Abstract:
An active voltage management device and a method for actively managing a voltage level of an energy storage device are provided. The active voltage management device comprises: a pair of input terminals adapted to be connected to the energy storage device; a reverse polarity protection circuit coupled to the pair of input terminals; a voltage comparator circuit adapted to compare a second voltage associated with the voltage level of the energy storage device to a reference voltage and to provide an output based upon the comparison of the second voltage to the reference voltage; and a transistor adapted to operate in a linear mode to dissipate energy from the energy storage device at a substantially constant current level, wherein output of the voltage comparator circuit is adapted to activate the transistor when the second voltage is greater than or equal to the reference voltage. The method comprises: receiving an input voltage from the energy storage device; providing reverse polarity protection from the energy storage device; comparing the a second voltage associated with the input voltage from the energy storage device to a reference voltage; and conducting a transistor in a linear mode to dissipate energy from the energy storage device at a substantially constant current level when the second voltage is greater than or equal to the reference voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application No. 60/866,408, filed 17 Nov. 2006, which is hereby incorporated by reference as though fully set forth herein. 
     
    
     BACKGROUND 
       [0002]    a. Field of the Invention 
         [0003]    The instant invention relates to a voltage management system for one or more energy storage cells. 
         [0004]    b. Background 
         [0005]    Energy storage devices are used to power many electrical devices. The energy storage devices may include one or more energy storage cells connected in series and/or parallel to provide an output voltage. The energy storage device can be charged to store energy in the energy storage device and can be discharged to provide that energy to a load. 
         [0006]    When the energy storage device is being charged one or more energy storage cells of the energy storage device may become overcharged. In order to prevent a potentially dangerous or harmful condition, the charging current is and energy stored in the one or more overcharged energy storage cells is dissipated from the cells until the voltage of the cell reaches a predetermined maximum voltage level. 
         [0007]    Similarly, when the energy storage device is being discharged, one or more of the energy storage cells of the energy storage device may reach a minimum desired charge level. In double layer capacitors and certain types of rechargeable batteries, for example, a predetermined minimum charge level may be desired to be maintained in each energy storage cell of the energy storage device. When this minimum charge level is reached, the discharge of the energy storage device may be stopped and/or a charging current may be applied to the energy storage device to recharge the one or more energy storage cells. 
       BRIEF SUMMARY 
       [0008]    In one embodiment, an active voltage management device for actively managing a voltage level of an energy storage device is provided. The active voltage management device comprises: a pair of input terminals adapted to be connected to the energy storage device; a reverse polarity protection circuit coupled to the pair of input terminals; a voltage comparator circuit adapted to compare a second voltage associated with the voltage level of the energy storage device to a reference voltage and to provide an output based upon the comparison of the second voltage to the reference voltage; and a transistor adapted to operate in a linear mode to dissipate energy from the energy storage device at a substantially constant current level, wherein output of the voltage comparator circuit is adapted to activate the transistor when the second voltage is greater than or equal to the reference voltage. 
         [0009]    In another embodiment, a method of actively managing a voltage level of an energy storage device is also provided. The method comprises: receiving an input voltage from the energy storage device; providing reverse polarity protection from the energy storage device; comparing the a second voltage associated with the input voltage from the energy storage device to a reference voltage; and conducting a transistor in a linear mode to dissipate energy from the energy storage device at a substantially constant current level when the second voltage is greater than or equal to the reference voltage. 
         [0010]    The foregoing and other aspects, features, details, utilities, and advantages of the present invention will be apparent from reading the following description and claims, and from reviewing the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Embodiments of the disclosed method and apparatus will be more readily understood by reference to the following figures, in which like reference numbers and designations indicate like elements. 
           [0012]      FIG. 1A  illustrates a block diagram of one embodiment of a system for balancing an electrical output of two individual energy storage cell elements. 
           [0013]      FIG. 1B  illustrates a block diagram of another embodiment of a system for balancing an electrical output of two individual energy storage cell elements. 
           [0014]      FIG. 2  illustrates a block diagram of one embodiment of a system for balancing an electrical output of four individual energy storage cell elements. 
