Patent Publication Number: US-9897663-B2

Title: Integrated DC/DC converter, electrochemical energy storage system, and methods for analyzing electrochemical impedance spectroscopy and working state of electrochemical energy storage apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims all benefits accruing under 35 U.S.C. § 119 from China Patent Applications No. 201410387807.5, filed on Aug. 8, 2014 and No. 201410389361.X, filed on Aug. 8, 2014 in the China Intellectual Property Office, the content of which is hereby incorporated by reference. This application is a continuation in part under 35 U.S.C. § 120 of international patent application PCT/CN2015/078178 filed May 4, 2015. 
    
    
     FIELD 
     The present disclosure relates to DC/DC converters and electrochemical energy storage systems, and particularly relates to an integrated DC/DC converter that is capable of monitoring working state of battery, and an electrochemical energy storage system including the integrated DC/DC converter. The present disclosure also relates to a method for analyzing electrochemical impedance spectroscopy of an electrochemical energy storage apparatus, and a method for monitoring working state of the electrochemical energy storage apparatus. 
     BACKGROUND 
     A hydrogen and oxygen proton exchange membrane fuel cell (PEMFC) is an electrochemical apparatus which directly transforms chemical energy to electrical energy. Unlike a conventional internal combustion engine, the energy conversion of the PEMFC is not confined by Carnot cycle, and has a higher theoretical energy conversion efficiency. The PEMFC produces water and no harmful emissions by using hydrogen and oxygen gases as reactants, which makes it attractive and popular in electric stations, vehicles, and mobile power sources. 
     The PEMFC produces a direct current with an output voltage smaller than 1 V (typically 0.7 V) per cell. A series connection of multiple PEMFC cells, which forms a PEMFC stack, achieves a higher voltage. One single PEMFC cell includes components such as gas diffusion layer (GDL) for anode, membrane electrode assemblies (MEA), and GDL for cathode. 
     The fuel cell power generation system comprises the PEMFC stack which is an essential member, and multiple auxiliary systems, such as air and hydrogen supplying systems, cooling system, power adjusting system, moisture adjusting system, and control system, to assist operation of the stack. The air supplying system inputs a suitable amount of oxidants, such as air, and controls the temperature, pressure, and flow rate of the air supplied. The hydrogen supplying system inputs hydrogen, and controls the pressure and flow rate of the hydrogen gas supplied. The cooling system maintains the temperature of the stack to a suitable level. The power adjusting system controls the output voltage and current of the stack to meet the needs of an electrical load. The moisture adjusting system adjusts the wetness of the air that is supplied to the stack, to be within an optimal range, neither too dry nor too wet. The control system controls each auxiliary system to achieve a best working state of the stack. 
     The water produced by the PEMFC as gas or liquid is expelled from cathode by an air flow. A high flow rate of the air supplied to the stack can efficiently expel water. However, when the stack has a low load, only a small amount of water is produced. A high flow rate of air may dry the proton exchange membrane, which causes degeneration in the performance of the proton exchange membrane. Yet, a relatively low flow rate of air may expel water inefficiently and cause the fuel cell to flood. Precise control of the flow rate and moisture of the air is difficult to achieve, especially for a stack which has a large amount of non-identical cells. 
     The working state, such as the moistness of the proton exchange membrane and the flooded or partly flooded state of the fuel cell has a relationship with equivalent circuit impedance of the fuel cell. By obtaining a measure of the equivalent circuit impedance in real time, the working state of the fuel cell can be precisely analyzed and adjusted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Implementations are described by way of example only with reference to the attached figures. 
         FIG. 1  is a block diagram of an embodiment of an electrochemical energy storage system. 
         FIG. 2  is an embodiment of an equivalent circuit diagram of an electrochemical energy storage cell. 
         FIG. 3  is a diagram showing electrochemical impedance spectroscopy of the equivalent circuit diagram shown in  FIG. 2 . 
         FIG. 4  is a schematic view of an embodiment of an electrochemical energy storage system. 
         FIG. 5  is a schematic view of an embodiment of an integrated DC/DC converter. 
         FIG. 6  is a schematic view of an embodiment of an application of the integrated DC/DC converter. 
         FIG. 7  is a schematic view of an embodiment of another application of the integrated DC/DC converter. 
         FIG. 8  is a circuit diagram of an embodiment of a second DC/DC converter. 
         FIG. 9  is a circuit diagram of an embodiment of a disturbance source. 
         FIG. 10  is a circuit diagram of another embodiment of the disturbance source. 
         FIG. 11  is a flow chart of an embodiment of a method for analyzing electrochemical impedance spectroscopy of an electrochemical energy storage apparatus. 
         FIG. 12  is a flow chart of an embodiment of an operation of an embodiment of a first DC/DC converter. 
         FIG. 13  is a flow chart of an embodiment of a method for generating a disturbance signal to be applied to an electrical current. 
         FIG. 14  is a flow chart of a continuation of  FIG. 13 . 
         FIG. 15  is a flow chart of an embodiment of a method of electrochemical impedance analysis. 
         FIG. 16  is a flow chart of an embodiment of a method for analyzing working state of an electrochemical energy storage apparatus. 
         FIG. 17  is a diagram showing a polarization curve of a disturbed output current of the fuel cell stack described in Example 1. 
         FIG. 18  is a diagram showing a disturbed output current signal and a response output voltage signal of the fuel cell stack described in Example 1. 
         FIG. 19  is a diagram showing an electrochemical impedance spectroscopy of the fuel cell stack described in Example 1. 
     
