Patent Publication Number: US-9837843-B2

Title: Voltage grouping of energy storage units

Description:
FIELD OF THE INVENTION 
     The present subject matter relates generally to energy storage systems and more particularly, to energy storage systems that can selectively couple energy storage units to buses. 
     BACKGROUND OF THE INVENTION 
     Energy storage systems (e.g., battery energy storage systems) have become increasingly used to deliver power either as part of standalone energy storage systems or as part of power generation systems (e.g., a wind farm, solar farm, gas turbine system) with an integrated energy storage system. Energy storage systems are unique in that energy storage systems have the ability to both deliver and reserve energy for particular services. Energy storage systems can include one or more battery banks that can be coupled to the grid or other load via a suitable power converter. 
     Multiple batteries can be coupled to the same power converter via the same conversion bus. However, batteries that operate at a lower voltage (e.g., due to cell failure) can experience overvoltage as a result of being coupled to the same bus as higher voltage batteries. While individual power conversion for each battery can help reduce overvoltage, it can lead to significant cost. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of the embodiments of the present disclosure will be set forth in part in the following description, or can be learned from the description, or can be learned through practice of the embodiments. 
     One example aspect of the present disclosure is directed to an energy storage system. The energy storage system includes a plurality of energy storage units and a plurality of buses. The energy storage system further includes a control system configured to receive one or more signals indicative of a voltage associated with each energy storage unit of the plurality of energy storage units. The control system can be configured to send one or more command signals to selectively couple each energy storage unit to one of the plurality of buses based at least on the voltage associated with the energy storage unit. 
     Another example aspect of the present disclosure is directed to a method of controlling an energy storage system. The energy storage system includes a plurality of energy storage units and a plurality of buses. The method includes receiving, by one or more control devices, one or more signals indicative of a voltage associated with each of the plurality of energy storage units. The method further includes detecting, by the one or more control devices, a change in the voltage of at least one of the plurality of energy storage units. The method includes sending, by the one or more control devices, one or more command signals to selectively coupling, by the one or more control devices, one or more of the plurality of energy storage units among the plurality of buses such that energy storage units associated with substantially similar voltage are coupled to the same bus. 
     Yet another example aspect of the present disclosure is directed to a control system for an energy storage system. The control system includes one or more processors and one or more memory devices. The one or more memory devices can store computer-readable instructions that when executed by the one or more processors cause the one or more processors to perform operations. The operations include receiving one or more signals indicative of a voltage associated with each of the plurality of energy storage units and detecting a change in the voltage of at least one of the plurality of energy storage units. The operations further include sending one or more command signals to selectively couple one or more of the plurality of energy storage units among the plurality of buses such that energy storage units associated with substantially similar voltage are coupled to the same bus. 
     Variations and modifications can be made to these example aspects of the present disclosure. 
     These and other features, aspects and advantages of various embodiments will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present disclosure and, together with the description, serve to explain the related principles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Detailed discussion of embodiments directed to one of ordinary skill in the art are set forth in the specification, which makes reference to the appended figures, in which: 
         FIG. 1  depicts an energy storage system according to example embodiments of the present disclosure; 
         FIG. 2  depicts a control system according to example embodiments of the present disclosure; 
         FIG. 3  depicts a flow diagram of an example method according to example embodiments of the present disclosure; 
         FIG. 4  depicts an energy storage unit according to example embodiments of the present disclosure; 
         FIG. 5  depicts an energy storage unit according to example embodiments of the present disclosure; 
         FIG. 6  depicts an energy storage unit according to example embodiments of the present disclosure; 
         FIG. 7  depicts an energy storage unit according to example embodiments of the present disclosure; and 
         FIG. 8  depicts a flow diagram of an example method according to example embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference now will be made in detail to embodiments of the present disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the present disclosure, not limitation of the present disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the present disclosure. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     Example aspects of the present disclosure are directed to selectively coupling energy storage units of an energy storage system to a plurality of buses to improve power conversion. More particularly, an energy storage system can include a plurality of energy storage units and a plurality of buses. Each energy storage unit can be associated with a voltage and each bus can be associated with a power converter. The energy storage system can include a master control system that can be configured to send one or more command signals to selectively couple each energy storage unit to one of the plurality of buses based at least on the voltage associated with the energy storage unit. 
