It happens to all of us eventually. You pull out your phone, maybe to show your friend that awesome new video, but as you're about to start watching it all those warning messages from your phone that you've been ignoring finally come back to haunt you. The screen fades to black, and your phone goes dead. You might slap your forehead and wish you'd charged your phone earlier, but as an engineer you might also be wondering why we can't simply design batteries that last longer. Well, it turns out that it's much harder than it sounds.

[Crash Course Engineering intro]

A battery's job is to provide a voltage, or energy supply, to push a current through a circuit, in the form of negatively charged electrons. When it can't provide enough voltage to power the circuit anymore, the battery's dead. And the length of its life mostly depends on the battery's three main parts: a negative terminal called an anode, a positive terminal called a cathode, and the electrolyte that lets charges flow between them. And anode and cathode are both electrodes, the bits that make contact between the chemistry of the battery and the circuit. They drive the electrochemical reactions that provide power to the circuit.

It all starts with a process called oxidation, which causes electrons to build up in the anode when the battery is connected to a circuit. Electrons tend to repel one another, because they have the same charge, so the anode is essentially pushing all those electrons away. At first, it might seem like the obvious place for them to go is straight to the cathode at other end of the battery, but that's exactly what the electrolyte is there to prevent—electrons can't flow through it. So instead, the anode's electrons flow through the circuit, delivering power to anything connected to it as they move toward the cathode, where another chemical reaction draws them in: reduction. In reduction, the cathode absorbs the electrons, which is what makes it the positive terminal in the power supply.

Meanwhile, inside the battery, other charges are moving. As all those electrons are released from the anode during oxidation, they leave positively charged ions behind, which can travel through the electrolyte to the cathode. Something similar happens at the cathode, too. As it gains electrons, it creates negative ions that travel to the anode. The chemical exchange releases energy, which sustains the oxidation and reduction reactions to keep the battery going. Eventually, the rate of oxidation and reduction decreases, which in turn decreases the voltage, until it finally gets to the point where it can no longer power the current through the circuit. In other words, the battery is out of power.

The amount of power you can cram into one battery mostly depends on its type. And there are lots of different types of batteries, because depending on what you're using them for you'll probably want different qualities. Obviously, you're not going to want to swap out the battery in your car every time it discharges, for example, so you'll want a rechargeable battery for that, even if a disposable one would last longer on a single charge. But there are less obvious examples too.

Mechanical engineers have to consider a battery's design, including its weight and shape, when incorporating it into a moving device like an automated drone or robot. Bioengineers, meanwhile, need to consider what chemicals go into their batteries. When you're implanting a battery-powered medical device into someone's body, the chemicals in the battery often come into close contact with the person's bloodstream, so it's important to make sure they won't cause them harm if the casing breaks. Some engineers are actually working on bio-sensing capsules that would use stomach acid as an electrolyte to generate power, and those would definitely need to be safe for the human body.

But if your main priority is maximizing the amount of energy you can cram into a given space, you'll probably want a "primary battery"—another name for disposables. Primary batteries give up their chemical energy as electrical power, and once they're done you won't get much more out of them. These sorts of batteries tend to be fairly cheap and lightweight, because, well, they're simple, and when they run out you can just grab a new one and you're ready to go. That makes them a convenient source of power for things like toys or remote controls. Plus, they have a high energy density, meaning they can store lots of energy for their size.

The power you can get from a battery—and therefore how long it lasts—depends on the voltage and current being delivered by the battery as it undergoes oxidation and reduction at its terminals. In an electrical circuit, the power—the energy delivered per unit time—is the product of the voltage and the current. So to get the total energy, you just add up all the power delivered to the circuit over the time the battery operates.

Primary batteries are also extremely consistent, which matters more than you might expect. Most circuits require a certain voltage to operate, so providing roughly a constant amount is pretty important over the course of a battery's life. In an ideal world, all batteries would deliver a perfect constant voltage for their entire life before dropping to zero. Unfortunately, real batteries don't have perfect chemical processes going on. Instead, you end up with what are called "discharge curves".

Initially the battery will provide more power than average, then it will drop off a little. After that, as the charge capacity of the battery is used, the voltage being supplied begins to drop off even more. That's because batteries themselves have what's called "internal resistance", a measure of how much of the chemical potential energy is lost to heat and other processes in the electrolyte rather than providing a voltage to the circuit. As ions are exchanged inside the battery, the internal resistance increases, preventing a perfect constant voltage from being delivered.