           [0015]      FIG. 3A , labeled as sub-parts  3 A- 1  and  3 A- 2 , illustrates a block diagram of a top-level view of a single cell balancing system. 
           [0016]      FIG. 3B  illustrates a block diagram of a low-level view of a single cell balancing system. 
           [0017]      FIG. 4A  illustrates a block diagram of a top-level view of a multiple cell balancing system. 
           [0018]      FIG. 4B , labeled as sub-parts  4 B- 1  and  4 B- 2 , illustrates a block diagram of a low-level view of a multiple cell balancing system. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    A system and method for actively managing one or more individual energy storage elements is provided. In one embodiment, for example, a voltage management system may actively manage a voltage level of an energy storage element by dissipating energy from the energy storage element when a voltage level of energy storage element is greater than and/or equal to a predetermined voltage level. The energy storage element may include one or more individual energy storage cells. The individual energy storage cells may include any type of rechargeable energy storage cell, such as a capacitor, a double layer capacitor, a rechargeable battery cell, and/or a hybrid cell. 
         [0020]      FIG. 1A  shows an embodiment of a system  100 A for managing an energy storage unit  102 , an active voltage management module element  105 , and a plurality of energy monitoring elements  106 . The energy storage unit  102  comprises a plurality of individual storage cells  111  and  119 . In one embodiment, the plurality of individual energy storage cells  111  and  119  comprise capacitors, although the energy storage cells  111  and  119  may comprise secondary batteries (e.g., lithium ion batteries, nickel cadmium batteries, lead-acid batteries), hybrid cells, or other types of energy storage devices. In this embodiment, the energy storage unit  102  provides an electrical payload output to a first terminal  101  and a second terminal  121 . The energy storage unit  102  comprises a maximum operating voltage, a nominal operating voltage, an actual operating voltage, and individual energy storage cell outputs for each of the plurality of energy storage cell elements  111  and  119  of the energy storage unit  102 . 
         [0021]    The energy storage unit  102  is operatively coupled to the at least one active voltage management module element  105  and, therefore, the plurality of individual storage cells  111  and  119  are operatively coupled to the active voltage management module element  105 . As will be described in greater detail below, the active voltage management module element  105  is adapted to dissipate energy from at least one of the individual storage cells  111  and  119  when a voltage level of the storage cell is greater than and/or equal to a predetermined threshold. 
         [0022]    The plurality of energy monitoring elements  106  is operatively coupled to the active voltage management module element  105 . As will be described in greater detail below, the plurality of energy monitoring elements  106  are adapted to monitor various aspects related to the electrical output or operating conditions of the plurality of energy storage elements  111  and  119 , and/or detect a change in the electrical output or operating conditions being monitored. In one embodiment, the plurality of energy monitoring elements  106  comprises four monitoring units  103 ,  113 ,  115 , and  117 . The plurality of energy monitoring elements  106  measures various electrical and/or physical parameters of the system  100 A. In one embodiment a life data summing stage  107  is operatively coupled to the plurality of energy monitoring elements  106 . The life data summing stage  107  may, for example, generate control signals and/or perform calculations based upon inputs received from the monitoring elements  106 . 
         [0023]      FIG. 1B  shows one embodiment of a system  100 B for managing an energy storage unit  102  comprising a plurality of individual energy storage cells  111  and  119 . In this embodiment, a plurality of energy monitoring elements  106  comprises a linearized temperature monitor element  103 , a voltage discharge monitoring element  113 , a nominal voltage monitoring element  115 , and a maximum voltage monitoring element  117 . The linearized temperature monitor element  103  measures a temperature of the voltage management module element  105 , an individual energy storage cell, and/or the energy storage unit  102 . The nominal voltage monitoring element  115  measures a nominal voltage output of energy storage unit  102 . In one embodiment, the nominal voltage output of each cell in the energy storage unit  102  is about 2.7 volts. Although specific monitoring elements are discussed, other types of monitoring elements may be used either instead of or in addition to the ones discussed above. As shown in  FIG. 1B , the system  100 B may be isolated (e.g., optically isolated) from a control system to protect the control system. 
