What is a battery?

What is a battery? According to my mum. (Hi, mum 👋) "a way of powering things". Now I hope she won't mind me telling you that she doesn't have an extensive scientific background. Despite this, she is not incorrect.

Power = work per unit time

So, yes, you can use a battery to do work using the electrical energy within it. But she is a wily old fox my mother and she added a very important caveat (even though she likely didn't know it, sorry mum). It is only "a way" of powering things you could equally use a diesel generator or a solar panel. So we are not quite at a completely satisfactory definition.

We have determined that a battery is "something that you can use to provide energy to something". So why isn't a solar panel a battery? It provides power to things…???

A key characteristic of a battery is its ability to store energy in chemicals so that you can use it later. A solar panel takes the light from the sun and instantly turns it into energy. Fantastic! But you can't save that energy and use it next week; you have to use it as soon as it is converted. So a battery must be able to store energy so you can use it anytime.

So why is a diesel generator, not a battery? It stores energy in the form of diesel (a chemical) and converts it to energy so you can power something on demand, just like a battery. The key missing word is "directly". A battery directly converts energy stored in chemicals to electricity. Where as a diesel generator has to ignite the diesel to create an explosion that drives a piston that spins a generator that outputs electricity. So, the energy changes from chemical to kinetic to electrical energy. There is an extra step in the middle.

We have jumped ahead slightly, though. It has been assumed that our energy's starting point is chemical energy, and the endpoint is electricity. This is the realm in which I predominantly work and is what most people are familiar with. This is how your standard AA battery works. But the forms of energy don't have to be chemical and electricity. You can have a thermal battery that stores energy as heat, which can then be released later (those of you who are old enough or social-economically unlucky enough to know the joys of a storage heater will begin to understand how this one works). Or you can use gravity to store energy. This is how a hydroelectric dam works. Pump water into a reservoir to store it and then let it fall back to a lower level. Then, the energy from this is used to turn a turbine. This is a stretch of our definition, as energy technically makes more than one conversion. Also, you are unlikely to be able to fit a hydroelectric plant into the next iPhone, but hopefully, you will see my point.

The umbrella term for all of these technologies is "energy storage". A battery like the one in your phone or laptop is, more specifically, an electro-chemical energy storage device. It is related to the conversion between energy in the form of electricity and chemicals. But that is a bit of a mouthful, and I doubt Duracell could have marketed the "electro-chemical energy storage device bunny". So, people use “battery” as a shorthand.
So we are sorted, Right?

Well, a comment in one of my previous posts mentioned fuel cells, which convert energy from chemicals directly into electricity. So either I leave it there, I disable comments on my posts so people stop making valid points, and we all move on, or we still have a bit of nuance to deal with.

I left you with the bombshell that a fuel cell could be seen as blurring the lines of our hard-earned definition. And under certain circumstances, it is kind of like a battery. For non-battery folks, a fuel cell is a device that takes “fuel,” for example, hydrogen gas, and passes it through an electrochemical cell where a chemical reaction occurs, resulting in an electrical current. On the face of it, this sounds a lot like a battery, but there is a key difference. The fuel is not stored inside the system itself, and a fuel cell generates energy as long as it is fed with fuel. On the other hand, with a battery, once you have extracted all the energy stored in the chemicals inside, it stops working. You then have to recycle it or recharge it.

The story has one more character - the “flow battery”. Basically, if a battery and fuel cell had a love child a flow battery would be their offspring. In a standard battery all the chemicals it will use are stored within it. Whereas, a fuel cell can be thought of as an engine or generator. It doesn’t store the energy itself; you feed it with fuel, and it converts it to energy. A flow battery stores its chemical energy in large tanks of chemicals that are pumped through the cell, and as they are pumped through, they undergo chemical reactions that produce energy. How much energy you can store is limited by the size of your chemical tank. When the tank is empty, you either dump out the waste and refill it with fresh chemicals or pump the used chemicals back through the cell while applying electricity to reverse the chemical reactions and restore the materials to their original state. This is similar to what occurs in a rechargeable battery when you recharge it. You feed it electricity, and the substances inside regenerate to how they started so you can use them again.

So our ultimate definition for a Battery is:
An electrochemical energy storage device that can directly convert chemical energy stored in the materials within into electrical energy.

How did I do? Can we do better? Also, I mentioned the term “Cell” over 10 times. What is a cell? Isn’t that Biology, I thought we were talking about batteries?