Download Characteristics of CHAdeMO Quick Charging System

Characteristics of CHAdeMO Quick Charging System
1. Necessary driving ranges for EV users
How long driving ranges do EVs need to be accepted by users?
We took a questionnaire to general automobile drivers about driving ranges EVs should have.
The answers were from 200 to 300km, which means drivers think they need EVs have the same
driving ranges as gas vehicles. About 40 to 50kWh are needed for battery to achieve this, and it
costs four to five millions yen with calculating by current average lithium battery unit price. In fact,
we can only draw a conclusion that it is difficult that markets accept EVs.
It is well known but the vehicle’s daily driving distance is as shown in Figure 1. The average daily
driving distance varies from regions to regions. Even in the United States in which long distance
driving is common, it is said that the average daily driving distance is below 50km. If EVs have more
than 100km driving distance, most needs of drivers can be filled theoretically. But, when we drive
EV and find that remaining driving range is less than 50km, we become upset.
Tokyo Electric Power Company introduced an EV with a driving range of 80km on a trial basis in
2007. Showed the driving record of the EV in Yokohama in October 2007, the area is compact and
the gas vehicle’s daily driving distance is around 40km. Then, the EV should be able to cover the
area easily. However, the driving distance of the EV in October was 200km and it was not used
often. The driver had been informed of its performance, but he hesitated to use it, since he was
worried in case the battery would be out.
Driving record in October 2007
Driving record in July 2008
To relieve this anxiety, we installed a quick charger. And the car usage was drastically increased
and the monthly driving distance became more than 1400km, which was longer than the one by gas
vehicle in the same area.
As an interesting fact, we’d like to emphasize that the driver hardly used the quick charger. Did
why the driving distance become longer without using the charger then? It was because the driver
wasn’t worried about the battery and could use the EV until its battery level got low.
Here is a summary we discovered. Quick chargers were expected to be used to charge and to
improve their driving ranges. However, even if they are actually not used, installation of the quick
chargers makes drivers feel safe and it can elicit the performance that EVs originally have enough.
As negative opinions toward quick chargers, it is often pointed out that the cost is high. However,
if quick chargers are not used often, there is no need for installing many chargers. Moreover, by
eliciting high performance of batteries, cost of the entire society using EVs becomes economical.
2. Optimal output for public charging infrastructure
In the past EV introduction trials, there were few cases that many quick chargers which powers
are more than 40kW were installed.
In California, the largest number of EVs and charging infrastructures were developed in the world
in order to meet ZEV mandate. Even now, hundreds of public charger stations have been installed
mainly in San Francisco and Los Angels, and most of their output is around 5kW.
In Tokyo area, it was a problem that there were only 6 charger stations and the output was only
2.5kW around 2000.
A problem of several kW chargers is that it takes more than an hour to charge for 50km driving
distance. Drivers stop by a station for charging while driving, but they can’t endure waiting for more
than an hour. It is not convenient. If EVs are charged at night, they have six to eight hours to get
charged. So it is not a problem about a low-power charger, but quick chargers are needed as
auxiliary charges while driving.
The cost of high-output charger is high, so desired output as public charging infrastructure is
determined by a balance between its charging rate and cost showing in Figure 4. To accomplish
charging to drive 50 to 60km for ten minutes, output of 40 to 50kW is needed assuming average
power consumption of EVs is around 7km/kWm.
On the other hand, basically, the cost of a charger increases in proportion to output. However, if
output of a charger increases, upgrade of electric power supply system may be necessary. A
charger has to be designed safer to meet higher power. Therefore, in fact, the cost of a charger will
increase more than output proportion. The effect of charging time reduction accordance with power
increase more than 50kW becomes saturate, then it isn’t economical considering cost benefit.
As stated above, 40 to 50kW is considered appropriate for the output needed for public charging
infrastructure to keep its cost rational.
3. Charging method to prevent battery degradation
It is concerned that batteries may be deteriorated because of quick chargers. Dominant factors of
deterioration of batteries, or capacity decrease are over voltage and high temperature. Capacity
decrease is caused by that the battery voltage or temperature increases to the level to accelerate
decay of electrode.
In recent EV, a battery management unit (BMU) always monitors each battery cells’ voltage in
addition to the entire system of lithium ion battery, and it prevents from exceeding limiting voltage.