         [0024]      FIG. 2  shows one embodiment of a system  200  for managing an energy storage unit  202 . The energy storage unit  202  comprises a plurality of individual energy storage cells  211 ,  219 ,  223 , and  225 . In this embodiment, a voltage management module element  205  is adapted to dissipate energy from at least one of the individual storage cells  211 ,  219 ,  223 , and  225  when a voltage level of the storage element is greater than or equal to a predetermined threshold. The system  200  further comprises a plurality of energy monitoring elements  206 , which measure and/or monitor an output of the voltage management module element  205 , an output of the energy storage unit  202 , and/or one or more operating conditions of the voltage management module element  205  and the energy storage unit  202 . A life data summing stage  207  is operatively coupled to the plurality of energy monitoring elements  206 . The life data summing stage  207  may, for example, generate control signals and/or perform calculations based upon conditions being monitored by one or more of the monitoring elements  206 . 
         [0025]    In one embodiment, the plurality of energy monitoring elements  206  comprises four monitoring units  203 ,  213 ,  215 , and  217 . The plurality of energy monitoring elements  206  measures various electrical conditions and/or physical parameters of the system  200 , such as voltage, current, and/or temperature. The plurality of energy monitoring elements  206  provide information to the system  200  regarding outputs of the plurality of individual energy storage cells  211 ,  219 ,  223 , and  225 . The system  200  may generate control signals and/or perform calculations based upon the conditions being monitored by the plurality of energy monitoring elements  206 , such as via the life data summing stage  207 . In one embodiment, the plurality of energy monitoring elements  206  detect a change in output voltage of the individual energy storage cells  211 ,  219 ,  223 , and  225 . The system  200  is also adapted to dissipate energy from at least one of the individual storage cells  211 ,  219 ,  223 , and  225  via the voltage management module element  205  when a voltage level of the storage cell is greater than or equal to a predetermined threshold. 
         [0026]      FIG. 3A  shows a top-level schematic diagram of an energy storage system  300 . The energy storage system  300  comprises an energy storage unit device  302 , a plurality of voltage management devices  304 , and a stop charge control block  306  that is a component of a life data summing element. The energy storage unit device  302  comprises a plurality of individual energy storage cells connected in series. Each of the individual energy storage cells is coupled to an individual voltage management device  304 . Each of the individual voltage management devices  304  is adapted to dissipate energy from the individual energy storage cell coupled to it when the voltage level of the cell is greater than or equal to a predetermined voltage. The voltage management devices  304 , for example, reduce the voltage of the individual cells when those cells have a voltage level greater than desired. 
         [0027]    The voltage management devices  304  may also generate a signal (e.g., a STOP_CHARGE signal) to be provided to a control system indicating that an overcharge condition has been reached in an energy storage cell. The signal is provided to the stop charge control block  306 . When the signal is asserted by the voltage management device  304 , a transistor Q 8  of the stop charge control block  306  is turned on to conduct current through an LED U 2  of an optical isolator. The output of the optical isolator, in turn, provides an isolated control signal to a system controller, such as via an open collector output configuration of the isolator. In the embodiment of  FIG. 3B , the stop charge control block  306  provides a control signal if any one of the voltage management devices  304  detects an overcharge condition on an energy storage cell. In other embodiments, the stop charge control block  306  may provide a stop charge control signal to a system controller if each of the voltage management devices  304  or a subset of the voltage management devices  304  detects an overcharge condition on their associated energy storage cells. 
         [0028]      FIG. 3B  shows a schematic diagram of a single cell voltage management circuit  304 . The single cell voltage management circuit  304  is coupled to a single energy storage cell (e.g., a capacitor, secondary battery, or hybrid cell) of the energy storage unit device  302  as shown in  FIG. 3A . The energy storage cell is coupled to the voltage management circuit  304  via a first electrical contact point  303  and a second electrical contact point  305 . The voltage management circuit  304  monitors an output voltage of the energy storage cell. In one embodiment, the voltage management circuit compares the output voltage to a reference voltage value. If the output voltage is greater than the reference voltage value, the voltage management circuit dissipates energy from the energy storage cell to reduce the voltage of the cell to a level less than or equal to the reference value. 