     More particularly, the master control system can be configured to receive one or more signals from each energy storage unit (and/or an individual control systems associated therewith) indicative of a voltage associated with each of the energy storage units. Based at least on the voltage, the master control system can be configured to send one or more command signal to selectively couple and/or de-couple (e.g., via switches, contactors, or other elements) each energy storage unit to one or more buses. Moreover, the master control system can be configured to group each energy storage unit such that energy storage units of substantially similar voltages are included in a same group. The master control system can be configured to send one or more command signals to couple energy storage units within the same group to the same bus to balance the open circuit voltages among the energy storage units coupled in parallel. 
     The master control system can also, and/or alternatively, be configured to detect a change in a voltage associated with an energy storage unit. The change in the voltage can be, for instance, a reduction in voltage due to an energy storage cell failure. In response to the change in the voltage, the master control system can be configured to send one or more command signals to de-couple one or more energy storage units from a first bus and to couple the one or more energy storage units to a second bus. The second bus can be coupled to other energy storage units of substantially similar voltages. 
     Selectively coupling energy storage units to conversion buses according to example aspects of the present disclosure can improve power conversion without incurring the significant cost of individually converting power for each energy storage unit. Moreover, by selectively coupling energy storage units of substantially similar voltages to the same bus, the present disclosure can help balance the open circuit voltage among the energy storage units and reduce overvoltage. 
     With reference now to the Figures, example embodiments of the present disclosure will now be discussed in detail.  FIG. 1  depicts an example energy storage system  100  according to example aspects of the present disclosure. The energy storage system  100  can be implemented in a standalone power system or can be implemented as part of a power generation energy system, such as a wind power generation system, solar power generation system, gas turbine power generation system, or other suitable system. 
     The energy storage system  100  can include a plurality of energy storage units  110 , such as battery energy storage units. Each energy storage unit  110  can include one or more string  112 . When an energy storage unit  110  includes more than one string  112 , the strings  112  can be coupled in parallel. Additionally, and/or alternatively, the plurality of energy storage units can be coupled in parallel, connecting strings of different energy storage units in parallel. Each string  112  can include a plurality of cells coupled in series. The term cell can refer to any energy storage device, such as, for example, a battery cell, fuel cell, electrochemical cell, rechargeable cell, ultrabattery, SMES, accumulator, capacitor, pack, etc. The energy storage unit  110  can contain one or more sodium nickel chloride batteries, sodium sulfur batteries, lithium ion batteries, nickel metal hydride batteries, sodium metal halide batteries or other similar devices. Three energy storage units, each with three strings, are illustrated in  FIG. 1  for purposes of illustration and discussion. Those of ordinary skill in the art, using the disclosures provided herein, will understand that any number of energy storage units and/or strings can be used in the energy storage system  100  within deviating from the scope of the present disclosure. 
     Each energy storage unit  110  can include a control system  120 , such as a battery management system (BMS). The control systems  120  can include one or more electronic control devices that monitor the plurality of the string(s)  112 , such as by protecting the string(s)  112  from operating outside a safe operating mode, monitoring a state of the cells, calculating and reporting operating data for the cells, controlling the cells environment, and/or any other suitable control actions. For example, in several embodiments, the control systems  120  are configured to monitor and/or control operation of the string(s)  112 , as described in further detail herein. The control systems  120  can also be configured to send and/or receive one or more signals. For instance, each control system  120  can be configured to monitor a voltage associated with one or more string(s)  112  and/or energy storage unit  110  and to send one or more signals indicative of the voltage associated with the energy storage unit  110 . The control systems  120  can be, for example, a logic controller implemented purely in hardware, a firmware-programmable digital signal processor, or a programmable processor-based software-controlled computer. 
