ULTRA-FAST ULTRACAPACITOR PACK/DEVICE CHARGER

FIELD OF THE INVENTION [0001] The present invention generally relates to ultracapacitors, and more

particularly to chargers for ultracapacitors.

BACKGROUND OF THE INVENTION [0002] Ultracapacitors, also known as double layer capacitors, DLCs,

supercapacitors or psuedocapacitors are devices that store electrical energy.

Ultracapacitor are increasingly being used to power consumer products, automotive

energy storage systems, military applications, etc. as the sole energy storage device or

they in combination with batteries.

[0003] After their charge is depleted, ultracapacitors are recharged. Ultracapacitors

have to go through a fairly large voltage swing to be used as an energy storage device

and must be charged carefully to prevent damage. Because ultracapacitors are sensitive

to being charged over their rated voltage, overcharging can result in significantly

reduced life or failure.

[0004] Another issue in recharging ultracapacitors is in capacitance variance. The

capacitance of an ultracapacitor will vary from its rated value, usually by no more than

±20%. Therefore, a series connected string of ultracapacitors will likely have cells with

different capacitances. When a series connected string is charged, the voltages of the

cells will become different from one another because cells with smaller capacitances will

charge more rapidly than cells with larger capacitances. This is apparent from Equation

1 (below) which relates the current, voltage, and capacitance of an ideal capacitor.

^• /- _* \ r> ^dv c

(1)

[0005] Charged ultracapacitors also experience leakage or self-discharge. This is

where energy is internally dissipated thereby reducing the ultracapacitor's voltage. All

ultracapacitors do not self-discharge at the same rate. Due to capacitance tolerances

and varying leakage, series connected ultracapacitors will often have voltages different

from one another.

[0006] Current ultracapacitor charging technology uses balancing circuits to try to

make every cell in an ultracapacitor array have equal voltage while the ultracapacitor

cells are being charged by a power source. In other words, the current method

"balances" the cells. This is done or could be done in five ways.

[0007] First, an active circuit is placed over two series connected cells. The circuit

compares the voltage of the two cells and then dissipates the energy in the cell that has

the highest voltage. Another balancing circuit is then used to balance one of those two

cells with the next cell in the series string. Circuits can be connected in this way to

balance many series connected cells. This type of balancing has the following

limitations: (1) because it takes time to balance all of the cells, the charging cannot be

done rapidly...rapid charging of the cells would not allow enough time to ensure proper

balancing and some cells in a series string could become over charged; and (2)

balancing circuits consume energy and reduce the voltage of every cell to the lowest

voltage cell, which wastes energy.

[0008] Second, an active bypass circuit is placed over each cell that dissipates energy

from the cell through a resistor when the cell gets close to its maximum rating.

However, if the cells are charged at a current rate that is higher than what the circuit

can bypass, the cell could become overcharged.

[0009] Third, a zener diode having a breakdown voltage close to the rated voltage of

the ultracapacitor cell is placed over each cell in a series string. When the cell becomes

close to its rated voltage the zener starts to conduct current. This has the same problem

as the active bypass circuit of the second way (above). It can only protect the cell if it

can bypass as much current as the cell is being charged by. This also wastes energy

close to the cells rated voltage because a zener does not have a distinct breakdown

voltage.

[0010] Fourth, a passive resistor with a small resistive tolerance is placed over each

cell. This causes a current to flow, which is significantly higher than the cells leakage

current. The higher voltage cells dissipate more energy because more current is drawn

through the resistor. Ohm's law, I=V/R, shows why this is true. This method consumes

a significant amount of energy from the ultracapacitors and balances the cells very

slowly. This slow balancing does not allow the cells to be charged from a low voltage to

their maximum voltage rapidly without possibly overcharging one or more of the cells.

[0011] Fifth, combinations of the above methods.

