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I'm trying to elaborate a simple and concise explanation (and conceptually correct) of how a battery charges and discharges, and why we say things like "the charge stored in the battery is x Ah". For now, I have this:

A battery stores potential chemical energy (potential energy because it is not being used, but can be used to do work), which can be converted into electrical energy by chemical reactions that occur inside it. This conversion happens when it is connected to a circuit.

Battery charging: For electrons to move from the positive terminal to the negative terminal of the battery, work must be done by an external source. This work, which is done on the electric charges that are on the positive terminal, results in accumulation of potential chemical energy in the battery, since it causes the reversal of the chemical reactions that release energy.

So , a battery does not store charge but rather energy. Despite this, in practice in many situations it is more important to know how much the battery will last and not how much energy it stores. So, although conceptually wrong, it is habitual to say "This battery stores a charge of x Ah" to indicate for how long a fixed voltage battery will be able to deliver a given current.


  • So, is this little fragment I've written completely correct? Is it possible to improve it without giving up simplicity and conciseness?

In addition, some sources claim that a battery stores electrical potential energy. If this is true then my statement "This conversion [chemical -> electrical] happens when it is connected to a circuit" will be wrong.

  • I have a question about this: does a battery have voltage when it is not being measured? In other words, does a battery maintain an electrical potential difference even when it is not being used?

  • If so, does it mean that the battery converts potential chemical energy into electrical energy all the time and intensifies this conversion when used?


1st improvement: (WARNING: This text still contains conceptual errors)

A battery stores charges (electrons), that stay bonded in the chemical compounds that form on the surface of its electrodes. Because of this, we also can say that the battery stores potential chemical energy (potential energy because it is not being used, but can be used to do work and chemical because this energy is stored in chemical compounds). Considering the fact that this potential energy is due to electrons, we can also say that a battery stores potential electrical energy. (Both measured in joules).

Due to the different amounts of bounded electrons in the electrode's chemical compounds, one electrode stays with excess of electrons, wile the other stays with depletion of electrons. Then, we call the first negative terminal and the last positive terminal. Having this in mind, we can say that the battery have a potential difference between its terminals. While we normally measure a difference of energy in joules, when dealing whit charges there's a more useful mesure: difference of energy (J) divided by amount of charge (C). This is called voltage or electrical potential difference or electrochemical potential difference.

Battery discharging:

When the battery is connected to a circuit, it starts the process of discharging. In this process, the electrical/chemical potential energy is released by the chemical reactions. In other words, the electrons bounded in the chemical compounds start to unbound and move around the circuit, until they reach the positive terminal. We have here the conversion of the potential electrical/chemical energy into electrical energy. This process lasts as long as there is a potential difference between the battery terminals.

Battery charging:

For electrons to move from the positive terminal to the negative terminal of the battery, work must be done by an external source. This work, which is done on the electric charges that are on the positive terminal, results in accumulation of potential electrical/chemical energy (and, consequently, accumulation of charge) in the battery, since it causes the reversal of the chemical reactions that release electrons. In other words, the electrons in the battery get bounded into new chemical compounds that form on the surface of electrodes due to external work. This process lasts until the voltage of the battery is lower than the applied charging.

The Ampere-hour unit:

One could say things like "This battery stores x J of energy", but in practice in most of the situations it's more important to know how much the battery will last, and not how much energy it stores. So, it's more habitual to say "This battery stores a charge of y Ah, because this indicates for how long a fixed voltage battery will be able to deliver a given current.


  • So, are those explanations conceptually correct now?
  • Does not the Charge Conservation Principle requires that the amount of charges entering in one terminal should be equal to the amount of charges coming out of the other terminal? If so, how can charges be stored in the battery when it is charging? What am I misunderstanding here?
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    $\begingroup$ Yes you are on the right track. You can also learn about the chemistry as well, for example a lead acid battery cell is 2 volts whereas a Li ion battery cell is 3.4v because Li is more reactive than Pb. If both batteries where rated at 1Ah of charge the Li one produces more energy, 1.7 times more! $\endgroup$ – PhysicsDave Apr 9 at 0:54
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    $\begingroup$ I think is not all wrong to say that it stores charge, since a loaded battery is trapping a defined amount of electrons with chemical bonds, and releases them upon discharge. The Ah are easier to measure and better defined; actual power output depends a lot on "internal resistance" and discharge mode/speed. $\endgroup$ – patta Apr 9 at 10:34
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    $\begingroup$ Yes the battery has its nominal potential when is not being used. When it is working , the potential drop a little $\endgroup$ – patta Apr 9 at 18:28
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    $\begingroup$ Yes there is voltage when not connected .... but the current is zero. Think of a positive charge separated by a glass pane from a negative charge, the force of attraction is still present but no current flows until the glass is removed. $\endgroup$ – PhysicsDave Apr 9 at 23:35
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    $\begingroup$ When the battery is connected, yes you can say that chemical energy is converted into electrical; voltage by itself is not energy; when you have current flowing, then the electrons do work (on a lamp or electric motor) $\endgroup$ – patta Apr 10 at 8:02
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After reading this link, in a site that deals, among other things, with electricity misconceptions, I found an explanation that pointed me the way to solve my misconceptions. I will reproduce below some fragments concerning my question.