     The energy storage system  100  can include a plurality of power converters  130 . The power converters  130  can each be configured to convert a DC voltage associated with an energy storage unit  110  to suitable AC power for the AC grid (e.g. 50 Hz or 60 Hz power). In some embodiments, the power converters  130  can include a combination of DC to DC converters and DC to AC converters. 
     The power converters  130  can include one or more electronic switching elements, such as insulated gate bipolar transistors (IGBTs). The electronic switching elements can be controlled (e.g., using pulse width modulation) to charge or to discharge the energy storage units  110 . In addition, the electronic switching elements can be controlled to convert the DC power received or provided to the energy storage units  110  to suitable AC power for application to utility grid  150  (e.g., 50 Hz or 60 Hz AC power). The power converters  120  can provide AC power to the grid  150  through a suitable transformer  140  and various other devices, such as switches, relays, contactors, etc. used for protection of the energy storage system  100 . 
     The energy storage system  100  can include a plurality of buses  160 . Each bus  160  can be associated with an individual power converter  130 . Each energy storage unit  110  can be coupled to a bus  160 . For instance, the energy storage system  100  can include a one or more switches  170 . The one or more switches  170  can be configured to couple and/or de-couple one or more energy storage units  110  to one or more buses  160 , as further described herein. The number of buses, power converters, and switches are illustrated in  FIG. 1  for purposes of illustration and discussion. Those of ordinary skill in the art, using the disclosures provided herein, will understand that any number of these components can be used in the energy storage system  100  without deviating from the scope of the present disclosure. 
     The energy storage system  100  can include a master control system  200  that is configured to monitor and/or control various aspects of the energy storage system  100  as shown in  FIGS. 1 and 2 . In accordance with various embodiments, the master control system  200  can include one or more control devices or separate units or can be part of the control systems  120  of the energy storage units  110 . 
     As shown, the master control system  200  can be in communication with the energy storage units  110 , control systems  120 , power converters  130 , buses  160 , and/or switches  170 . The master control system  200  can be configured to send and/or receive one or more signals to and/or from the energy storage units  110 , control systems  120 , power converters  130 , buses  160 , and/or switches  170 . For instance, the control systems  120  and/or energy storage units  110  can be configured to send one or more signals indicative of the voltage associated with the energy storage unit  110  to the master control system  200 . The master control system  200  can be configured to receive the one or more signals from the energy storage units  110  and/or the control systems  120  indicative of a voltage (e.g., an open circuit voltage) associated with each of the energy storage units  110 . 
     The master control system  200  can be configured to send one or more command signals to each energy storage unit  110  and/or control system  120 . For instance, the master control system  200  can be configured to send one or more command signals to each energy storage unit  110  and/or control system  120  to selectively couple and/or de-couple each energy storage unit  110  to one or more of the plurality of buses  160  based at least in part on the voltage associated with the energy storage unit  110 . The master control system  200  can send one or more command signals to the energy storage  110  and/or control system  120  to couple the energy storage unit  110  to a bus  160 A,  160 B,  160 C. The energy storage unit  110  and/or control system  120  can receive the one or more command signals and can adjust the switch  170  to couple the energy storage unit  110  to a bus  160 A,  160 B,  160 C. For instance, the control system  120  can be configured to adjust the switch  170  to an open position (e.g., that does not allow current to flow from the energy storage unit  110  through the bus  160 ) and/or a closed position (e.g., that allows current to flow from the energy storage unit  110  through the bus  160 ) with respect to each of bus  160 A,  160 B,  160 C. By way of example, to de-couple the energy storage unit  110  from the bus  160 A, the control system  120  can adjust the switch  170  to be in an open position with respect to bus  160 A. To couple the energy storage unit  110  to bus  160 B, the control system  120  can adjust the switch  170  to be in a closed position with respect to bus  160 B. 