    
    
     DETAILED DESCRIPTION 
     It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures, and components have not been described in detail so as not to obscure the related relevant feature being described. Also, the description is not to be considered as limiting the scope of the embodiments described herein. The drawings are not necessarily to scale and the proportions of certain parts may be exaggerated to better illustrate details and features of the present disclosure. 
     Several definitions that apply throughout this disclosure will now be presented. 
     The term “comprise” or “comprising” when utilized, means “include or including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in the so-described combination, group, series, and the like. The terms “comprise”, “comprising”, “include”, and “including” do not suggest that another undisclosed feature, element, component, or step must be required. Therefore, when using the term “comprise”, “comprising”, “include”, or “including”, the term “consist of” or “consisting of” can also be applied. The term “connect” or “connected” can mean both “directly connect” or “directly connected” and “indirectly connect” or “indirectly connected.” 
     Referring to  FIG. 1 , one embodiment of an electrochemical energy storage system  20  comprises an electrochemical energy storage apparatus  22 , a control system  24 , and an integrated DC/DC converter  200 . The control system  24  is capable of maintaining a stable electric energy output of the electrochemical energy storage apparatus  22 . The integrated DC/DC converter  200  is electrically connected to the electrochemical energy storage apparatus  22 , and is capable of regulating the electric energy output from the electrochemical energy storage apparatus  22  to satisfy an electrical load. 
     The electrochemical energy storage apparatus  22  can comprise one or a plurality of electrochemical energy storage cells. The electrochemical energy storage cell generates electric energy from a chemical reaction. The electrochemical energy storage cell comprises a cathode, an anode, and an electrolyte separator located between the cathode and the anode. Referring to  FIG. 2 , the electrochemical energy storage cell can be represented by a circuit (“equivalent circuit”) consisting of a Nernst voltage (E Nernst ), an anodic double layer capacitor (C dl,A ), an anode resistor (R A ), a cathodic double layer capacitor (C dl,CA ), a cathodic resistor (R CA ), and proton exchange membrane as a resistor (R Ω ). A parallel connection is made between the C dl,A  and R A  to form an anodic RC circuit. Another parallel connection is also made between the C dl,CA  and R CA  to form a cathodic RC circuit. The E Nernst , cathodic RC circuit, R Ω , and anodic RC circuit are connected in series. Referring to  FIG. 3 , an electrochemical impedance of the equivalent circuit shown in  FIG. 2  satisfies equations (1) and (2): 
     
       
         
           
             
               
                 
                   
                     
