I’m a professor who studies batteries and electrochemistry. To understand why batteries come in many different sizes and shapes – and serve many purposes – look to the past, at how batteries originated and how they have developed over the years.

The first batteries were made in the 1800s, and they were quite simple. One of the first demonstrations was a series of metal discs soaked in brine, which Italian scientist Alessandro Volta found created an electric current. The first lead-acid battery was made of a few pieces of lead in a jar of sulfuric acid. The modern versions are not that different. They’re just easier to manufacture and contain various additives to improve performance.

In all cases, batteries perform in the same manner: a voltage difference between two dissimilar electrodes produces an electric current, which can be discharged to power a device. Rechargeable batteries can then reverse this current to charge back up. Inside the battery, the electric current is accompanied by the flow of ions through a liquid, the electrolyte.

The passage of each electron in the current is accompanied by the transport of one ion through the electrolyte. Electrodes that can store more ions lead to batteries that can hold more charge and therefore last longer on a single charge. Electrodes that are engineered for faster ion storage lead to batteries that can discharge faster, for high-power applications. Lastly, being able to charge and discharge many times without degrading leads to batteries with long lifetimes.

Lead-acid batteries

The lead-acid battery was the first rechargeable battery invented back in 1859 by Gaston Plante, who experimented with lead plates in an acidic solution and found that the flow and storage of electric current could be reversed.

A lead-acid battery has to be big enough to provide enough charge to start a car. It also has to be usable in cold climates and last many years. Since the electrolyte is a corrosive acid, the external casing has to be tough to protect people and car parts from any possible harm. Knowing all this, it makes sense that modern lead-acid batteries are blocky and heavy.

Alkaline batteries

On the other hand, household devices like calculators and digital scales can afford to use smaller batteries because they don’t require a lot of charge. These are primarily non-rechargeable alkaline batteries that have been used for decades. The standardized cell sizes are AAAA, AAA, AA, C and D, as well as button and coin cells and many others. The sizes are related to how much charge they store – the bigger the battery, the more it holds – and the sizes of the devices they power.

Sometimes, you may find alkaline batteries sold in rectangular shapes, like common 9-volt batteries, but open the outer casing and you’ll find that they are simply a few cylindrical cells connected together inside. Cylindrical batteries have been around so long and used so widely that it just doesn’t make sense for the companies to manufacture anything different – it would require an investment to change their manufacturing facilities, something they’d rather not do.

Lithium-ion batteries

Nickel-cadmium batteries were the first widely used rechargeable batteries for household electronics and were popular through the end of the 20th century. But they had their pitfalls. Cadmium is very toxic, and the batteries suffered from a “memory effect,” which decreased their lifetime.

For many decades, lithium was studied for potential use in rechargeable batteries because of its unique properties as a lightweight metal that stores a lot of energy. Sony first commercialized the lithium-ion battery in 1991.

The company made cylindrical cells because these were the easiest to manufacture. In the 1990s, Sony was making lots of camcorders and tapes and thus had lots of equipment for roll-to-roll manufacturing. It was natural to repurpose this equipment to produce rolls of battery electrodes, which are made by casting films on sheets of copper or aluminium and then rolling them up into a “jelly roll” cylinder.

The thick casing of these cylindrical cells is mechanically strong, and to add another layer of safety they have a pressure relief valve. Very quickly, these early lithium-ion cells took over the portable electronics market, especially for laptops and cellphones, because they stored more energy and lasted longer than nickel-cadmium rechargeable batteries.

Factors that shape batteries

Batteries are made in certain sizes and shapes for reasons of cost and manufacturability, but in other cases because of legacy manufacturing processes. Market demand also plays a role.

For example, electric vehicles didn’t take off until Tesla started making cars using cylindrical lithium-ion battery cells rather than the rectangular pouch or prismatic cells other EV makers have used. Pouch and prismatic cells can be packed closely together, but because cylindrical cells were already being mass-produced for portable electronics, Tesla was able to make lower-cost EVs in the 2010s.

Read More: Lithium-ion batteries don’t work well in the cold − a battery researcher explains the chemistry at low temperatures

What shapes and sizes batteries will take in the future depends not only on how much energy they store, but also on market economics – how easy it is to make each type of cell, how much it costs to make them and what they’re used for. And those factors are a mix of innovation and history.

• Wesley Chang is an Assistant Professor of Mechanical Engineering and Mechanics, Drexel University
• This article first appeared in The Conversation

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This is because it’s a heavily narrative-driven game, and one that comes with more twists and turns than you’d get in a typical M. Night Shyamalan film. Each twist brings about new emotional responses, and how you handle those emotional responses will ultimately mould your opinion about the game. In my case, the whole playthrough was one of highs and lows. There were points where I just wanted to put the controller down and mute the keywords on Twitter, but there were times that I couldn’t take my eyes off the screen, tears welling up. 