     Additionally and/or alternatively, the master control system  200  can be configured to group each energy storage unit  110  based at least on the voltage associated with each energy storage unit  110 . For instance, the master control system  200  can be configured to group each energy storage unit  110  such that energy storage units associated with substantially similar voltages are included in a same group (e.g., similar open circuit voltage groups). In some embodiments, the number of groups can be equal to the number of power converters. The master control system  200  can be configured to send one or more command signals to the energy storage units  110  to couple the energy storage units within the same group to the same bus  160 . The control system  120  may adjust the switch  170 , according to the one or more command signals, such that an energy storage unit  110  is coupled to the same bus as other energy storage units in the group. The energy storage units  110  may be assigned to different groups over time as the energy storage units  110  degrade at different rates or if one or more energy storage units  110  are added to and/or replaced in the energy storage system  100 . 
     The master control system  200  can be configured to detect a change in a voltage of the energy storage unit  110 . For instance, the master control system  200  can be configured to detect a change in the voltage based on one or more signals received from the energy storage units  110  and/or the control systems  120 . In response to the change in voltage, the master control system  200  can be configured to send one or more command signal to de-couple one or more energy storage unit  110  from a first bus  160 A (e.g., by adjusting the switch  170  from a closed position to an open position with respect to bus  160 A) and to couple the one or more energy storage units  110  to a second bus  160 B (e.g., by adjusting the switch  170  from an open position to a closed position with respect to bus  160 B). The second bus  160 B can be coupled to other energy storage units associated with substantially similar voltages. In this way, the master control system  200  can help prevent overvoltage conditions. 
     Referring particularly to  FIG. 2 , the master control system  200  can have any number of suitable control devices. The master control system  200  can include a system level controller for the energy storage system  100  and/or a controller of one or more individual energy storage unit  110  or control system  120 . As shown, for example, the master control system  200  (and/or control system  120 ) can include one or more processor(s)  212  and one or more memory device(s)  214  configured to perform a variety of computer-implemented functions and/or instructions (e.g., performing the methods, steps, calculations and the like and storing relevant data as disclosed herein). The instructions when executed by the processor(s)  212  can cause the processor(s)  212  to perform operations according to example aspects of the present disclosure. For instance, the instructions when executed by the processor(s)  212  can cause the processor(s)  212  to implement one or more control interfaces. 
     Additionally, the master control system  200  can include a communications interface  216  to facilitate communications between the master control system  200  and the various components of the energy storage system  100 . For example, the communications interface can permit the transmission of signals to and/or from energy storage units  110 , control systems  120 , power converters  130 , buses  160 , and/or switches  170 . The signals can be communicated using any suitable communications protocol. As such, the processor(s)  212  can be configured to receive and/or send one or more signals from the energy storage units  110 , control systems  120 , power converters  130 , buses  160 , and/or switches  170 . 
     As used herein, the term “processor” refers not only to integrated circuits referred to in the art as being included in a computer, but also refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits. Additionally, and/or alternatively, the memory device(s)  214  can generally include memory element(s) including, but not limited to, computer readable medium (e.g., random access memory (RAM)), computer readable non-volatile medium (e.g., a flash memory), a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), a digital versatile disc (DVD) and/or other suitable memory elements. Such memory device(s)  214  can generally be configured to store suitable computer-readable instructions that, when implemented by the processor(s)  212 , configure the master control system  200  to perform the various functions as described herein. 
       FIG. 3  depicts a flow diagram of an example method  300  for controlling an energy storage system according to example embodiments of the present disclosure. The method can be implemented in any suitable energy storage system, such as the energy storage system  100  of  FIG. 1 . In addition,  FIG. 3  depicts steps performed in a particular order for purposes of illustration and discussion. Those of ordinary skill in the art, using the disclosures provided herein, will understand that various steps of any of the methods disclosed herein can be omitted, rearranged, modified, expanded, or adapted in various ways without deviating from the scope of the present disclosure. 