[0012] The prior art is replete with examples employing such methods. For instance,

U.S. Patent No, 6,664,766 shows a supercapacitor balancing system and method

(balancing); U.S. Patent No. 6,265,851 shows an ultracapacitor power supply for an

electric vehicle (bypass); U.S. Patent No. 6,836,098 shows a battery charging method

using supercapacitors at two stages; U.S. Patent No. 7,042,187 shows a control circuit;

and U.S. Patent No. 6,847,192 shows a power supply for an electrical load.

BRIEF DESCRIPTION OF THE DRAWINGS [0013] Fig. IA is a schematic view of a first ultracapacitor configuration that can be

charged by the present invention.

[0014] Fig. IB is a schematic view of a second ultracapacitor configuration that can

be charged by the present invention.

[0015] Fig. 1C is a schematic view of a third ultracapacitor configuration that can be

charged by the present invention.

[0016] Fig. ID is a schematic view of a fourth ultracapacitor configuration that can

be charged by one or more embodiments of the present invention.

[0017] Fig. 2 is system block diagram for one embodiment of the present invention.

[0018] Fig. 3 is a flow chart illustrating the MCU charging program of one

embodiment of the present invention.

[0019] Fig. 4 is a schematic representation of one possible embodiment of the present

invention with parallel resistors.

[0020] Fig. 5 is a schematic representation of one possible embodiment of the present

invention with series resistors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS [0021] While the invention is susceptible of various modifications and alternative

constructions, certain illustrated embodiments thereof have been shown in the

drawings and will be described below in detail. It should be understood, however, that

there is no intention to limit the invention to the specific form disclosed, but, on the

contrary, the invention is to cover all modifications, alternative constructions, and

equivalents falling within the spirit and scope of the invention as defined in the claims.

[0022] In the following description and in the figures, like elements are identified

with like reference numerals. The use of "or" indicates a non-exclusive alternative

without limitation unless otherwise noted. The use of "including" means "including,

but not limited to," unless otherwise noted.

[0023] The present invention is an ultra-fast ultracapacitor pack/device charger

(hereinafter "charger"). The charger allows many ultracapacitors that may be connected

in several different configurations to be charged from a voltage as low as zero volts to

their rated voltage very rapidly without cell damage. Ultracapacitor cells typically fail

by a significant increase in effective series resistance (ESR), this cell damage can occur in

various ways including: (1) when one or more of the cells becomes charged past the

rated voltage of the cell as specified by the manufacturer, as well as (2) through physical

damage (damage due to dropping, impact, etc.) to the cell. Cell damage can be defined

as when the ultracapacitor cells characteristics are outside of what is identified by the

manufacturer. Embodiments of the present invention also allow for the detection of

failed cells. This technology allows ultracapacitors to be used in many applications

such as rechargeable consumer products, automotive energy storage systems, military

applications, etc.

[0024] There are many different embodiments of the present invention. The term

"charger" used herein to represent generally the present invention and as such mention

of one "charger" having a certain element is not intended to be a limitation that all

"charger" embodiments have said element unless indicated otherwise.

[0025] One embodiment of the present invention's charger allows one or more

ultracapacitors to be charged from a totally depleted state to a fully charged state very

rapidly (< 15 seconds / kilojoule) without overcharging any of the cells, and is

particularly useful for charging any type of capacitor with a capacitance greater than 1/2

Wh/1. This charger embodiment can charge ultracapacitors that are in series, in parallel

or combinations of series and parallel. Figs. 1 A-ID show some of the possible

ultracapacitor configurations that can be charged by this embodiment of the charger.

Further, "string of ultracapacitor cells" is intended to mean "an individual

ultracapacitor cell and/or a plurality of interconnected ultracapacitor cells."

[0026] While it is preferred that the charger be physically separate from the

ultracapacitors (except for the necessary electrical connections), the charger may be

integrated directly into the ultracapacitor device if desired.

[0027] In one embodiment, the charger consists of a power electronic DC-DC

converter that converts an automotive 12- volt power source to a lower voltage and

regulates the current to charge the ultracapacitor cells. Other power sources and

voltages can be used to power the charger.

