This episode of Real Engineering is brought
to you by Brilliant. A problem solving website that teaches you
to think like an engineer. Tesla has grown rapidly over the past decade,
when it became the first American automotive company to go public since Ford in 1956. The attraction towards Tesla is undeniable. Their cars are slick, their acceleration is
insane and perhaps most importantly, their brand represents a movement towards renewable
energy. Tesla has attracted thousands of well intentioned
people who want to play their part in saving the world, but there have been a niggling
questions on the minds of many EV owners and EV naysayers. When is that expensive battery going to need
to be replaced, and at what cost. As existing Teslas begin to age, and more
exotic and demanding models of Teslas come to the fore, like the Tesla Truck and the
Roadster 2. These issues are going to become more prominent, These batteries do NOT come cheap, but they
are getting cheaper. This chart shows the cost per kilowatt hour
for Tesla powerpacks, and the market average. Both dropping dramatically as technology advanced,
and manufacturing volumes increased. But that storage capacity slowly creeps away
as the battery is used, slowly degrading the range of your electric vehicle. Tesla currently offers a warranty to all Model
3 owners that cover it below 8 years or 160,000 kilometres, whichever comes first. Guaranteeing a retention of capacity of at
least 70% when used under normal use. If it falls below that, they will replace
your battery for free. Finding out what is considered normal use
is pretty difficult, but they seem to be reasonable with it going by customer satisfaction reports. From our graph earlier, it’s estimated that
Tesla is achieving a cost of 150$ per kwH of battery packs, so the 50 kWh battery pack
of the base model would cost around 7,500 dollars to replace, so they must be pretty
confident on those numbers. As a massive recall of the approximately 193
thousand Model 3s currently shipped would ruin Tesla. [3] Ultimately these batteries are unlikely
to drop below the warranties guarantee in those 160,000 kilometres, but even so improving
batteries is obviously just a wise business decision to retain those customers in future. This is just one of a myriad of factors that
influenced Tesla’s recent landmark acquisition of Maxwell Technologies for $218 million dollars. A rare Tesla acquisition that sets Tesla up
for not just cheaper batteries, but better batteries. That will be lighter, have greater range,
and live a longer life. It wouldn’t be the first time an automotive
company underestimated their battery degradation. When the Nissan Leaf debuted in 2010, the
battery production they needed simply did not exist, and neither did the technical expertise
required to design battery packs. In those days lithium ion batteries cost about
400 dollars per kWh for laptop grade batteries, and up to 1000 dollars per kWh for ones with
the longevity needed for an electric vehicle. To minimise costs Nissan decided to start
production of their own batteries, and opted for a small 24 kWh battery, giving it a range
of just over 100 kilometres. Suitable for city driving, and that’s about
it. But customers soon realised that this paltry
range was dwindling quickly. Within just 1-2 years of driving, the Leafs
battery capacity was dropping up to 27.5 percent under normal use. [4] Despite careful in-house testing Nissan
overlooked some crucial test conditions when developing their battery, and because of this
they made some crucial design errors. To learn why this degradation happens, we
first need to understand how lithium ion batteries work. A lithium ion battery, like all batteries,
contains a positive electrode, the anode, and a negative electrode, the cathode, separated
by an electrolyte. Batteries power devices by transporting positively
charged ions between the anode and cathode, creating an electric potential between the
two sides of the battery and forcing electrons to travel through the device it is powering
to equalise the electric potential. Critically, this process is reversible for
lithium ion batteries, as the lithium ions are held loosely, sitting into spaces in the
anode and cathodes crystal structure. This is called intercalation. So, when the opposite electric potential is
applied to the battery it will force the lithium ions to transport back across the electrolyte
bridge and lodge themselves in the anode once again. This process determines a huge amount of the
energy storage capabilities of the battery. Lithium is a fantastic material for batteries,
with an atomic number of 3, it is the 3rd lightest element and the lightest of the metals. Allowing it’s ions to provide fantastic
energy to weight characteristics for any battery. But, the energy capacity of the battery is
not determined by this, it is determined by how many lithium ions can fit into these spaces
in the anode and cathode. For example, the graphite anode requires 6
carbon atoms to store a single lithium ion, to form this molecule (LiC6). This gives a theoretical maximum battery capacity