The brilliant methodology in this game makes it challenging enough to enjoy as a videogame, while pulling you in with a heart-wrenching story of revenge. The first installment of this game managed that quite well, so naturally, the developers wanted to build on that reputation. 

It’s not ALL story

This time around, the zombies in The Last of Us Part II are utterly terrifying. Developer Naughty Dog obviously went out of their way to make the zombie noises as creepy and human as possible, while it’s still clear these are really monsters. Zombies that have recently turned have eerie human-like sounds, of people who are clearly in pain, while clickers are just the scariest things we’ve ever encountered… and that’s including the bloaters and newly introduced Shamblers.

The sneak element is also just done so well. Playing as Ellie, you still have the ability to sense nearby zombies or humans — which gives the player a few different strategies to consider. Unlike Joel, Ellie is very nimble and equipped with her trusty knife, can take on enemies double her size. Easy. 

Combat is done exceptionally well, with intense encounters bringing the need for more strategic gameplay. This is no Souls-level combat system, but considering the little resources you have in a post-apocalyptic zombie-infested world, they’re doing just fine. 

Always switch your melee weapon

Now, because the first game had the duo-dynamic with Joel and Ellie, this time the developers brought that dynamic back. This is a spoiler-free review (because everyone should experience this game for themselves), so we won’t go into too much detail here. But throughout the game you’ll play with a ‘sidekick’ about 70% of the time. This includes about four different people on different occasions, each with their own pros and cons. 

Initially, this is the one mechanic I was not convinced worked well. It felt like the person was more of a liability than of much use, and it gave the game a weighed-down feeling. Like, if you’re going to add a gaming element, that’s supposed to aid in gameplay, make it aid in gameplay. 

But after teaming up with the other characters later in the game, it became clear that this is, in fact, a useful mechanism. So the mechanic redeemed itself, luckily.

Which difficulty you play the game on will influence your need to manage resources sparingly. I played it through on ‘Normal’, which actually gave me ample resources and ammunition to comfortably finish the game. There were quite a few times I ran out of ammo and just bonked everything over the head with a melee (always switch your melee to a stronger one, trust). Great strategising right there, buds. 

But it is all about story

If we say this game is narrative-drive, it’s no over-promise. The thing that kept pulling me in day-after-day was that story. If you’re at all a person who’s after a good story, and loves reading a good book which you keep going back to, The Last of Us Part II is 100% your type of shit. 

On a personal level, Ellie is the first videogame character I could relate to on a personal level. This isn’t about political views or sexual orientation, but in how people around her view her as a person. She starts out as a misunderstood teen in a settlement of survivors. Some of those survivors have particularly conservative views, while Ellie just does her part to keep the community safe, focusing on her role as a cog in this broken, but functioning system. 

As you progress through the game, Ellie grows as an individual, making decisions no young adult should make. But this is a different world where literally everything wants to kill you and the people you love. 

I’m not crying, you’re crying

The writers have gone out of their way to put you in the shoes of a survivor, making you feel like one of the last people on earth, and the decisions that go with that. In this world, there are many remaining, most of whom have established and joined a variety of factions. Most of whom are also trying to kill each other. Following some more traumatic events (oh, you’ll encounter so many of those), character growth is inevitable. 

When issuing the review code, developers accompanied it with a short message, including a few things to keep in mind when playing through this title. It reads “The themes of The Last of Us Part II are based on a universal truth – that just as we are capable of unconditional love, we are also capable of unforgivable hate. And those two feelings are inextricably linked.” Oh, how true that is. 

Even though on its surface, The Last of Us Part II is a story of revenge on its surface there are so many more underlying themes of redemption, finding love in a loveless world and making decisions that could impact a lot of human lives. All while killing a buttload of zombies on a daily basis. Because this is a zombie game, remember? 

The Last of Us Part II Verdict

I consider myself a relatively objective person, never really getting sucked too deeply into narratives of games. This, however, is the first game that made me feel emotions I’ve never thought I could feel. At one point, at about the 75% mark into the game an intense sequence of events that the game forces you to play through brought me to tears. I felt a feeling of utter relief, sadness and happiness all at once. Without me noticing it at first, tears started rolling down my face. At this point it was clear that the writers had done their job — this is a good game. 

If you’re not one to listen to online squabble and feed into the conspiracies making the rounds in gaming groups, The Last of Us Part II is a brilliant playthrough for anyone who enjoys a challenging game filled with heart-wrenching storylines, well-built characters (pun intended) and beautiful graphics.

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