[0028] In the embodiment of Fig. 2, the charger is composed of a

microcontroller/microprocessor (hereinafter "MCU"), a power electronic circuit, analog

closed loop current circuit, and an analog conditioning circuit that allows the MCU to

measure each voltage of one or more ultracapacitor cells connected in series.

[0029] The analog closed loop current circuit regulates the current into the

ultracapacitor cells that was commanded by the MCU. The closed loop control is

accomplished by measuring the feedback current from the DC-DC converter and

comparing it to the commanded current (Iref) and then outputting a pulse-width-

modulated (PWM) signal to the DC-DC converter. Other power electronic converter

topologies could be used.

[0030] The MCU measures the voltage of each ultracapacitor cell in a series string

via the analog conditioning circuit. Based on these voltages, the MCU commands a

current to the DC-DC converter. This current is commanded using a PWM signal that is

filtered with a low-pass filter to a nearly constant voltage signal. This voltage is called

Iref (shown in Fig. 2).

[0031] The MCU calculates total string voltage by summing all of the ultracapacitor

cell voltages. A current (charging) profile is then followed based on the total string

voltage and/or individual voltage. This maintains a safe maximum current for the

power electronic converter while the ultracapacitor cells charge from a voltage as low as

zero to their maximum. The MCU also measures the voltage of the source to ensure it is

within a specified range.

[0032] The MCU measures the voltage of each ultracapacitor cell during charging

and determines the cell with the largest voltage. When the cell with the largest voltage

reaches its maximum value, the MCU commands the DC-DC converter to stop

charging. This prevents any of the cells from becoming charged beyond their

maximum voltage while also allowing them to be charged from a totally depleted state

to a totally charged state quickly.

[0033] The charger uses a hysteresis type scheme to prevent the converter from

oscillating on and off while at the completely charged state (not shown in Figure 3).

Figure 3 is a flow chart illustrating the MCU charging program. Figure 3 excludes, for

simplicity, a number of steps, including but not limited to, the user LED indicator

control, and the process of preventing operation due to the source voltage not being

within a specified range.

[0034] The MCU also performs several other tasks. It measures the supply voltage

to ensure it is within a specified range and prevents operation if it is not. It operates

two light emitting diodes (LED) that indicate if the charger is on, if there is an error, if

ultracapacitors are connected, if it is charging, and if the ultracapacitors are fully

charged. Additionally, the MCU enables or disables the DC-DC converter.

[0035] Ultracapacitor cells typically fail by either a significant increase in effective

series resistance (ESR) or by significant decrease in capacitance. The MCU can

determine either type of cell failure because it closely monitors cell voltage. The MCU

measures the voltage and knows the current for every cell. Therefore, it can determine

when one of the cells has failed because it will have a significantly higher voltage while

it is being charged.

[0036] An analog conditioning circuit is interfaced between the series connected

ultracapacitor cells and the MCU's analog-digital-converter (ADC). The analog

conditioning circuit consists of several op-amp circuits that measure the difference

between the positive terminal and negative terminal of an ultracapacitor cell and

outputs a signal proportional to this voltage. The output is a signal that is relative to

ground. The analog conditioning circuit also employs a low-pass filter to filter out

higher frequencies.

[0037] The analog conditioning circuit is scalable and the output can be multiplexed

to accompany many ultracapacitor cells. The current embodiment (shown in Fig. 2)

measures six series connected cells. The voltage measurements are multiplexed by the

microcontroller. Because the analog conditioning circuit is designed to be scalable the

overall charger design is scalable to accommodate many cells.

[0038] In one embodiment of the present invention, (1) the charger measures the

voltage over every cell; (2) the charger's microcontroller detects problems with

ultracapacitor arrays, calculates maximum cell voltages, determines total pack voltage,

and controls a power electronic converter; (3) the microcontroller allows the current

from the power electronic converter to follow a profile (this minimizes charge time and

operates the converter safely); (4) items 1-3 above enable the charger to safely charge

ultracapacitor arrays very quickly; (5) because the microcontroller monitors the voltage

over each cell and the current going through them, it can determine a cell failure; (6) the

design is scalable to accommodate many ultracapacitor cells and configurations; and (7)

the charger is designed to be as small as possible to allow for integration into products

using ultracapacitors.