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     wherein Z (ω) is the electrochemical impedance of the equivalent circuit, which varies depending on angular frequency ω. When the electrochemical energy storage cell outputs a direct current (i.e., ω=0) satisfying the equation (2), the electrochemical impedance of the equivalent circuit is equal to a total internal resistance R internal  of the electrochemical energy storage cell. 
     By detecting the impedances of the equivalent circuit during working of the electrochemical energy storage apparatus  22 , working conditions (such as temperature and moisture) of each component in the electrochemical energy storage apparatus  22  can be analyzed, and the working conditions can be dynamically adjusted to effectively improve the performance of the electrochemical energy storage apparatus  22 . The electrochemical energy storage cell can be at least one of a fuel cell, a lithium battery, and a supercapacitor. In one embodiment, the electrochemical energy storage cell is a fuel cell, and the electrochemical energy storage apparatus  22  is a fuel cell stack formed by connecting a plurality of fuel cells in series. 
     The control system  24  is selected to suit the type of the electrochemical energy storage apparatus  22 . For example, when the electrochemical energy storage apparatus  22  is a lithium ion battery pack, the control system  24  can be a lithium ion battery management unit to detect temperatures and electrical readings, such as voltages and currents, of lithium ion battery pack or cells, and to unify the lithium ion battery cells. Referring to  FIG. 4 , in one embodiment of the electrochemical energy storage system  20 , the electrochemical energy storage apparatus  22  is a fuel cell stack. The control system  24  is designed for the fuel cell stack, and comprises a hydrogen supplying system  12 , an air supplying system  14 , a cooling system  16 , a recycling system  18 , a temperature and moisture detecting system, and a working condition adjusting system. The working condition adjusting system adjusts the working conditions of the fuel cell stack according to readings taken by the other systems. The air supplying system  14  comprises an air compressor  142 , a heat radiator  144 , a humidifier  146 , and a first flowing control member  148 . The recycling system  18  comprises a condenser  182  and a second flowing control member  184 . Air is compressed by the air compressor  142 , cooled by the heat radiator  144 , moistened by the humidifier  146 , and then introduced to the fuel cell stack, which is the electrochemical energy storage apparatus  22 . The oxygen gas from the cathode of the fuel cell stack chemically reacts with the hydrogen ions from the anode to produce electric energy and water. Due to the chemical reaction, the oxygen amount decreases and the water amount increases in the air at the cathode. The air expelled from the fuel cell stack is dried by the condenser  182  to recycle the water. The flow rate and pressure of the air can be controlled by the air compressor  142 , the first flowing control member  148 , and the second flowing control member  184 . The temperature and moisture in the air can be detected by the temperature and moisture detecting system which delivers data of readings to the working condition adjusting system. The working condition adjusting system controls the temperature of the air through the heat radiator  144 , and the moisture of the air through the humidifier  146 . 
     Referring to  FIG. 5 , the integrated DC/DC converter  200  comprises a first DC/DC converter  202 , a second DC/DC converter  204 , a first voltage sensor  206 , a second voltage sensor  208 , a first current sensor  210 , a second current sensor  212 , a third current sensor  214 , a fourth current sensor  216 , and a controller  218 . The first and second DC/DC converters  202 ,  204  are connected in parallel. Each of the first and second DC/DC converters  202 ,  204  has an input end and an output end. The electrochemical energy storage apparatus  22  has an output end to output electric energy. The electrical load has an input end to receive the electric energy output from the electrochemical energy storage apparatus  22 , and the integrated DC/DC converter  200  performs adjustment and control. The input end of the first DC/DC converter  202  is electrically connected to the output end of the electrochemical energy storage apparatus  22 . The output end of the first DC/DC converter  202  is electrically connected to the input end of the electrical load. The first voltage sensor  206  is electrically connected in parallel with the input end of the first DC/DC converter  202  to detect an output voltage of the electrochemical energy storage apparatus  22 . The second voltage sensor  208  is electrically connected in parallel to the output end of the first DC/DC converter  202  to detect an output voltage of the first DC/DC converter  202 . The first current sensor  210  and the output end of the electrochemical energy storage apparatus  22  are connected in series. The first current sensor  210  is capable of detecting an output current of the electrochemical energy storage apparatus  22 . The second current sensor  212  and the input end of the second DC/DC converter  204  are connected in series. The second current sensor  212  is capable of detecting an input current of the second DC/DC converter  204 . The third current sensor  214  and the output end of the first DC/DC converter  202  are connected in series. The third current sensor  214  is capable of detecting the output current of the first DC/DC converter  202 . The fourth current sensor  216  and the output end of the second DC/DC converter  204  are connected in series. The fourth current sensor  216  is capable of detecting the output current of the second DC/DC converter  204 . The controller  218  receives signals from the first voltage sensor  206 , the second voltage sensor  208 , the first current sensor  210 , and the third current sensor  214 , and is capable of controlling the electric energy output from the electrochemical energy storage apparatus  22  through the first DC/DC converter  202  to the electrical load. The controller  218  also controls the second DC/DC converter  204  to be on or off. The controller  218  can also receive signals from the second current sensor  212  and the fourth current sensor  216 . When the second DC/DC converter  204  is on, the controller  218  controls the second DC/DC converter  204  to add an electrical disturbance to the output current of the electrochemical energy storage apparatus  22  to achieve an electrochemical impedance spectroscopy of the electrochemical energy storage apparatus  22 . 
     The first and second DC/DC converters,  202  and  204 , can be of any known type, such as at least one of a buck DC/DC converter, a boost DC/DC converter, and a buck-boost DC/DC converter. In one embodiment for use in a vehicle, the first DC/DC converter  202  can be a high power DC/DC converter that is capable of meeting a vehicle power requirement. In one embodiment, the high power of the first DC/DC converter  202  can be greater than or equal to 20 kW (e.g., 20 kW to 80 kW). The first DC/DC converter  202  is capable of adjusting the output current of the electrochemical energy storage apparatus  22  to meet and satisfy the electrical load. 
     The second DC/DC converter  204  is a signal disturbance source, adding a disturbance to the output current of the electrochemical energy storage apparatus  22  to detect the electrochemical impedance spectroscopy of the electrochemical energy storage apparatus  22 . The second DC/DC converter  204  can be a high frequency DC/DC converter, which, while decreasing the influence on and disturbance to the electric energy output, detects the electrochemical impedance spectroscopy of the electrochemical energy storage apparatus  22 . The frequency of the second DC/DC converter  204  can be in a range from about 0.1 Hz to about 1 kHz. 