         [0029]    As shown in  FIG. 3B , a reverse polarity protection circuit  310  is connected to the first electrical contact point  303  and the second electrical contact point  305 . The reverse polarity protection circuit  310  protects the voltage management circuit  304  if the energy storage cell is connected in the wrong orientation or if the voltage of the cell goes negative during discharge. In the particular embodiment shown in  FIG. 3B , the reverse polarity protection circuit comprises a p-channel MOSFET Q 1 A. A drain of the MOSFET Q 1 A is connected to the first electrical contact point  303 , and a gate of the MOSFET Q 1 A is connected to the second electrical contact point  305 . If the voltage level at the gate (i.e., at the second electrical contact point  305 ) is greater than the voltage level at the drain (i.e., at the first electrical contact point  301 ), the MOSFET Q 1 A prevents current from flowing from the energy storage device. If the voltage level at the drain is greater than the voltage level at the gate, the MOSFET Q 1 A allows current to flow from the energy storage device to the voltage management circuit  304 . The low on-resistance of the MOSFET Q 1 A provides a very low loss reverse polarity protection circuit  310 . Although one embodiment of a reverse polarity protection circuit is shown in  FIG. 3B , one skilled in the art would recognize from this disclosure that other embodiments could also be used. 
         [0030]    In the embodiment shown in  FIG. 3B , the voltage management circuit  304  also comprises a resistor voltage divider including resistors R 18 A and R 21 A, a filter capacitor CIA, and a voltage comparator U 1 A. The resistor voltage divider including resistors R 18 A and R 21 A provides a fraction of the voltage level provided to the circuit  304  as an input voltage to the voltage comparator U 1 A. The filter capacitor C 1 A forms a low pass filter with resistor R 18 A that prevents oscillations and suppresses transients. 
         [0031]    The voltage comparator U 1 A comprises an integrated reference voltage comparator. The comparator U 1 A is configured in an open drain output configuration that pulls an inverted output OUT low until a voltage threshold is reached at the input to the comparator. When the input voltage reaches the voltage threshold, however, the comparator U 1 A sinks current at the inverted output OUT. The voltage comparator receives an input voltage from the resistor voltage divider and compares that input voltage to the threshold voltage level of the comparator. The inverted output OUT provides an output based on the comparison of the input voltage to the threshold voltage of the comparator. In the embodiment shown in  FIG. 3B , for example, the threshold voltage comprises about a 2.2 volt trigger corresponding to an energy cell voltage of about 2.8 volts. The inverted output OUT of the voltage comparator U 1 A is low when the input voltage provided by the voltage divider is less than the threshold voltage of the comparator. The inverted output OUT of the voltage comparator sinks current from the transistor Q 5 A to turn the transistor Q 5 A on when the input voltage is greater than or equal to the threshold voltage of the comparator. The transistor Q 5 A in turn turns on the transistor Q 2 A which also in turn turns on the transistor Q 4 A. 
         [0032]    In the embodiment shown in  FIG. 3B , the voltage comparator U 1 A also provides a hysteresis window that prevents the voltage management circuit  304  from oscillating. In one embodiment in which a resistor R 19 A is not included in the circuit, the hysteresis of the voltage comparator U 1 A can be preset at a predetermined level (e.g., 110 mV). By adding the resistor R 19 A, the hysteresis window can be increased by a voltage level depending on the value of the resistor  19 A added to the circuit  304 . 
         [0033]    In one embodiment, the transistor Q 4 A operates in a constant current linear mode to dissipate energy from the energy storage cell coupled to the voltage management circuit  304  at a constant rate of discharge. By using the transistor in a constant current linear mode to dissipate energy instead of primarily relying on a resistor to dissipate the majority of the energy from the energy storage cell, the discharge of current can be held constant regardless of the voltage level of the energy storage cell and further allows the resistors of the circuit  304  to be sized smaller than if the resistors were used as the primary discharge mechanism. A transistor Q 7 A can also be used to provide an overcurrent protection for the transistor Q 4 A. 