     At ( 302 ), the method includes receiving one or more signals indicative of a voltage associated with each of the plurality of energy storage units  110 . For instance, the control systems  120  can monitor a voltage associated with the energy storage units  110  and/or string(s)  112 . The control systems  120  and/or energy storage units  110  can send one or more signals indicative of the voltage associated with an energy storage unit  110  to the master control system  200 . The master control system  200  can receive one or more signals, for instance, from the each energy storage unit  110  and/or control system  120 , indicative of a voltage (e.g., open circuit voltage) associated with each of the energy storage units  110 . 
     At ( 304 ), the method includes detecting a change in the voltage of at least one of the plurality of energy storage units  110 . For instance, the master control system  200  can detect a change in the voltage. The change in the voltage can be a reduction in voltage (e.g., open circuit voltage) due to a cell failure. In example embodiments, the master control system  200  can detect the change in the voltage based on the one or more signals received by the master control system  200 . In addition, and/or in the alternative, the master control system  200  can monitor a voltage associated with each energy storage unit  110  and detect a change in voltage based on such monitoring. 
     At ( 306 ), the method includes sending one or more command signal to selectively couple one or more of the plurality of energy storage units  110  among the plurality of buses  160  such that energy storage units  110  associated with substantially similar voltage are coupled to the same bus  160 . The master control system  200  can send one or more command signal to the energy storage units  110  and/or control systems  120  to selectively couple each energy storage unit  110  to a bus  160  by, for instance, closing a switch  170 . Each energy storage unit  110  can be coupled to a bus  160  such that energy storage units associated with substantially similar voltages are coupled to the same bus. 
     As one example, the method  300  can include grouping each energy storage unit  110  based at least on the voltage associated with each energy storage unit  110 , such that energy storage units associated with substantially similar voltages are included in a same group. The method  300  can include sending one or more command signal to couple the energy storage units  110  within the same group to the same bus. The master control system  200  can group each energy storage unit  110  such that energy storage units associated with substantially similar voltages can be included in the same group. Moreover, the master control system  200  can send one or more command signals to the energy storage units  110  and/or control systems  120  to couple (e.g., via the switches, contactors, or other elements) the energy storage units  110  within the same group to the same bus. In this way, the master control system  200  can help prevent overvoltage. 
     As another example, the method  300  can include sending one or more command signals to de-couple one or more energy storage units  110  from a first bus  160 A and sending one or more command signals to couple the one or more energy storage units  110  to a second bus  160 B of the plurality of buses in response to the change in the voltage. The master control system  200  can send one or more command signals (e.g., to the energy storage units  110  and/or control systems  120 ) to de-couple one or more energy storage units  110  from a first bus  160 A. For instance, the energy storage units  110  and/or control systems  120  can de-couple the energy storage units  110  from a first bus  160 A by adjusting the switch  170  from a closed position to an open position with respect to the first bus  160 A. 
     The master control system  200  can send one or more command signals (e.g., to the energy storage units  110  and/or control systems  120 ) to couple the energy storage unit  110  to a second bus  160 B in response to the change in the voltage. For instance, based on the change in voltage (e.g., a reduction in voltage) the master control system  200  can select a second bus  160 B that can be coupled to one or more energy storage units associated with substantially similar voltages as the energy storage unit  110  associated with the change in voltage. The energy storage unit  110  and/or control system  120  can couple the energy storage unit  110  to the second bus  160 B by adjusting the switch  170  from an open position to a closed position with respect to second bus  160 B. In this way, the master control system  200  can help prevent overvoltage of an energy storage unit  110  that experiences a change in voltage (e.g., reduction in voltage). 