     Referring to  FIG. 6 , in one application, the input end of the integrated DC/DC converter  200  can be electrically connected to the electrochemical energy storage apparatus  22 , such as a PEMFC stack, a lithium ion battery pack, or a supercapacitor. The output end of the integrated DC/DC converter  200  can be electrically connected to the electrical load as shown in  FIG. 5 . In addition to the electrical load, another electrochemical energy storage apparatus, such as another lithium ion battery pack or supercapacitor, also can be electrically connected to the output end of the integrated DC/DC converter  200 . The electrochemical energy storage apparatus  22  connected to the input end of the integrated DC/DC converter  200  is then a first electrochemical energy storage apparatus. The electrochemical energy storage apparatus connected to the output end of the integrated DC/DC converter  200  is then a second electrochemical energy storage apparatus. The second electrochemical energy storage apparatus and the electrical load can be connected in parallel. The second electrochemical energy storage apparatus can maintain the output voltage of the integrated DC/DC converter  200 , and can be used in other applications, such as providing a transient high power output or restoring energy from the electrical load. 
     Referring to  FIG. 7 , in another application, the input end of the integrated DC/DC converter  200  can be electrically connected to the first electrochemical energy storage apparatus  22 , the apparatus  22  being a PEMFC stack, a lithium ion battery pack, or a supercapacitor. The output end of the integrated DC/DC converter  200  can be electrically connected to an electric motor and to the second electrochemical energy storage apparatus. The electric motor can be an AC asynchronous motor, a permanent-magnet synchronous motor, or a DC motor. The electric motor and the second electrochemical energy storage apparatus are electrically connected in parallel. The second electrochemical energy storage apparatus can maintain the output voltage of the integrated DC/DC converter  200 , and can be used in other applications, such as providing transient high power output or restoring energy from the motor. The motor can have many applications, such as motive power source or a braking energy recovery device of a vehicle. 
     Referring to  FIG. 8 , in one embodiment, the second DC/DC converter  204  is a boost DC/DC converter, comprising an inductor L 1 , a diode D 1 , a switch G 1 , and a capacitor C 1 . One end of the inductor L 1  is an input terminal such as a positive input terminal of the second DC/DC converter  204 . The other end of the inductor L 1  is electrically connected to the anode of the diode D 1 . The cathode of the diode D 1  is an output terminal such as a positive output terminal of the second DC/DC converter  204 . Switches in the second DC/DC converter  204  can each be a transistor with base, collector, and emitter terminals. The switch G 1  has the base electrically connecting to the controller  218 , the collector electrically connecting to the anode of the diode D 1 , and the emitter being both the other input terminal such as a negative input terminal and the other output terminal such as a negative output terminal of the second DC/DC converter  204 . The capacitor C 1  has one end electrically connected to the cathode of the diode D 1 , and the opposite end electrically connected to the emitter of the switch G 1 . The switch G 1  can be an insulated gate bipolar transistor (IGBT). 
     The working process of the second DC/DC converter  204  is as follows: 
     (1) when the switch G 1  is on, the current having the input voltage U in  goes through the inductor L 1  and is linearly increased by the inductor L 1  depending on the characteristics of the inductor. The electric energy is stored in the inductor L 1 . The inductor L 1  and the switch G 1  form a conducting loop. The anode of the diode D 1  electrically connects to the negative input end of the second DC/DC converter  204 . The cathode of the diode D 1  electrically connects to the positive output end of the second DC/DC converter  204 . The diode D 1  blocks current in the reverse direction and the capacitor C 1  discharges electric energy to the electrical load. 
     (2) When the switch G 1  is off, the inductor L 1  does not immediately drop the current at the moment the switch G 1  switches off, but forms an electric potential having a direction the same as the input voltage U in . The electric energy stored in the inductor L 1  gradually releases, charges the capacitor C 1 , and provides energy to the electric load through the diode D 1 . The inductor L 1 , diode D 1 , capacitor C 1 , and electrical load form a conducting loop. 
     (3) When the switch G 1  periodically switches between on and off, the on and off switching of the switch G 1  at different moments is controlled by the controller  218 . Electric energy is delivered from the input end U in  to output end U o  of the second DC/DC converter  204  to generate current disturbance signals. 
     The first voltage sensor  206  and the first current sensor  210  can detect an overall electrical view of the electrochemical energy storage apparatus  22  from the readings. 
     The fourth current sensor  216  can cooperate with the second current censor  212  to monitor an efficiency of the second DC/DC converter  204 , and detect a current change of the output current of the second DC/DC converter  204 . A detected current change of the output current of the second DC/DC converter  204  is conducted to the controller  218  and used to evaluate the influence on the electrical load, a great influence on the electrical load should be avoided. 
     The controller  218  receives the data from the above disclosed sensors and controls the first and second DC/DC converters  202 ,  204  based on the requirements from the electrical load and the analyzing of the electrochemical impedance spectroscopy. 
     When the integrated DC/DC converter  200  is in a normal working state without a need for analyzing the electrochemical impedance spectroscopy, an electric current is supplied to the first DC/DC converter  202  (i.e., at the “on” state), the second DC/DC converter  204  is cut off from the circuit of the integrated DC/DC converter  200  (i.e., at the “off” state), and the controller  218  controls the first DC/DC converter  202  to adjust the output of the electrochemical energy storage apparatus  22  according to the data from the first voltage sensor  206 , the second voltage sensor  208 , the first current sensor  210 , and the third current sensor  214 , to satisfy the need of the electrical load. 
     When there is a need to analyze the electrochemical impedance spectroscopy of the electrochemical energy storage apparatus  22 , currents are supplied to both the first DC/DC converter  202  and the second DC/DC converter  204  in the circuit of the integrated DC/DC converter  200  (i.e., both are at the “on” state). While performing the same adjusting process to the output of the electrochemical energy storage apparatus  22  through the controlling of the first DC/DC converter  202  in a normal working state, the controller  218  also receives data from the second current sensor  212  and the third current sensor  214 . In one embodiment data is also received from the fourth current sensor  216 . Based on the received data, the controller  218  controls the second DC/DC converter  204  to adjust the output current of the electrochemical energy storage apparatus  22  by the current disturbance from the second DC/DC converter  204  thereby achieving an electrochemical impedance spectroscopy of the electrochemical energy storage apparatus  22 . 