         [0034]    In one embodiment, for example, the transistor Q 4 A may draw approximately 300 mA. In this embodiment, the transistor Q 4 A may dissipate at least the majority of the energy dissipated from the energy storage cell. The resistors R 9 A and R 10 A also dissipate energy from the energy storage cell, but in one embodiment dissipate less than half of the total energy dissipated from the energy storage cell. 
         [0035]    The voltage management circuit  304  shown in  FIG. 3B  also provides an indicator, such as a light emitting diode (LED) D 2 A, to indicate when the voltage management circuit is actively dissipating energy from the energy storage cell (or when the voltage management circuit is not actively dissipating energy from the energy storage cell). In the embodiment shown in  FIG. 3B , for example, the LED is activated by transistor Q 2 A described above. 
         [0036]    The voltage management circuit  304  also comprises a control signal STOPCHARGE via transistors Q 6 A and Q 3 A to indicate when the circuit  304  is actively dissipating energy from an energy storage cell. The control signal, for example, may be used to control a charging current being applied to the energy storage cell connected to the voltage management circuit  304 . 
         [0037]    In one embodiment, the voltage management circuit  304  draws a low quiescent current when the circuit is not actively dissipating energy from an energy storage cell. The voltage management circuit  304 , for example, may draw a quiescent current of approximately 50 μA. Where the dissipation current is approximately 300 mA, for example, a ratio of the dissipation current to the quiescent current is approximately 6000. In other embodiments, for example, the ratio of the dissipation current to the quiescent current is greater than approximately 1000, greater than approximately 2000, greater than approximately 4000, greater than approximately 5000, or greater than approximately 6000. 
         [0038]      FIG. 4A  shows a top-level schematic diagram of a multi-cell voltage management system  400 . The multi-cell voltage management system  400  comprises a plurality of multi-cell voltage management circuits  408  that are each coupled to a plurality of energy storage cells of an energy storage unit  402 . In one embodiment, for example, the energy storage unit is a module that comprises eighteen energy storage cells connected in series. In this embodiment, the multi-cell voltage management system  400  comprises three voltage management circuits  408  each coupled to six of the series-connected energy storage cells. 
         [0039]    Each of the multi-cell voltage management circuits  408  each monitor the voltage of the plurality of energy storage cells coupled to the circuit  408 . If the monitored voltage of the plurality of energy storage cells is greater than or equal to a predetermined threshold voltage, the multi-cell voltage management circuit dissipates energy from the plurality of energy storage cells. In the embodiment shown in  FIG. 4A , the individual multi-cell voltage management circuits  408  may also provide control signals. These control signals may, for example, be used to stop a charging current from being applied to the plurality of energy storage cells, initiate a charging current to be applied to the plurality of energy storage cells, provide a warning or indicator or an over-voltage condition, or provide another type of warning or ameliorative action. 
         [0040]    Low voltage control block  404  and stop charge block  406  operate similarly to stop charge control block  306  described above with reference to  FIG. 3A  to provide isolated control signals to a system controller for a low voltage control signal and a stop charge control signal, respectively. The low voltage control signal, for example, may be used to disconnect one or more energy storage cell from a load and/or to initiate a charge operation to recharge one or more energy storage cell. Low voltage control block  404  and stop charge control block  406  each comprise a resistor-capacitor (RC) filter on the input of the transistor Q 6 . The RC filter reduces noise on the control line entering the control block. 
         [0041]      FIG. 4B  shows an individual voltage management circuit  408  of the voltage management system  400  shown in  FIG. 4A . The individual voltage management circuit  408  comprises a reverse polarity protection circuit  403 , a voltage regulator circuit  405 , a first comparator circuit  406 , a second comparator circuit  407 , and an energy dissipation circuit  409 . The voltage management circuit  401  further comprises a first electrical contact point  411  and a second electrical contact point  413 . A plurality of energy storage cells (e.g., a series and/or parallel string of energy storage cells) may be connected between the first electrical contact point  411  and the second electrical contact point  413 . The voltage management circuit  408  may be external to a module including the energy storage cells or may be integrated within the module. 