The rate at which the voltage begins to drop over the course of the battery's life depends on its chemistry, and the combination of chemicals in primary batteries tend to deliver pretty consistent voltage. For example, zinc and manganese dioxide do a good job as the anode and cathode in a primary battery with potassium hydroxide and water as the electrolyte. That combination is what we call an "alkaline" battery; it's what's in those AAs you probably have sitting in a drawer somewhere. The problem is you can't recharge primary batteries—once they're dead, they're dead—and that's a big trade-off.

That's why we also have "secondary batteries", the technical term for the kind that can be recharged. When you use a secondary-type battery, it discharges like a primary type battery with one important difference. The anode, cathode, and electrolyte are all chosen so that the oxidation and reduction reactions at the terminals can be reversed to restore the ions back to their original electrodes.

That won't happen on its own, but if you connect a separate power supply—like from an electrical outlet—you can recharge the batteries. The external power source applies the reverse of the current the battery would deliver, which sends the ions in the electrolyte back to the electrodes they originally came from. Once the chemicals are back where they started, the battery is ready to discharge again.

Being able to recharge a battery has enormous advantages. For one thing, you don't need to buy a new battery for your phone every day, which is nice. But it also means you can store power from different sources. That's useful for things like cars, where you can store energy from something like gasoline to provide electrical power wherever else it's needed, like in your headlights. On a broader scale, being able to store energy when you have a surplus is incredibly important for making renewable sources of energy practical.

A rechargeable battery that needs to deliver a high voltage—like in a car—might use a combination of lead electrodes and sulfuric acid electrolyte. Those batteries are somewhat expensive, but they have a good energy density. But your phone or laptop almost certainly use a different type of rechargeable battery: lithium-ion.

"Lithium-ion" actually describes a whole class of batteries, but they all tend to have a cathode or anode based on a lithium compound. When the battery discharges, lithium ions are exchanged in the electrolyte, hence the name. These types of batteries have become popular for a couple of reasons. Even though they're very expensive, they're rechargeable and have a high energy density. That makes them great for supplying high voltages and lots of power with a relatively small amount of space.

Still, it's pretty easy to end up in a situation where your phone runs out of battery in the middle of the day, because as we've found ways to make more powerful components smaller and smaller, battery tech hasn't really been able to keep up. Don't get me wrong: people are trying. Companies have put tons of effort into making lithium-ion batteries as energy dense and efficient as they possibly can; it's just been difficult to do much more than we already have.

Engineers also have to take safety into account, because when lithium-ion batteries break they can pose a real safety hazard. People who bought the Galaxy Note 7 learned that one the hard way. Certain chemicals called "carbonates" found in lithium-ion batteries tend to be quite flammable. Just as troubling is the fact that thin strands of lithium can build up in the electrolyte as the batteries are repeatedly used. Those strands are called "dendrites", and if they build up too much and connect the anode and cathode, they can short-circuit the battery, and even set it on fire.

So, apart from little tweaks here and there, lithium-ion batteries seem to have basically hit their limit. These days, when phone or laptop companies promise better battery life, that's often because they have designed the rest of the hardware to draw less energy, not found a way to get the battery to provide more. If we're going to surmount all these problems with lithium-ion batteries, we're going to have to explore the chemical landscape a little.

At the cutting edge of battery engineering, researchers are experimenting with new chemical combinations. For example, swapping out the carbon-based anode used in most lithium-ion batteries for a silicon-based anode could store much more energy in a given volume. That could mean a longer battery life. And introducing tiny particles of silicon dioxide into the electrolyte could stop dendrites from growing so quickly by making them travel further within the battery, making them safer for long-term use. So far, though, no one's been able to make these changes work on a commercial scale.

But those are only a couple of the ideas being put to the test right now; there are plenty more in the works. And someday, maybe fighting over the power outlets in coffee shops will be a thing of the past.

In this episode, we looked at batteries. We saw how they provide power by discharging ions between a cathode and an anode, and how reversing that process gives us a way of charging them. We saw that batteries deliver voltage differently over time, leading to discharge curves, and some of the work being done to improve the properties of batteries for portable electronics. Next time, we'll see how batteries and other fields of engineering come together when we look at robotics.

Crash Course Engineering is produced in association with PBS Digital Studios, which also produces Reactions, a show that uncovers the chemistry all around us and answers the burning questions you always wanted to ask, like "Why does bacon smell so good?" and "How can I get my smartphone battery to last longer?" Check it out in the link in the description.

Crash Course is a Complexly production, and this episode was filmed in the Dr. Cheryl C. Kinney Studio with the help of these wonderful people. And our amazing graphics team is Thought Café.