13-12-2012, 04:15 PM
Effective Charging Method for Ultracapacitors
[attachment=43677]
Abstract
One of the advantages of ultracapacitors is its high power capability, which is applicable for high rate of charging
and discharging operation like motor starting and regenerative braking of an electric vehicle. This paper presents
a new charging method for ultracapacitors. Comparing with batteries, ultracapacitor can accept a wide range of
charging current and can be fully charged within a few minutes. Common chargers for ultracapacitors are usually
equipped with current transducers and closed loop circuitry for current control, which are expensive and complicated.
The proposed circuit consists of a minimum number of components. It does not require any current transducer
or dedicated voltage/current control circuitry. A simple open-loop control system is applicable for the whole
charging stage. It is free of stability problem and protects itself from being overloaded by ultracapacitor with zero
initial charge. This paper presents the design and operation of the hardware circuit. Both simulation and experimental
results are included.
INTRODUCTION
High energy density batteries, like lithium-ion (Li-ion)
and nickel metal hydride (NiMH) battery, and fuel cells
have been developed for many years. They are successfully
utilized as energy sources for electric vehicles
(EVs). However, their power densities are inadequate
under certain operation conditions, namely quick acceleration,
hill-climbing and regenerative braking. The
ultracapacitor is a high power density energy storage
device that can deliver high short-term discharging current
and acquire burst of charging current. It does not
have the drawbacks of batteries like poor temperature
coefficient, limited charging and discharging cycle, and
critical charging current. It can be used in tandem with
batteries for performance improvement of EVs.
ULTRACAPACITOR
Ultracapacitors are also known as supercapacitors. Two
main types of ultracapacitors are pseudocapacitor and
double layer capacitor. Their structures are somewhere
like a battery, which contains electrolyte with electrodes
immersed. The positive and negative electrodes are separated
by a separator. The electrodes are made with porous
material. There are pores with size in terms of nanometer
where ions can travel freely. For the doublelayer
ultracapacitor, there is no Faradic reaction between
the material and the electrolyte. For the pseudocapacitor,
Faradic reaction does occur. If an ultracapacitor is
charged with constant current, the voltage across the electrodes
will rise linearly with time as an idea capacitor.
CHARGER FOR ULTRACAPACITORS
Basically, common charging circuits for batteries are applicable
for ultracapacitors, but several issues should be
taken into consideration. A linear mode power supply
is inappropriate since its efficiency is low. Energy loss
is very large since the voltage difference between the
supply source and the ultracapacitor is great, especially
under zero initial charge condition. It reduces the mileage
of an EV, and upsets the advantages of an
ultracapcitor-battery combination system. A switching
mode dc-dc converter with pulse width modulation
(PWM) is a good candidate. A battery is usually charged
for hours and then the charger can be shut down or even
be disconnected from the power source. An
ultracapacitor is under repetitive charging with a much
shorter time constant. It can be completely discharged
within a few minutes or even just a couple of seconds,
and then fully charged again within a short period. The
charger needs to operate in such a way that it provides a
maximum charging current for a short period and then
reduces to zero until next charging cycle. This pulsating
operation imposes stresses on a PWM charger.
[attachment=43677]
Abstract
One of the advantages of ultracapacitors is its high power capability, which is applicable for high rate of charging
and discharging operation like motor starting and regenerative braking of an electric vehicle. This paper presents
a new charging method for ultracapacitors. Comparing with batteries, ultracapacitor can accept a wide range of
charging current and can be fully charged within a few minutes. Common chargers for ultracapacitors are usually
equipped with current transducers and closed loop circuitry for current control, which are expensive and complicated.
The proposed circuit consists of a minimum number of components. It does not require any current transducer
or dedicated voltage/current control circuitry. A simple open-loop control system is applicable for the whole
charging stage. It is free of stability problem and protects itself from being overloaded by ultracapacitor with zero
initial charge. This paper presents the design and operation of the hardware circuit. Both simulation and experimental
results are included.
INTRODUCTION
High energy density batteries, like lithium-ion (Li-ion)
and nickel metal hydride (NiMH) battery, and fuel cells
have been developed for many years. They are successfully
utilized as energy sources for electric vehicles
(EVs). However, their power densities are inadequate
under certain operation conditions, namely quick acceleration,
hill-climbing and regenerative braking. The
ultracapacitor is a high power density energy storage
device that can deliver high short-term discharging current
and acquire burst of charging current. It does not
have the drawbacks of batteries like poor temperature
coefficient, limited charging and discharging cycle, and
critical charging current. It can be used in tandem with
batteries for performance improvement of EVs.
ULTRACAPACITOR
Ultracapacitors are also known as supercapacitors. Two
main types of ultracapacitors are pseudocapacitor and
double layer capacitor. Their structures are somewhere
like a battery, which contains electrolyte with electrodes
immersed. The positive and negative electrodes are separated
by a separator. The electrodes are made with porous
material. There are pores with size in terms of nanometer
where ions can travel freely. For the doublelayer
ultracapacitor, there is no Faradic reaction between
the material and the electrolyte. For the pseudocapacitor,
Faradic reaction does occur. If an ultracapacitor is
charged with constant current, the voltage across the electrodes
will rise linearly with time as an idea capacitor.
CHARGER FOR ULTRACAPACITORS
Basically, common charging circuits for batteries are applicable
for ultracapacitors, but several issues should be
taken into consideration. A linear mode power supply
is inappropriate since its efficiency is low. Energy loss
is very large since the voltage difference between the
supply source and the ultracapacitor is great, especially
under zero initial charge condition. It reduces the mileage
of an EV, and upsets the advantages of an
ultracapcitor-battery combination system. A switching
mode dc-dc converter with pulse width modulation
(PWM) is a good candidate. A battery is usually charged
for hours and then the charger can be shut down or even
be disconnected from the power source. An
ultracapacitor is under repetitive charging with a much
shorter time constant. It can be completely discharged
within a few minutes or even just a couple of seconds,
and then fully charged again within a short period. The
charger needs to operate in such a way that it provides a
maximum charging current for a short period and then
reduces to zero until next charging cycle. This pulsating
operation imposes stresses on a PWM charger.