BATTERIES STORE CHARGE, AND THIS CHARGE FLOWS IN WIRES? No.

The word "charge" has more than one meaning, and the meanings contradict each other. The "charge" in a battery is energy (chemical energy), while the "charge" that flows inside wires is part of matter, it is electron particles. And those wires, even though full of charge... are neutral and uncharged! The term "charge" refers to several different things: to net-charge, to quantities of charged particles, and to "charges" of energy. If you are not very careful while using the word "charge" in teaching, you might be spreading misconceptions.

[...]

Another one: as you "charge" a battery, you cause an electric current to appear in the electrolyte, and this motion of electric charges causes chemical reactions to occur upon the surfaces of the battery's plates. Chemical "fuel" accumulates, but charge does not: the charges flow into (or out of) the surfaces of the plates and do not accumulate there. (The path for electric current is through the battery. Through, and back out again.) A "charge" of chemical energy is stored in the battery, but electrical charge is not. And when a battery is being "discharged", it's chemical fuel drives a process which pumps charge through the battery. During discharge the battery's fuel will eventually be exhausted, but the total electric charge within the battery will never decrease!

Here's a way to imagine the process: a battery is like a spring-driven "wind up" water pump. Send water backwards through this pump, and you wind up the internal spring. Then, provide a pathway between the inlet and the outlet of the pump, and the spring-motor will pump the water in a circle. But now think for a moment: the water is the charge, yet our wind-up pump does not store water! When we "charge" our wind-up pump, we send the charge (water) through the pump, and this stores energy by winding up the spring. Same with a battery: to "charge" a battery, we send electrical charges through the battery and back out again. This causes the chemicals on the battery plates to store energy, just like winding up the spring in our spring-powered water pump. See how "charging" and "charges" can create a horrible mess of misunderstandings? When this mess gets into the K-12 textbooks, and educators start teaching it to kids, the kids end up believing that Electricity is far too complicated for them to understand. Yet the fault does not lie with the students!!!!


So, correcting the misconceptions of my understanding and answering my questions:

  1. A battery is a device that is able to store electrical energy in the form of chemical energy, and convert that energy into electricity;
  2. We can also say that the battery stores chemical potential energy (potential energy because it is not being used, but can be used to do work and chemical because this energy is stored in chemical compounds);
  3. We can't say that the battery stores electric potential energy. A battery tends to maintain the electric potential energy at each of its terminals, but the energy stored in the battery is not from the separation of charges, it's from the redox reactions (chemical energy). Compare with the capacitor, whose energy is stored in the form of electric potential energy, due to the separation of charges in its plates.
  4. The chemical reactions that occur inside a battery occur even when it is disconnected from a circuit. Thus, the batteries have voltage even if they are not being used or its voltage measured. (See Explanation of Point 4 below)
  5. A battery have a fixed amount of charge inside it. When it is charging, it does not store the electrons that enters into the (+) terminal. The same amount that enters the (+) terminal, leaves the battery by the (-) terminal. There is no violation of the Charge Conservation Principle here.
  6. When one say things like "This battery stores 80 Ah of charge", that means that 80 Ah (288.000 Coulombs) entered and leaved the battery when it was charged, inducing the reversion of redox reactions, and if the battery is connected to a circuit it can make up to 80 Ah (288.000 Coulombs) of charge pass through it. And this unit (Ah) is used in practice because it gives an idea of how much the battery will last: it indicates for how long a fixed voltage battery will be able to deliver a given current.

Explanation of Point 4:

One of its terminals - the anode - can "produce" electrons by oxidation reaction, making the environment surrounding it negative, and the other - the cathode - can "consume" electrons by reduction reaction, making the environment surrounding it positive. These processes can occur (in a limited way) even when the battery is disconnected from a circuit and thus we "always" (until equilibrium is reached) have a charge separation, resulting in an electric potential difference (voltage) .

To better understand this, I will cite an answer given by Alfred Centauri:

Consider for a moment, a cell that is not connected to a circuit, i.e., there is no path for current external to the cell.

The chemical reactions inside the cell remove electrons from the cathode and add electrons to the anode. Thus, as the chemical reactions proceed, an electric field builds between the anode and cathode due to the differing charge densities.

It turns out that this electric field acts to reduce the rate of the chemical reactions within the cell. At some point, the electric field is strong enough to effectively stop the chemical reactions within the cell. The voltage across the terminals of the cell, due to this electric field, is then constant and this is the open-circuit voltage of the cell.

If an external circuit is connected to the cell, electrons flow from the anode through the external circuit and into the cathode, reducing the difference in charge densities which in turn reduces the electric field just enough such that the chemical reactions can once again take place to maintain the electric current through the circuit.

The larger the external current, the greater the required rate of chemical reactions and thus, the lower the voltage across the terminals. As long as the circuit current is significantly less than the maximum current the chemicals reactions can sustain, the voltage across the battery terminals will be close to the open circuit voltage.


Further reading + References:

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