of 372 mAh per gram. Silicon however can do better. A single silicon atom can bind 4.4 lithium
ions, giving it a theoretical maximum battery capacity 4200mAh per gram. This seems great, and can provide increases
in battery capacity, but it also comes with drawbacks. As those 4.4 lithium ions lodging themselves
into the silicon crystal lattice causes a volume expansion of 400% when charging from
empty to full. This expansion creates stress within the battery
that damages the anode material, that will eventually destroy it’s battery capacity
over repeated cycles. Battery designers are constantly looking for
ways to maximise this energy density of their batteries while not sacrificing longevity
of the battery. So what exactly is being damaged in the batteries
that causes them to slowly wither away? When researchers began investigating what
caused the Nissan Leaf’s rapid battery degradation, they began by opening the battery and unrolling
the batteries contents. They found that the electrode coatings had
become coarse over their life, clearly a non-reversible reaction was occurring within the cell, the
change was expected. In fact the chemical process that caused it
is vital to the operation of the battery. When a battery is charged for the very first
time a chemical reaction occurs at the electrolyte electrode interface, where electrons and ions
combine. This causes the formation of a new layer between
the electrode and electrolyte called the solid electrolyte interphase. The name is exactly what it suggests, it’s
a layer formed by the liquid electrolyte reacting with electrons to form a solid layer. Thankfully, this layer is permeable to ions,
but not electrons. So it initially forms a protective layer over
the electrode that allows ions to enter and insert themselves via intercalation, but it
is impermeable to electrons. [10] Preventing further reaction with the
electrolyte. At least that’s the idea under normal conditions. [11] The problem is, under certain conditions this
layer can grow beyond just a thin layer of protective coating, and result in the permanent
lodgement of the lithium that provides the battery with its energy storage. This process is not entirely well understood
and is outside the scope of this video, but we can identify some factors that increase
the rate of this formation. The expansion of the silicon electrode battery
we mentioned earlier causes the fracture of the SEI layer, exposing fresh layers of electrode
to react with the electrolyte. Charging rate and temperature can also accelerate
the thickening of this layer. NASA performed their own in depth study of
this effect, and released a report in 2008 titled “Guidelines on Lithium-ion Battery
Use in Space Applications” sharing their findings. [12] The temperature that the battery is charged
and discharged at plays a massive role in the batteries performance. Lowering the temperature lowers chemical activity,
but this is a double edged sword. Lowering the chemical activity negatively
affects the batteries ability to store energy. Which is why batteries have lower ranges in
cold countries, but lowering the chemical activity also decreases the formation rate
of that SEI layer. This is on of reason the Nissan Leaf’s battery
lost a huge amount of capacity over just 2 years in many countries. Nissan performed most of its testing in stable
laboratory conditions, not over a range of possible temperatures. Because of this they failed to realise the
disastrous effect temperature would have on the life of the battery, and failed to include
a thermal management system, which is common place in any Tesla. This of course reduces the energy density
of the battery. Adding tubing, the glycol needed to exchange
heat, along with the heat pumps and valves needed to make a thermal management system,
not only adds weight, but it draws energy away from the battery to operate. But it plays a vital part in maintaining the
performance of the battery. Nissan’s choice to not include a thermal
management system, even in the 2019 version, makes it a poor choice for anyone living in
anything but a temperate climate. Ofcourse, just cycling the battery though
it’s charged and discharged states is one of the biggest factor in degrading the battery. Every time you cycle the battery you are giving
the SEI layer opportunities to grow. Minimising the number of times a cell is cycled
will increase it’s life, and maintaining an ideal charge and discharge voltage of about
4 volts minimises any resistive heating that may cause an increase in chemical activity. This is where Maxwell technologies comes into
play. Maxwell has two primary technologies that
Tesla will be taking advantage of. The first is what Maxwell are known for, their
ultracapacitors. Ultracapacitors serve the save fundamental
job as batteries, to store energy, but they function in an entirely different way and
are used for entirely different purposes. The fundamental difference between a capacitor
and a battery is that a battery stores energy through chemical reactions, as we saw for
lithium ion batteries earlier this is done through insertion into the crystal lattice. Capacitors instead store their energy by ions
