DR. PAVOL BAUER: Ladies and gentlemen, one
of the main concerns about electric vehicles is about charging time.
Via fueling, a combustion engine car takes around two minutes.
Charging an electric vehicle battery takes much longer.
Why is it so?
In this lecture, we will talk about basics of electric vehicle
charging process, look at the basic equations,
and get an introduction to smart charging.
Let us first start with the charging process.
In this image, we can see the battery charging current, battery voltage,
and energy stored in the battery is a function of time
when the battery is charged.
As seen in the image, when the charging of a battery starts,
it typically charges at a constant current equal to or less
than a nominal current of the battery.
During this period, the voltage of the battery
increases as the battery gets charged.
This is referred to a constant current or CC charging region.
Fast charging is done in this CC region.
When charging a battery, there is a maximum
charging current and charging voltage for safe operation of the battery
and to ensure a long lifetime.
When the voltage reaches the maximum set point, usually
at the state of charge of roughly 80%, the electric voltage charger
changes to the constant voltage region or CV region,
but the voltage is maintained while the current is gradually reduced to zero.
Charging in this region typically takes a long time
as the charging current reduces.
Charging is stopped once the current is close to zero.
Let us now look at the fuel general equations for charging.
First, the charging power, P charge, is the product of the charging current
and the battery voltage, V batt.
A battery is, hence, charged by controlling the charging current.
Therefore, faster charging can be achieved
by increasing the charging current.
Typical units used for electric vehicle charging power
are kilowatts, volts for voltage, and amperes for current.
Next, the energy delivered to the battery during the charging, E charge,
can be estimated as the time integral of the charging power, P charge.
The typical units use for charging time, charging power,
and energies are hours, kilowatts, and kilowatt hours, respectively.
If the charging power is constant, this simply [INAUDIBLE]
to the product of the charging power, P charge, and the charging time, T
charge.
For example, a 10 kilowatt charger used for two hours
delivers 20 kilowatt hours.
The next important parameter is the charge rate,
often referred to as a C rate.
It is the ratio of the charging power to the nominal energy
capacity of the battery.
As the charging current increases, so does the C rate.
The importance of C rate is that as the C rate increases,
battery losses and temperature of the battery increases as well.
This eventually reduces the lifetime of the battery.
Hence, to ensure the long lifetime of the battery,
lower C rates are preferred.
Now that we have looked at the charging process,
let us now see what is smart charging and how
it is different from the conventional charging.
When we connect an electric vehicle to an EV charger,
charging starts as soon as the electric vehicle is plugged in.
The charging occurs at a fixed charging power.
And the charging occurs till the electric vehicle charging battery
is full.
This is often referred as an uncontrolled charging
or conventional charging.
Now, what is the advantage of this?
In the case of multiple cars connected at the same time,
then all the cars will charge at the same time when they are plugged in.
This may lead to a peak leading on the electricity grid.
If the peak is too high, then power lines and transformers
in the distribution network will have to be upgraded,
so more cables more transformers.
Further, this kind of charging has no correlation
to renewable energy generation or energy prices,
as the charging is dependent on the time of connection
and always happens at a fixed power.
Smart charging can help overcome these disadvantages.
For example, if many cars are connected at the same time at a workplace,
smart charging can help to plan and spread the charging power over the day.
Here, in this figure, one car is charged at a time and the others sequentially.
Alternatively, the cars can be charged at the same time,
but with a lower power for each car, such
that the charging is spread over the eight hours of work time.
Or the smart charging system can monitor the electricity network
and charge more cars when there is less demand on the electricity network.
The most important use of smart charging is the ability
to allow the charging of electric vehicles
from renewable energy sources and the possibility
to reduce the cost of charging this way.
In this graph, wind generation, solar generation, energy prices,
the load on the grid, and the electric charging profile
are shown as a function of time.
The wind and solar power generation have their maximum
at the start and middle of the day, respectively.
While the energy prices and load on the grid
are reaching its peak at the end of the day.
With smart charging, electric vehicles can
be controlled to charge from the wind generation in the morning.
Then the charging can be increased to charge from both the solar and wind
generation in the afternoon.
Finally, the charging power can be reduced
in the evening when the loading on the grid and the energy prices are high.
It means smart charging can allow the sustainable
charging of electric vehicles from renewable energy sources
and help to reduce the cost of charging.
Finally, let us look at how smart charging is implemented.
In case of both, AC and DC charging, control and communications
are established between the electric vehicle and the charging station
using protocols like IEC 61851 and ICO 15118.
Through this, the charging current can be continuously controlled
in time and in magnitude to cater the different smart charging applications.
To summarize, the charging process of an electric vehicle battery
has two distinct regions--
the constant current and constant voltage charging region.
Fast charging is typically done in the constant current region.
The C rate is a vital charging parameter as high C
rates lead to increase in battery losses and lowering of the battery lifetime.
Smart charging facilitates the control of charging
power of the electric vehicle.
Through this, electric vehicles can be controlled to charge from the sources,
like wind or solar, when energy prices are lower and charge at a time
when the loading on the electricity network is low,
this way easing the load on the electric network.