[0039] The embodiment of Fig. 2 uses a DC-DC converter that receives a current

command from the MCU to regulate the current into the ultracapacitors. One possible

variation of the design is to use a switched resistive network to control current into the

ultracapacitors from a DC voltage source. The DC voltage source could be one of the

following: (1) a battery; (2) an AC to DC converter; and/or (3) a DC to DC converter. In

cases (2) and (3) these provide an internal control scheme that regulates the output

voltage.

[0040] The resistive network consists of one or more resistors and one or more

switches operated by the MCU. The resistive network is connected to the voltage

source and (via MCU) is used to regulate the current that flows in the ultracapacitor

cells. Figure 4 shows one possible embodiment with parallel resistors and Figure 5

shows one possible embodiment with series resistors. The MCU controls the switches,

which modify the resistance between the DC source and the ultracapacitors. The

amount of current flowing into the ultracapacitors is then (Equation 2):

Vs -Vuc

Iuc = (2) network

Where: Vs is the source voltage, Vuc is the voltage over the ultracapacitors (total string

voltage) , and Rnetwork is the total resistance of the switched resistive network.

[0041] The current is regulated in steps by changing the resistance (Rnetwork)

between the source (Vs) and the ultracapacitors. The higher number or switched

resistors provides a higher resolution of resistance and therefore better control of

current. As the voltage of the ultracapacitor string increases, the MCU reduces the

resistance, Rnetwork. The MCU can calculate how much current will flow into the cells

because the voltages of the ultracapacitors and source are measured, and the resistance,

Rnetwork, is known based on different switch configurations.

[0042] The present invention's components can be integrated into one device, or

could be spread among more than one device. For instance, the charger may be

separate from the components including the ultracapacitors. As such, the ultracapacitor

device would need to be connected to the charger to be charged, and disconnected from

the charger when charging is complete. Alternatively, the charger could be built

directly into the device containing the ultracapacitors. Further "connecting"/

"disconnecting" is intended to include both physical connections as well as electrical

connections (such as switches).

[0043] The following paragraphs describing three particular embodiments of the

present invention. These embodiments are not exclusive.

[0044] The first embodiment is an ultracapacitor charging method for charging a

string of ultracapacitor cells. The method comprising the steps of: (1) measuring the

voltage of each ultracapacitor cell; (2) determining a maximum voltage level of each

ultracapacitor cell; (3) determining which ultracapacitor cell has the largest voltage; (4)

charging said string of ultracapacitor cells through use of a charger, said charger

connected to a voltage source; (5) monitoring the voltage of the ultracapacitor cell

having the largest voltage; and, (6) stopping the charging said string when the

ultracapacitor cell with the largest voltage reaches its maximum voltage level.

[0045] It is preferred that the method further comprise: (1) first connecting said

string of ultracapacitor cells to said charger, (2) last disconnecting said string of

ultracapacitor cells from said charger. It is further preferred that the voltage of each

ultracapacitor cell is monitored during the charging step to determine if one or more of

the ultracapacitor cells has failed and if so alerting a user of said method of said failure.

These ultracapacitors can be connected in series, parallel and/or a combination of

series/parallel.

[0046] A second embodiment is a method of charging a string of ultracapacitors

comprising the following steps: (1) connecting a string of ultracapacitors to a charger,

said charger connected to a voltage source; (2) measuring the voltage of each of the

ultracapacitors in said string; (3) charging said string of ultracapacitors using said

charger; (4) monitoring the voltage of each ultracapacitor; (5) determining which

ultracapacitor cell has the largest voltage; (6) monitoring the voltage of the

ultracapacitor cell with the largest voltage during charging; (7) stopping charging of

said string of ultracapacitors when the ultracapacitor cell with the largest voltage

reaches its maximum voltage ; and, (8) disconnecting said charger from said string of

ultracapacitors. Further steps optionally including: (9) calculating the total string

voltage by summing the voltages of said ultracapacitor cells, (10) following a current

(charging) profile based upon the total string voltage; and (11) monitoring the voltage

over each ultracapacitor cell during charging, monitoring the current passing through

each ultracapacitor cell, and thereby determining if one or more of the ultracapacitor

cells has failed.