clinging onto the surface of the electrode. This is a standard ultracapacitor schematic. On each side we have an aluminium current
collector with thin graphite electrodes on each, separated by an electrolyte and an insulating
separator to prevent the passage of electrons. In an uncharged state ions float in the electrolyte. When a voltage is applied during charging,
ions drift towards their opposite charge and cling to the surface, holding the charge in
place. When a device is then connected to the capacitor
this charge can quickly leave while the ions drift back into the electrolyte. The key limiting factor for ultracapacitors
is the surface area available for this to happen, and nanotechnology has allowed for
amazing advances in the field. This is what the inside of a ultracapacitor
looks like, it contains hundreds of layers of these electrode pairs. But even with this enormous surface area,
ultracapacitors simply cannot compete with batteries when it comes to energy density. Even Maxwell’s best ultracapacitors have
an energy density of just 7.4 Wh/kg [13] while the best guess for Tesla’s current energy
density is about 250 Wh/kg. Counter to what corporate owned tech channels
may tell you, ultracapacitors are not intended to be a replacement for batteries. They are intended to work in conjunction with
batteries. Ultracapacitors strength is their ability
to quickly charge and discharge without being worn down. This makes them a great buffer to place between
the motors and the battery. Their high discharge rate will allow them
to give surges of electricity to the motors when rapid acceleration is needed, and allow
them to charge quickly when breaking. Saving the battery from unnecessary cycles
and boosting its ability to quickly provide current when needed for acceleration. This is going to be a massively important
technology for two upcoming Tesla vehicles. The Tesla Roadster, which will boast an acceleration
of 0-60 in just 1.9 seconds, which a normal battery would struggle to achieve the discharge
rate needed without damaging itself. The second vehicle is the Tesla Truck. I have made a video in the past noting that
the Tesla Truck is going to be limited in its range and cargo hauling ability as a result
of the heavy batteries it will need, as trucks are limited in weight to about 40 metric tonnes
in most countries. This ultracapacitor technology will boost
its ability to regain energy from breaking significantly, and thus allow its battery
capacity to decrease, in turn allowing the truck to swap batteries for cargo. The second technology Maxwell has been toting
as their next big breakthrough is dry coated batteries. [9] This is a manufacturing advancement that
Maxwell claims will reduce the cost of manufacturing. A factor Tesla has been working fervently
to minimize with the growth of the gigafactory. So what are dry coated batteries. Currently in order to coat their current collectors
with the electrode material Tesla, in partnership with Panasonic’s patented technology, must
use first dissolve the electrode material in a solvent which is then spread over current
collector, both are then passed through an oven for drying, where the solvent evaporates
leaving just the electrode material behind. This adds cost of the manufacturing procedure
as the solvent is lost in the process, and the baking process takes energy. On top of this the solvent is toxic, so removing
it from the process would benefit the environment. Maxwell instead uses a binding agent and conductive
agent, which I assume will work similarly to electrostatic painting. Where a metal being painted will be given
a negative charge, while the paint will be given a positive charge as it is sprayed attracting
it to the metal where it will cling to it. This painting process also eliminates the
solvents needed in paint. In this paper, published by Maxwell technologies,
they detail how their dry coating manufacturing techniques could result in a high energy storage
capacity of the electrodes, due to a denser and thicker coating. Resulting a potential increase in battery
capacity to 300 Watt hours per kilogram, 20% up from our best estimates of Tesla’s current
specs. Only time will tell if this claim can be realised
at an industrial scale. Perhaps, more importantly to Tesla, they now
own this manufacturing technique. Currently Panasonic owns the manufacturing
process for Tesla, there is a literally a line of demarcation in the gigafactory separating
Panasonic and Tesla, denoting the point at which the ownership of batteries transfers
hands. Having to buy their batteries from Panasonic
adds cost, that Tesla will want to avoid in future and this step could allow for full
vertical integration of their battery manufacturing. Thereby making electronic vehicles more affordable
to the everyday consumer. All of this technology is powered by incredibly
smart engineers working to solve really interesting problems, and with so much focus on battery
technology across the entire tech industry there’s a high demand for qualified engineers. For anyone looking to build or advance their
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