     In one embodiment, the electrochemical energy storage apparatus  22  comprises a plurality of electrochemical energy storage cells, and the integrated DC/DC converter  200  further comprises a voltage inspecting device  220 . The voltage inspecting device  220  is capable of acquiring a voltage of each electrochemical energy storage cell, and sending all the voltage data to the controller  218 . By using the voltage inspecting device  220 , an electrochemical impedance spectroscopy of each electrochemical energy storage cell in the electrochemical energy storage apparatus  22  can be obtained. 
     The electrical disturbance source is not limited to the second DC/DC converter  204 . Any circuit that is capable of generating a current disturbance signal at a required frequency can be used as the disturbance source. The disturbance source can be electrically connected to the first DC/DC converter  202  in parallel, controlled by a switch, and generate the current disturbance signal by switching on and off. 
     Referring to  FIG. 9 , in one embodiment, the disturbance source  204   a  comprises an inductor L 1   a , a capacitor C 1   a , a switch G 1   a , and a diode D 1   a . One end of the inductor L 1   a  is electrically connected to the positive input terminal of the disturbance source  204   a , and the other end of the inductor L 1   a  is electrically connected to the emitter of the switch G 1   a . The capacitor C 1   a  and the input end of the disturbance source  204   a  are electrically connected in parallel. The cathode of the diode D 1   a  is electrically connected to the emitter of the switch G 1   a . The anode of the diode D 1  is electrically connected to both a negative input terminal and a negative output terminal of the disturbance source  204   a . The base of the switch G 1   a  is electrically connected to the controller  218 . The collector of the switch G 1   a  is electrically connected to the positive output terminal of the disturbance source  204   a . The switch G 1   a  can be an IGBT. 
     Referring to  FIG. 10 , in another embodiment, the disturbance source  204   b  comprises resistors R 1   b , R 2   b , a transformer T 1   b , and switches G 1   b , G 2   b , G 3   b , and G 4   b . The transformer T 1   b  comprises primary and secondary windings. The primary winding has one end electrically connected to the positive input terminal of the disturbance source  204   b  and the other end electrically connected in series to the resistor R 1   b  and the negative input terminal of the disturbance source  204   b . The secondary winding has one end electrically connected in series to the resistor R 2   b  and the emitter of the switch G 1   b  and the other end electrically connected to the emitter of the switch G 2   b . The switches G 1   b , G 2   b , G 3   b  and G 4   b  form a bridge circuit. The bases of the switches G 1   b , G 2   b , G 3   b , and G 4   b  are all electrically connected to the controller  218 . The emitter of the switch G 1   b  is electrically connected to the collector of the switch G 3   b . The collector of the switch G 1   b  is electrically connected to the collector of the switch G 2   b , and is also electrically connected to the positive output terminal of the disturbance source  204   b . The emitter of the switch G 2   b  is electrically connected to the collector of the switch G 4   b . The emitter of the switch G 3   b  is electrically connected to the emitter of the switch G 4   b , and is also electrically connected to the negative output terminal of the disturbance source  204   b . The switches G 1   b , G 2   b , G 3   b , and G 4   b  can be IGBTs. 
     The on and off switching of the switches G 1 , G 1   a , and G 1   b ˜G 4   b , controlled by the controller  218 , allows the disturbance sources  204   a ,  204   b ,  204  to generate current disturbance signals having desired frequencies and amplitudes. 
       FIG. 11  presents a flowchart in accordance with an illustrated example embodiment. The embodiment of a method  300  for analyzing electrochemical impedance spectroscopy of an electrochemical energy storage apparatus  22  based on the integrated DC/DC converter  200  is provided by way of example, as there are a variety of ways to carry out the method  300 . Each block shown in  FIGS. 11 to 14  represents one or more processes, methods, or subroutines carried out in the exemplary method  300 . Additionally, the illustrated order of blocks is by example only and the order of the blocks can be changed. The exemplary method  300  can begin at block S 1 . Depending on the embodiment, additional steps can be added, others removed, and the ordering of the steps can be changed. 
     At block S 1 , the second DC/DC converter  204  is electrically conducted, and meanwhile the controller  218  controls the second DC/DC converter  204  to generate a current disturbance signal. The current disturbance signal has a frequency. 
     At block S 2 , the current disturbance signal disturbs the output current of the electrochemical energy storage apparatus  22 . 
     At block S 3 , the disturbed output current and disturbed output voltage of the electrochemical energy storage apparatus  22  are detected. 
     At block S 4 , an impedance corresponding to the frequency of the current disturbance signal is calculated based on the current disturbance signal, the disturbed output current, and the disturbed output voltage. 
     At block S 5 , the frequency of the current disturbance signal is varied. The output current of the electrochemical energy storage apparatus  22  is disturbed again by the varying frequency of the current disturbance signal thereby achieving an electrochemical impedance spectroscopy of the electrochemical energy storage apparatus  22 . 
     Before and during the analyzing of the electrochemical impedance spectroscopy, the first DC/DC converter  202  works normally to output electric energy from the electrochemical energy storage apparatus  22  to the electrical load. Referring to  FIG. 12 , the working process of the first DC/DC converter  202  comprises steps shown in blocks S 1   a  to S 1   c.    
     At block S 1   a , a control mode of the first DC/DC converter  202  and a target output value are selected according to the need of the electrical load. 
     At block S 1   b , the output current and output voltage of the electrochemical energy storage apparatus  22 , and the output current and output voltage of the first DC/DC converter  202  are detected. 
     At block S 1   c , the output current and output voltage of the first DC/DC converter  202  detected in block S 1   b  are compared with the target output values, wherein 
     if the output current and output voltage are substantially equal to the target output values, then the first DC/DC converter  202  continues the same output of electric energy to the electrical load, and 
     if the output current and output voltage are not substantially equal to the target output values, then the controller  218  controls switches in the first DC/DC converter  202  to increase the output current and output voltage of the first DC/DC converter  202  to the target output values. The output current and output voltage can be up to 5% larger or 5% smaller than the target output values. 
     At block S 1   a , the control mode, such as a current output or a voltage output, is selected according to the need of the electrical load. The output currents or output voltages of the electrochemical energy storage apparatus  22  and the first DC/DC converter  202  in the following steps are detected in the selected mode. The target output value is determined according to the need of the electrical load. 
