicated by the red arrow. BAT3 is short-circuited while BAT4 is not.

But I don't notice heat or explosions or anything like that when I connect batteries in series.

That's because no current is flowing as in Figure 2a.

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From the comments:

How come in Figure 2a there is no current flowing between the positive terminal of BAT1 and the negative terminal of BAT2.

Because there is no closed circuit.

I thought BAT1's positive terminal has a deficiency of electrons while BAT2's negative terminal has an excess of electrons.

No, the net charge in a battery is zero! There is no surplus or missing charge at either terminal.

So shouldn't the excess electrons in BAT2 flow towards the positive terminal of BAT1, thus creating a current?

Current can only flow when there is a circuit. As shown in Figure 1 the circuit is open so no current can flow.

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I thought a battery separates itself into two sides, one positive and one negative, with a barrier in between.

If that were true then batteries suspended by a thread around their middles would all line up due to electrostatic attraction. (They don't.)

Answer 3

A short (of the type that you mean, that causes high current and thus heat) is caused by a high voltage being directly connected to a low voltage.

The positive terminal of a battery is higher voltage with respect to its own negative terminal. But that doesn't give it any particular relationship to a second battery.

So, if you take a 5V battery and ground the negative end, the positive end will be at 5V wrt ground. Then you take a second 5V battery and put its negative end on the positive end of the first battery, and its positive end will be at 10V wrt ground. But the positive end of battery1 and the negative end of battery2 are at the same voltage--and there's nothing trying to force different voltages--so no sparks.

Answer 4

I think it is an excellent question given how basic electricity is taught in school.

To all those who answer "because the circuit is incomplete", you are correct of course. But remember that in elementary school, a battery is drawn as an excess of "+" on the positive pole and an excess of "-" on the negative pool. We are told that "+" and "-" attract. Now, the wire from the negative pole of one battery to the positive of a second battery, should of course provide a free path to give these to each other. Right? What's keeping them back? This "incomplete circuit" talk should not interfere with the simple model of plus and minus connecting.

Given the model that is presented to the students, this should be an obvious problem. And I am a bit surprised that the question seems to hard to understand.

The problem is in the model. A battery does not have an excess of + or - in a "room" waiting to be released as if it was cows in a pen. Even though it's often drawn that way to illustrate. Nor is a wire a freeway with electrons being cars.

A battery has the ability to shove electrons from positive pole to negative pole. They aren't free floating in some "room". The battery chemistry progresses by transporting electrons from positive to negative.

A wire is better understood as a pipe with marbles. The pipe cannot have space between the marbles. Marbles being electrons. You need to shove one in the one end in order to be able to retrieve one out the other end.

With this model in mind it's clear that connecting two batteries are both pulling the marbles in the same direction, but the marbles still cannot move, thus not expending any capacity on the batteries. Hence also illustrating the need to complete the circuit.

Answer 5

A battery looks like this:

^{simulate this circuit – Schematic created using CircuitLab}

A battery is like an ideal voltage source. An ideal voltage source produces the same voltage no matter what. It doesn't matter if you sink 1 million Amps out of it, it will still give you the same voltage. If you put two ideal voltage sources in series, the voltages add. If you revers the polarity of one, it will fight the other one and the voltage will be subtracted.

A simmilar case applies to batteries. The ways that a battery is not like an ideal source is:

• It doesn't have unlimited current sourcing abilities
• The voltage drops the more current you pull from it (in the short term) it has series resistance
• The batterys voltage will drop the more energy you pull from it.
• It has internal resistance that is high (if you leave it there for a long time it will slowly degrade the battery.

So if you put two of the same batteries in series, it will double the voltage.

If you do this in the real world, it is important to match the batteries (which matches the series resistance, otherwise you can have more voltage running through one battery than the other and it will damage it or explode.

The middle doesn't 'short' because the voltage of the battery is pushing against the voltage of the other battery.

Answer 6

Reading all these replies I'm kinda scared for all these people thinking they know how electricity works yet they can't peoperly explain it. I'm new to all this, but from my limited understanding it works like this: In order for the battery's positive side to be able to accept electrons - the negative side needs to be able to release electrons, otherwise the force of attraction inside the battery doesn't allow the battery to neither receive or give away any electrons to/from the outside world, basically making the battery neutral to the outside world, the battery's charge is the difference between its own 2 plates, for the outside world each terminal of the battery is neutral on its own. All my life I couldn't figure out batteries because I thought batteries use some kind of static electricity/self capacitance, like they have something and then they lose it, but now I think I figured that they're actually closed - self sufficient systems when compared to the outside world. This is my understanding so I might be wrong, but I don't claim to be an expert.

Answer 7

I'm no pro but it is easier than it looks. Please follow through. When you take one battery, you connect its positive with its negative together, it "explodes". If you take two batteries, connect positive of the one with negative of the other, nothing happens. Why?

In the first case, you created a closed loop, a complete circuit. In second, the circuit is not complete because the two batteries have no relation with each other as you have not completed the circuit yet (i.e one terminal of the source connected to other terminal through some load/resistance.) It will be complete once you connect the other two terminals, again ending up in an "explosion".

By definition, you are shorting the two middle terminals. You don't get heat or explosion simply because the circuit is not complete.