       FIGS. 4-7  depict an example energy storage unit  110  according to example aspects of the present disclosure. The energy storage unit  110  can include one or more strings  112 A,  112 B and a control system  120  (e.g., a battery management system). In the event that the energy storage unit has more than one string, the strings  112 A,  112 B can be coupled in parallel. Each string  112 A,  112 B can include a plurality of cells  113 ,  114  coupled in series. In  FIGS. 4-7  and/or the embodiments described herein, the strings  112 A and  112 B can be associated with the same energy storage unit  110  and/or difference energy storage units  110 . Additionally, and/or alternatively, as shown in  FIGS. 4-7  and described herein, the control system  120  can include one or more control devices of an energy storage unit  110  and/or one or more control devices of different energy storage units  110 . 
     Each string  112 A,  112 B can be associated with a selectively adjustable tap location to control the number of cells  113 ,  114  in the string that provide power to a power system. While each cell  113 ,  114  can be configured to provide power to a power system, the control system  120  can control which of the cells  113 ,  114  do, in fact, provide power to the power system by adjusting the tap location. For instance, each cell  113 ,  114  can be associated with a tap  115 ,  116  (e.g., switch, transistor, contactor, etc.) configured to prevent and/or allow the cells  113 ,  114  (and all cells previously in series) to provide power to a power system. The tap location can be a location at which a tap is closed, thereby allowing cells to provide power through a tap. 
     By way of example, as shown in  FIG. 4 , tap  115 B is closed and can allow cells  113 A and  113 B (and any cells coupled in series below  113 A) to provide power to a power system, through tap  115 B. Tap  115 C is open and can prevent cell  113 C from providing power to a power system. The tap location can be associated with the location of tap  115 B through which the cells  113 A,  113 B (and any cells coupled in series below  113 A) can provide power to a power system. In this way, the cells  113 A,  113 B can be included in the number of cells that provide power to a power system, while cell  113 C can be excluded from the number of cells that provide power to a power system. 
     The control system  120  can be configured to be in communication with each of the string(s)  112  and the taps  115 ,  116 . The control system  120  can be configured to monitor a voltage associated with each of the string(s)  112 A,  112 B and to detect a change in a voltage associated with one or more of the string(s)  112 A,  112 B. The change in the voltage can be a reduction in voltage associated with a cell failure. The control system  120  can be configured to adjust the tap location for at least one of the string(s)  112 A,  112 B in response to the change in the voltage. 
     In one example, one or more energy storage units  110  can include a first string  112 A comprising a first plurality of cells  113 A,  113 B,  113 C coupled in series and a second string  112 B comprising a second plurality of cells  114 A,  114 B,  114 C coupled in series. The first string  112 A and the second string  112 B can be coupled in parallel. If a first cell  113 B of the first string  112 A fails (e.g., fails short) there can be a change in the voltage (e.g., a reduction in the voltage) associated with the first string  112 A. The control system  120  can detect the change in the voltage and can be configured to adjust the tap location for at least one of the string(s)  112 A,  112 B to balance the open circuit voltage. 
     For instance, when the change in the voltage is a voltage reduction associated with the first string  112 A, the control system  120  can be configured to adjust the tap location associated with the first string  112 A to increase the number of cells  113  of the first string  112 A that provide power through the tap location. The control system  120  can adjust the tap location associated with the first string  112 A by adjusting a first tap  115 B from a closed position (e.g.,  FIG. 4 ) to an open position (e.g.,  FIG. 5 ) and adjusting a second tap  115 C from an open position (e.g.,  FIG. 4 ) and to a closed position (e.g.,  FIG. 5 ). Such adjustment can allow cell  113 C to provide power through the tap location (e.g., at the closed second tap  115 C of  FIG. 5 ). In this way, the voltage associated with the first string  112 A can be increased such that it can be substantially similar to the voltage associated with the first string  112 A prior to the cell failure. Moreover, by adjusting the tap location associated with the first string  112 A, the control system  120  can balance the open circuit voltage among the plurality of strings such that the open circuit voltages among the plurality of strings are substantially similar (e.g., within 20% of each other). For instance, the voltage of the first string  112 A can be substantially similar to the voltage of the second string  112 B. Such balancing can help avoid overvoltage of the first string  112 A. 