     At block S 1   c , when the output current and output voltage are not substantially equal to the target output values, the time periods of on and off of the switches in the first DC/DC converter  202  can be controlled by the controller  218  to have the electrochemical energy storage apparatus  22  output a current or voltage, or both, to be equal to the target output current or voltage. 
     Referring to  FIG. 13  and  FIG. 14 , the step shown in block S 1  can further comprise steps shown in blocks S 11  to S 16 . 
     At block S 11 , a determination is made as to whether an analyzing of the electrochemical impedance is required, wherein 
     if the answer is yes, the second DC/DC converter  204  is electrically conducted, and meanwhile the step in block S 12  is performed; and 
     if the answer is no, the second DC/DC converter  204  is cut off (i.e., short) from the circuit. At this situation, the second DC/DC converter  204  is not electrified. 
     At block S 12 , a frequency of the current disturbance signal used in the analyzing of the electrochemical impedance is selected. 
     At block S 13 , an amplitude of the current disturbance signal to correspond to the required frequency is determined. 
     At block S 14 , the current disturbance signal is defined according to the amplitude and the frequency. 
     At block S 15 , the output current of the electrochemical energy storage apparatus  22  and the input current of the second DC/DC converter  204  are detected. 
     At block S 16 , a determination is made whether the input current of the second DC/DC converter  204  is substantially equal to the current disturbance signal, wherein 
     if the answer is no, then the controller  218  adjusts the switches in the second DC/DC converter  204  to achieve the input current of the second DC/DC converter  204  being substantially equal to the current disturbance signal. The time periods of on and off of the switches in the second DC/DC converter  204  are adjusted. For the second DC/DC converter  204   s  shown in  FIG. 8  and  FIG. 9 , the “on” time of the switches is extended to increase the input current of the second DC/DC converter  204 , and vice versa. 
     The process of block S 12  can further comprise determining whether the frequency used in the analyzing of the electrochemical impedance is a single frequency, wherein 
     if the frequency is a single frequency, then steps in blocks S 13  to S 16  are performed; and 
     if there are multiple frequencies, then steps in blocks S 12   a  to S 12   d  are performed: in S 12   a  each amplitude of the current disturbance signal is determined corresponding to each of the multiple frequencies; 
     in S 12   b , a plurality of current disturbance signals are formed; 
     in S 12   c , the plurality of current disturbance signals are superposed into a mixed current disturbance signal; and 
     in S 12   d , steps S 15  to S 16  are performed. 
     At step S 15 , the purpose for detecting the output current of the electrochemical energy storage apparatus  22  is to determine whether or not the amplitude of the disturbed output current of the electrochemical energy storage apparatus  22  is the same as the amplitude of the current disturbance signal. If not the same, the current disturbance signal can be further adjusted to render the amplitude of the disturbed output current of the electrochemical energy storage apparatus  22  the same as that of the current disturbance signal. 
     At block S 16 , a confirmation can be further made, based on the disturbed output current of the electrochemical energy storage apparatus  22 , that the current disturbance signal does not cause a shortfall in the level of power required by the electrical load. 
     At block S 1 , the current disturbance signal can be a sine wave having a relatively small amplitude, which both avoids a shortfall in power to the electrical load and forms a linear relationship with the response of the integrated DC/DC converter  200 , to facilitate the mathematical processing of the detected data. 
     The value of the amplitude of the current disturbance signal can be 1% to 10% of the output current of the electrochemical energy storage apparatus  22 . In one embodiment, the value of the amplitude of the current disturbance signal is 5% of the output current of the electrochemical energy storage apparatus  22 . 
     At block S 2 , the disturbance is carried out by applying the current disturbance signal to the output current of the electrochemical energy storage apparatus  22 . The electrochemical energy storage apparatus  22  can generate a signal in response (i.e., the disturbed output voltage) having the same frequency as the current disturbance signal. The electrochemical impedance at the selected frequency can be calculated by using the signal in response generated by the electrochemical energy storage apparatus  22  and the current disturbance signal. 
     Referring to  FIG. 15 , to precisely calculate the electrochemical impedance at the selected frequency, the process of blocks S 3  can further comprise processes in blocks S 31  to S 34 . 
     At block S 31 , the output current of the electrochemical energy storage apparatus  22  and the input current of the second DC/DC converter  204  are continuously recorded for a period of time. There is a response time period between the moment of applying the current disturbance signal to the output current of the electrochemical energy storage apparatus  22  and the generation of the signal in response. Therefore, the output current of the electrochemical energy storage apparatus  22  and the input current of the second DC/DC converter  204  are previously recorded for a period of time as a history, to determine if they are disturbed by the instant disturbance current signal. The period of time at block S 31  is related to the selected frequency. At a relatively high frequency, the period of time can comprise relatively greater number of sine wave periods (such as 10 periods). At a relatively low frequency, the period of time can comprise fewer sine wave periods (such as 2 periods or less). In one embodiment, the period of time can comprise one to ten periods of the sine wave. At block S 31 , the output current of the first DC/DC converter  202  can be further monitored to ensure a continuation of sufficient power to the electrical load. 
     At block S 32 , based on the recorded currents, a determination is made as to whether an analysis of the current disturbance signal can be made, to calculate the electrochemical impedance, wherein 
     if the answer is no or not, then block S 31  process is applied again; and 
     if the answer is yes, then the process in block S 33  is applied. 
     At block S 32 , a determination is made that a signal in response (i.e., the disturbed output current/voltage) is received, thus enabling an analysis of electrochemical impedance to be carried out. 
     At block S 33 , the output current and output voltage of the electrochemical energy storage apparatus  22  are continuously recorded for further period of time, which satisfies the electrochemical impedance analysis and ends as early as possible to decrease power consumption. In one embodiment, the period of time at block S 33  can be smaller than 0.2 seconds. A wave filtering and a Fourier transforming can be further applied to the recorded output current and output voltage at block S 33 . 
     The disturbed output current i formed by applying the current disturbance signal to the output current of the electrochemical energy storage apparatus  22  can be calculated by equation (3).
 