[0047] A third embodiment being a charger for charging a string of ultracapacitors.

The charger comprising: (1) a voltage source; (2) a MCU, said MCU measuring the

voltage of each ultracapacitor cell via an analog circuit during charging to determine

which ultracapacitor cell has the largest voltage, said MCU commanding current to said

string of ultracapacitors based on the measured voltages, said MCU monitoring the

voltage of the ultracapacitor cells and when the ultracapacitor cell with largest voltage

reaches its maximum voltage the MCU commands the voltage source to stop charging

the ultracapacitor string; and (3) a first analog circuit, said first analog circuit for

allowing the MCU to measure the voltage of each ultracapacitor cell, said first analog

circuit measuring the difference between the positive and negative terminals of each

ultracapacitor in said string and outputting a signal proportional to the voltage to the

MCU. Optionally the charger could comprise a second analog circuit regulating the

current into the ultracapacitor cells that was commanded by the MCU. The voltage

source could be a DC-DC converter.

[0048] In one version of this embodiment, the MCU monitors the voltage over each

ultracapacitor cell and the current passing through each ultracapacitor cell to determine

if one or more of the ultracapacitor cells has failed. In another version, the MCU

calculates the total string voltages by summing the voltages of said ultracapacitor cells,

and wherein based upon said total voltage said MCU allows the current from the power

electronic converter to follow a charging profile based upon total output voltage. In yet

another version, the MCU monitors the voltage over each ultracapacitor cell and the

current passing through each ultracapacitor cell to determine if one or more of the

ultracapacitor cells has failed, and wherein said MCU calculates the total string voltages

by summing the voltages of said ultracapacitor cells, and wherein based upon said total

voltage said MCU allows the current from the power electronic converter to follow a

charging profile based upon total output voltage.

[0049] The exemplary embodiments shown in the figures and described above

illustrate but do not limit the invention. It should be understood that there is no

intention to limit the invention to the specific form disclosed; rather, the invention is to

cover all modifications, alternative constructions, and equivalents falling within the

spirit and scope of the invention as defined in the claims. For example, while the

exemplary embodiments illustrate an ultracapacitor charger, the teachings of the

invention is not limited to use with ultracapacitors and may be used with other power

sources. While the invention is not limited to use with ultracapacitors, it is expected

that various embodiments of the invention will be particularly useful in such devices.

Hence, the foregoing description should not be construed to limit the scope of the

invention, which is defined in the following claims.

[0050] While there is shown and described the present preferred embodiment of the

invention, it is to be distinctly understood that this invention is not limited thereto, but

may be variously embodied to practice within the scope of the following claims. From

the foregoing description, it will be apparent that various changes may be made

without departing from the spirit and scope of the invention as defined by the following

claims.

[0051] The purpose of the Abstract is to enable the public, and especially the

scientists, engineers, and practitioners in the art who are not familiar with patent or

legal terms or phraseology, to determine quickly from a cursory inspection, the nature

and essence of the technical disclosure of the application. The Abstract is neither

intended to define the invention of the application, which is measured by the claims,

nor is it intended to be limiting as to the scope of the invention in any way.

[0052] Still other features and advantages of the present invention will become

readily apparent to those skilled in this art from the following detailed description

describing preferred embodiments of the invention, simply by way of illustration of the

best mode contemplated by carrying out my invention. As will be realized, the

invention is capable of modification in various obvious respects all without departing

from the invention. Accordingly, the drawings and description of the preferred

embodiments are to be regarded as illustrative in nature, and not as restrictive in

nature.