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.
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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.
An 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 the
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 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 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, or 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 a roughly 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, and 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 in water as the electrolyte.
That combination is what we call an alkaline
battery.
It’s what’s in those double AAs you probably
have sitting in a drawer somewhere.
Problem is, you can’t recharge primary batteries
– once they’re dead, they’re dead.
And that’s a big tradeoff.
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 the 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’ve designed the rest
of the hardware to draw less energy, and 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.
Check out our Augmented Reality Poster at DFTBA.com!
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 Doctor Cheryl C. Kinney Studio with the help of these wonderful people.
And our amazing graphics team is Thought Cafe.