 i=I   1   +ΔI ×sin(2π f×t+φ   1 )  (3)
 
     wherein, I 1  is the standard output current of the electrochemical energy storage apparatus  22  when the current disturbance signal is not applied, ΔI is the amplitude of the current disturbance signal, f is the selected frequency of the current disturbance signal, t is the period of time, and φ 1  is the original phase of the current disturbance signal. 
     The disturbed output voltage u, responding to the current disturbance, can be calculated by equation (4).
 
 u=U   1   +ΔU ×sin(2π f×t+φ   1 +φ)  (4)
 
     wherein, U 1  is the standard output voltage of the electrochemical energy storage apparatus  22  when the current disturbance signal is not applied, ΔU is the amplitude of the voltage response disturbance signal corresponding to the current disturbance signal, and f is the frequency of the response signal, which is equal to the selected frequency of the current disturbance signal. t is the period of time, φ 1  is the original phase of the current disturbance signal, and the value φ is the lacking phase of the response signal compared to the current disturbance signal. 
     At block S 34 , the electrochemical impedance and phase at the selected frequency are calculated based on the output current and output voltage. 
     The electrochemical impedance of the electrochemical energy storage apparatus  22  at the selected frequency f can be calculated by this equation (5). 
     
       
         
           
             
               
                 
                   
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     wherein 
               Δ   ⁢           ⁢   U       Δ   ⁢           ⁢   I           
is the amplitude of the electrochemical impedance at the selected frequency f, and j is an imaginary unit.
 
