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I'm 15 and I recently started electronics and just had a question about batteries.

(I'm going to use a 9v battery as an example)

From what I understand and from what I've read, a 9v battery creates a voltage (potential difference) by doing 9 joules of work (9 joules of chemical energy into 9 joules of electrical potential energy) to pull electrons away from their atoms and their normal state to a point of high potential energy, therefore creating a potential difference, or voltage.

Could someone tell me if this is correct please?

Thanks

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    \$\begingroup\$ Does this answer your question? What exactly is voltage? \$\endgroup\$ – bem22 Aug 23 '20 at 1:12
  • \$\begingroup\$ The existing answers plus bem22's link to this answer together probably provide exactly what you want. Your Joule concept is not correct in this context. A Joule is a measure of energy - 1 Watt for 1 second or eg 1 Amp x 1 Volt for 1 second. With a 9 volt battery if you supplied say 3 amps for 60 seconds you'd supply 9V x 4A x 60s = 2160 Joule. || People MAY vote to close this answer as a duplicate (I hope not) - don't be discouraged if this happens. Keep on with the good questions and learning :-). \$\endgroup\$ – Russell McMahon Sep 9 '20 at 4:24
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Pretty close but what you describe much more accurately reflects a capacitor than a battery. In a capacitor, you add energy to pull electrons away from where they want to be thus storing potential energy, and that potential energy is released when the electron is released and allowed to pop back. You do move electrons in a similar way in a battery, but the electron don't stay "free". They got locked into chemical bonds instead.

A charged battery has a different high-energy chemical compound on each side. They are high energy and therefore unstable and want to release the energy so that the elements can re-form into lower energy compounds which are more stable. One side requires more electrons to go from high energy to low energy, while the other side has excess electrons when going from high energy to low energy.

The elements on each side of the battery can form one of two compounds: a low energy compound and a high energy compound. But the the compounds between the two sides of the battery have opposing characteristics:

  • On one side, the low energy compound requires more electrons to form than its corresponding high energy compound contains.
  • On the other side, the low energy compound requires fewer electrons to form its chemical bonds than its corresponding high energy compound actually has.

The high energy compounds are prevented from reforming into the low energy compounds if the battery is left disconnected because the elements on one side require more electrons to form the low energy compound than the high energy compound actually contains, while on the other side the high energy compound contains more electrons than the low energy compound needs and has no way to get rid of the extra electrons if the low energy compound were to form.


An uncharged battery has only low energy compounds on both sides of the battery. When you charge the battery, you are adding energy to force those electrons from one side of the battery to the other:

  • You rob electrons from the side with low energy compounds that need them (yet its corresponding high energy compound does not need them),
  • and move those electrons to the side with low energy compounds that does not need them (yet its corresponding high energy compound does need them).

In the presence of an electron deficiency or surplus on the correct side, this breaks up the low energy compounds and forces the high energy compounds to form in their place. This effectively stores that energy in the chemical bonds of those compounds thus charging the battery. It's the chemistry version of winding up a spring.

These compounds are high energy and therefore unstable. When you give a path for electrons to flow between the two materials formed by those chemical reactions, they have an easy way to re-enter the lowest energy state (as so many things in the universe try to do) which is more stable by undoing the chemical reaction and as a useful byproduct, charge from side to the other and releases the energy you initially stored in the chemical bonds.

Then you come full circle, where:

  • the side with high energy compound that needs those electrons no longers has them while its corresponding low energy compound doesn't need them,
  • while on the other side, the high energy compound has electrons but doesn't need them while its corresponding low energy compound does need them.

So the high energy compounds on both sides get broken up and recombine to form the low energy compound, producing a dead battery.


What makes a battery rechargeable is that these chemical reactions are reversible. If the chemical reaction is not reversible then you have a non-rechargeable battery where the materials used to produce the battery must be prepared outside of the battery in their "charged" form.

I hope that gives you a clearer picture of what is going on inside. You need to take a basic chemistry class to see how the moving pieces actually work inside the battery (with why certain elements are used and with the ions and electrons moving back and forth).

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The work done by the battery is that needed to let the current flow in the circuit. The battery if not connected to a circuit does not do any work even if there exist a potential difference of 9 volt across it. There is of course an unavoidable discharge phenomenon which entails the transformation of chemical energy into heat, but this is a secondary effect. The work, in particular, is not 9 joules, but 9V times the current times the time during which the current flows. Moreover, in a battery, charge carriers are not only electrons but also ions. The battery is a system where a chemical reaction happens. In general a chemical reaction releases or absorbs energy depending on the direction. When a current goes in a circuit from the positive to the negative pole of the battery, the direction of the reaction in the battery is such that an energy is released in the form of work done on the circuit. A reverse current entails the opposite direction for the reaction if the battery is rechargeable (that is, if chemically speaking the reaction is reversible): in such a case the energy is absorbed by the reaction and hence by the battery, thanks to an equal work done by the circuit. For a current to flow in the positive-to-negative direction the potential difference across the circuit must be less than the battery voltage at no load. For the current to flow in the opposite direction such a potential difference must be higher than the battery voltage at no load. So the voltage at no load, that you are saying in your case is of 9V, is the exact voltage across the battery that is not able to let a current flow in either direction. So those 9V are the result of a thermochemical equilibrium where the tendency to let current and reaction flow in a direction is equal to the tendency to let current and reaction proceed in the opposite direction. Internally those 9V are created by a distribution of charges that creates an electric field such that if you want to move a small quantity of positive charge from the negative to the positive pole of the battery open-circuited following whatever path at a slow speed you need to do a work that when expressed in joules equals 9V times the value of that charge in coulomb, and hence a work of 9 J per positive charge unit.

P.S. I notice that you use voltage and potential difference interchangeably, but voltage is more general than potential difference. Look at here

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