Battery temperature is important regarding quick charging. If a large current is flowed, battery
temperature becomes high because of internal resistance. However, since BMU also monitors
battery temperature, if a charger current is controlled depending on battery temperatures,
deterioration of battery can be avoided.
Since charging speed can be varied depend on battery characteristics and its circumstance, if
charging procedure is standardized, charging current is determined based on the lowestperformance battery and the worst usage environment. Following such standard, public quick
chargers’ original purpose can’t be achieved and charging time can’t be shorten even if battery
performance is improved in the future.
In CHAdeMO standard quick charger, as described in Figure 5, a control of charging current
receives instruction from an EV via CAN bus, and the quick charger sets current to meet the
command value from an EV. By this mechanism, optimal and the fastest charging become possible
in response to battery performance and usage environment. The thing we are proposing as
CHAdeMO standard charger is the mechanism of signal transaction between EV and charger for
safety. Interface shapes of connecters are needed to be standardized to charge any kinds of EV as
public chargers too.
Other parts are not included in the standardization for creativeness of charger designers and
manufactures. In other words, it is all left to charger makers’ preferences to design AC/DC
converters circuit and connectors hardware except interface geometry.
In business development using quick chargers, authentification of EVs and billing method will be
taken into consider. However, since these services’ reliability is different from CHAdeMO
standardization to control charging process safe enough, it is appropriate to make these systems
independent of the charge control.
4. Impact on electric power grid
Many people have concern about the impact on electric power grid since output of a quick charger is high
such as 50kW.
They misunderstand that EV would become widely used in the future, and peak load would increase
because everyone would use quick chargers at once during daytime. Also they are afraid that if batteries are
developed and the amount of on-board battery increases, usage frequency of high-output chargers will
increase too, and as the result, huge investments is necessary to enhance the distribution grid. This concern
is caused by the idea that EV would be used like gas vehicles.
Charging, fuel servicing for EVs, is taken mainly at night by using low-output charger. Therefore, as stated
before, quick chargers are not used very often. So it is unlikely to happen that electric power grid can’t stand
the burden because quick chargers are used at once. Furthermore, when quick chargers are installed, a
capacity of distribution grid is taken into consider. As shown in Figure 6, in the case of installing quick
chargers, facilities being supplied with electricity by more than tens of thousands of ultra high voltage should
be chosen. In such facilities , 50kw is the small percentage of the entire receiving capacity and it doesn’t give
any effect. Even in facilities being supplied with electricity by a few thousand of voltage, installation of quick
chargers is possible. A problem is the case when quick chargers are installed under low-voltage distribution
grid like residential areas. But there is no occasion that quick charging is needed at home charger use.
Some people worry that low-output chargers become inconvenient because they require more than 10
hours charging time for nighttime charging in a garage if battery performance are developed in the future and
the amount of batteries increases. They are not sure if enhancement of distribution grid would be possible to
solve this problem. This is also a misunderstanding of the way of EV usage. Even a gas vehicle that can run
more than 300km on one fuel charging, the average daily driving distance is below 50km. Even if EV’s driving
range were increased to 300km in the future, daily driving distance wouldn’t be changed because it is settled
by people’s life style of driving and not by size of fuel tank or battery.
The amount of electricity to drive 50km is around 7kWh. Assuming that EV is charged every night, they can
be charged less than four hours even if low-capacity, 2kW, on-board charger is used. There might be some
drivers who worry that battery gets completely empty, but there is no problem if a sufficient quantity of
electricity is charged in one night for next day’s driving. If drivers need to fill up by any means, they can only
use the nearest quick charger.
Like this, deployment of quick charging infrastructures can help to limit needs for increasing output of
average chargers at home. In Figure 7 and Figure 8, we estimated the effect to low-voltage distribution system
when the percentage of EV users is 5% and 20%. If output of on-board charger can be kept around 2kW,
there is no need to enhance distribution grid even when EV’s share becomes 20%.
On the other hand, output of EV’s on-board charger increases, enhancement of distribution grid becomes
necessity and this becomes a factor that increases the cost when people use a new EV.
Installation of appropriate amount of public quick charging infrastructures can bring remarkable effects to
moderate needs for increasing capacity of facilities for charging EV at night and impact to low-voltage grid at
the time EVs widely spread.