     By varying the frequency, the electrochemical impedance of the electrochemical energy storage apparatus  22  at different frequencies can be calculated. Thereby, an electrochemical impedance spectroscopy of the electrochemical energy storage apparatus  22  can be achieved. When the electrochemical energy storage apparatus  22  comprises a plurality of electrochemical energy storage cells, the output voltage and output current of each of the electrochemical energy storage cells can be detected, and the electrochemical impedance spectroscopy of each of the electrochemical energy storage cells can be achieved by the above method. 
       FIG. 15  presents a flowchart in accordance with an illustrated example embodiment. The embodiment of a method  400  for analyzing the working state of the electrochemical energy storage apparatus  22  is provided by way of example, as there are a variety of ways to carry out the method  400 . Each block shown in  FIG. 15  represents one or more processes, methods, or subroutines carried out in the exemplary method  400 . Additionally, the illustrated order of blocks is by example only and the order of the blocks can be changed. The exemplary method  400  can begin at block T 1 . Depending on the embodiment, additional steps can be added, others removed, and the ordering of the steps can be changed. 
     At block T 1 , a typical electrochemical impedance spectroscopy is provided. The typical electrochemical impedance spectroscopy comprises a plurality of typical frequencies and corresponding impedances, which are capable of reflecting the working state of a specific element of an ideal electrochemical energy storage apparatus. The typical frequencies and corresponding impedances for reflecting the working state of each element in the ideal electrochemical energy storage apparatus can be provided. 
     At block T 2 , an actual electrochemical impedance spectroscopy of the target electrochemical energy storage apparatus  22  is obtained by the above mentioned method  300 . The type of the target electrochemical energy storage apparatus  22  in the actual spectroscopy is the same type as the ideal electrochemical energy storage apparatus. 
     At block T 3 , the actual electrochemical impedance spectroscopy is compared with the typical electrochemical impedance spectroscopy to analyze the working states of specific elements of the target electrochemical energy storage apparatus  22 . 
     At block T 1 , the typical electrochemical impedance spectroscopy can be obtained by achieving a number of electrochemical impedances corresponding to the number of typical frequencies of the ideal electrochemical energy storage apparatus which is in a relatively good working state and in a relatively ideal working environment. The typical electrochemical impedance spectroscopy can be obtained by using the above method  300  for analyzing electrochemical impedance spectroscopy. In the typical electrochemical impedance spectroscopy, the typical frequencies and corresponding impedances can reflect a good working state of each element of the ideal electrochemical energy storage apparatus. 
     At block T 3 , by comparing the typical electrochemical impedance spectroscopy with the actual electrochemical impedance spectroscopy, the working state of each element of the target electrochemical energy storage apparatus  22  can be evaluated to determine whether any changes should be made to keep the target electrochemical energy storage apparatus  22  in a good working state. 
     In one embodiment, only one or more electrochemical impedances, be it or they the actual target or typical, at specific frequencies relating to the working state of each element of the electrochemical energy storage apparatus are achieved. 
     The integrated DC/DC converter of the present disclosure can freely adjust the output of the electrochemical energy storage apparatus, and can in real time monitor and adjust the working state of the electrochemical energy storage apparatus. By the second DC/DC converter applying current disturbance signals having different frequencies to the output end of the electrochemical energy storage apparatus, and detecting the disturbed output current and voltage of the electrochemical energy storage apparatus, an electrochemical impedance spectroscopy of the electrochemical energy storage apparatus can be obtained. The electrochemical impedance spectroscopy enables the working state of the electrochemical energy storage apparatus to be analyzed. According to the analysis, the working conditions/environment of the electrochemical energy storage apparatus can be adjusted to maintain the apparatus working in a good state. The integrated DC/DC converter has low cost, small size, and can be used in vehicle. 
     Example 1 
     The electrochemical energy storage apparatus  22  in Example 1 is a fuel cell stack. Referring to  FIG. 16  and  FIG. 17 , a current disturbance signal having a relatively small amplitude is used to disturb the output current of the fuel cell stack. The fuel cell stack can reveal a linear character at working point A when the amplitude of the current disturbance signal is relatively small. The electrochemical impedance of the fuel cell stack can be calculated by equation (5). Referring to  FIG. 18 , the specific frequencies and corresponding impedances reflect the working state of elements in the fuel cell stack. 
     Specifically, an impedance of the fuel cell stack at a relatively low frequency f 0  represents the impedance caused by substance transfers within the fuel cell stack, which reflects transferring speed of the reacting substances to reach the catalyst layer in the fuel cell. A typical frequency of the f 0  is 0.1 Hz. The impedance at the low frequency f 0  increases when the cathode/anode or the gas diffusion layer are blocked by liquid water, or when the reactant gases (e.g., H2 and O2) have a low pressure. 
     An impedance of the fuel cell stack at a medium frequency f 1  characterizes the dynamics of catalyst in the fuel cell. A typical frequency of the f 0  is 4 Hz. The impedance at the medium frequency f 1  increases when the catalyst is absent or rendered invalid (e.g., catalyst poisoning by CO gas). 
     An impedance of the fuel cell stack at a relatively high frequency f 2  represents the capacitive impedance in the fuel cell. A typical frequency of the f 2  is 1 kHz. The impedance at the high frequency f 2  increases when the assembly of the fuel cell is loose or the current collector is corroded. Meanwhile, the relatively high frequency f 2  also indicates the amount of water in the proton exchange membrane. The impedance at the high frequency f 2  increases when the proton exchange membrane of the fuel cell is saturated with water or dry. 
     Depending on the embodiment, certain of the steps of methods described may be removed, others may be added, and the sequence of steps may be altered. It is also to be understood that the description and the claims drawn to a method may comprise some indication in reference to certain steps. However, the indication used is only to be viewed for identification purposes and not as a suggestion as to an order for the steps. 
     The embodiments shown and described above are only examples. Even though numerous characteristics and advantages of the present technology have been set forth in the foregoing description, together with details of the structure and function of the present disclosure, the disclosure is illustrative only, and changes may be made in the detail, especially in matters of shape, size, and arrangement of the parts within the principles of the present disclosure, up to and including the full extent established by the broad general meaning of the terms used in the claims. It will therefore be appreciated that the embodiments described above may be modified within the scope of the claims.