         [0042]    The reverse polarity protection circuit  403  is the same as the reverse polarity protection circuit  310  described above with respect to  FIG. 3B . 
         [0043]    The voltage regulator and reference circuit  405  comprises a voltage regulator and a voltage reference. The voltage regulator comprises a zener diode D 2 , a voltage regulator U 1 , a filter capacitor C 2 , and a voltage clamp diode D 1 . The zener diode D 2  protects the voltage regulator U 1  from an input voltage that is too high for the voltage regulator U 1 . In one embodiment, for example, the zener diode has a breakdown voltage of about 18 volts. The voltage regulator steps down the input voltage from the bank of energy storage cells and provides a fixed output voltage (e.g., about five volts). The zener diode D 3  sets the reference voltage (e.g., about 2.5 volts). 
         [0044]    The voltage reference comprises a resistor R 3  and a reference zener diode D 3 . The voltage reference provides a reference voltage VREF from the output voltage of the voltage regulator U 1  and provides the reference voltage VREF to the first and second comparator circuits  407  and  408 . 
         [0045]    The first comparator circuit  406  comprises a voltage divider and an op-amp. In the embodiment shown in  FIG. 4B , for example, the voltage divider comprises a resistor voltage divider including resistors R 1  and R 7 . The op-amp U 2 A compares a voltage provided by the voltage divider and the reference voltage VREF. If the voltage provided by the voltage divider is greater than or equal to the reference voltage VREF, the output of the op-amp U 2 A turns on transistors Q 2  and Q 3 . In the particular embodiment shown in  FIG. 4B , the op-amp output is driven low to turn on p-type transistors Q 2  and Q 3  although other embodiments are also possible. The transistor Q 3  (e.g., a p-channel bipolar transistor in the embodiment shown in  FIG. 4B ) provides a control signal STOPCHARGE that may be used to turn off a charging current from being applied to the bank of energy storage cells coupled to the voltage management circuit  408 . The transistor Q 2  (e.g., a p-channel MOSFET in the embodiment shown in  FIG. 4B ) is part of a buffer circuit formed by the transistor Q 2  and the op-amp U 2 B that in turn turns on dissipation transistor Q 4  that provides constant current dissipation of energy from the bank of energy storage cells coupled to the individual voltage management circuit  408  via contact points  411  and  413 . 
         [0046]    The second comparator circuit  407  also comprises a voltage divider and a comparator. The voltage divider in this embodiment is a resistor voltage divider including resistors R 13  and R 15 . The comparator comprises an op-amp U 2 C that compares a voltage provided by the voltage divider to the reference voltage VREF described above. When the voltage provided by the voltage divider is less than or equal to the reference voltage VREF, the comparator turns on transistor Q 5  to provide a low voltage warning signal LOW_WARN. The low voltage warning signal LOW_WARN may be used, for example, to disconnect the energy storage cells from a load and/or to initiate a charging current to re-charge the energy storage cells. In one embodiment, for example, the low voltage warning signal LOW_WARN may be used to indicate that the energy storage cells are at approximately fifty percent of their rated energy storage capacity, although other embodiments may be used depending on the type of energy storage cells being used. 
         [0047]    Although embodiments have been described above with a certain degree of particularity, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention. All directional references (e.g., upper, lower, upward, downward, left, right, leftward, rightward, top, bottom, above, below, vertical, horizontal, clockwise, and counterclockwise) are only used for identification purposes to aid the reader&#39;s understanding of the present invention, and do not create limitations, particularly as to the position, orientation, or use of the invention. Joinder references (e.g., attached, coupled, connected, and the like) are to be construed broadly and may include intermediate members between a connection of elements and relative movement between elements. As such, joinder references do not necessarily infer that two elements are directly connected and in fixed relation to each other. It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not limiting. Changes in detail or structure may be made without departing from the spirit of the invention as defined in the appended claims.