     Additionally and/or alternatively, when the first string  112 A experiences a change in voltage (e.g., reduction in voltage), the control system  120  can be configured to reduce the voltage associated with the second string  112 B. For instance, the control system  120  can adjust the tap location associated with the second string  112 B to decrease the number of cells  114  of the second string  112 B that provide power through the tap location. The control system  120  can adjust the tap location associated with the second string  112 B by adjusting a first tap  116 B from a closed position (e.g.,  FIG. 6 ) to an open position (e.g.,  FIG. 7 ) and adjusting a second tap  116 A from an open position (e.g.,  FIG. 6 ) to a closed position (e.g.,  FIG. 7 ). Such adjustment can prevent the cell  114 B from providing power through the tap location (e.g., at the closed second tap  116 A) and can reduce the voltage associated with the second string  112 B. The voltage associated with the second string  112 B can be reduced such that it is substantially similar to the voltage associated with the first string  112 A after the cell failure. Additionally, and/or alternatively, the control system  120  can adjust a tap location associated with each string (other than the first string  112 A) of the plurality of strings in a similar manner to reduce the voltage associated with each string. In this way, the control system  120  can balance the open circuit voltages among the plurality of strings such that the open circuit voltages among the plurality of strings are substantially similar. 
       FIG. 8  depicts a flow diagram of an example method  800  for controlling a tap location associated with an energy storage unit according to example embodiments of the present disclosure. The method  800  can be implemented in any suitable energy storage unit, such as the energy storage unit  110  of  FIGS. 4-7 . In addition,  FIG. 8  depicts steps performed in a particular order for purposes of illustration and discussion. Those of ordinary skill in the art, using the disclosures provided herein, will understand that various steps of any of the methods disclosed herein can be omitted, rearranged, modified, expanded, or adapted in various ways without deviating from the scope of the present disclosure. 
     At ( 802 ), the method  800  includes monitoring a voltage associated with a first string  112 A comprising a first plurality of cells  113  coupled in series. At ( 804 ), the method  800  includes monitoring a voltage associated with a second string  112 B comprising a second plurality of cells  114  coupled in series. The first string  112 A and the second string  112 B can be coupled in parallel. For instance, a control system  120  can monitor the voltage associated with the first string  112 A and/or the voltage associated with the second string  112 B. The voltage can be an open circuit voltage. In addition, and/or in the alternative, the control system  120  can monitor a voltage associated with one or more cells of the first and/or second plurality of cells  113 ,  114 . 
     At ( 806 ), the method  800  includes detecting a change in a voltage associated with the first string  112 A. For instance, the control system  120  can detect a change in the voltage associated with the first string  112 A. The change in the voltage can be a voltage reduction associated with the first string  112 A that can occur when a cell  113  fails (e.g., fails short). 
     At ( 808 ), the method  800  includes adjusting a tap location for the first string  112 A or second string  112 B in response to the change in the voltage. For instance, the control system  120  can adjust a tap location for the first string  112 A and/or second string  112 B. In one example, the method  800  can include adjusting the tap location associated with the first string  112 A to increase the number of cells that provide power through the tap location associated with the first string  112 A. Prior to the change in the voltage of the first string  112 A, the first string  112 A can be associated with a tap location at a first tap  115 B that is closed, as shown in  FIG. 4 . When the voltage associated with the first string  112 A changes (e.g., reduces), the control system  120  can adjust the tap location associated with the first string  112 A by adjusting the first tap  115 B from a closed position to an open position and adjusting a second tap  115 C from an open position to a closed position, as shown in  FIGS. 4 and 5 . By adjusting the tap location to the second tap  115 C, the control system  120  can allow an additional cell  113 C (that was not providing power through the tap location prior to the change in voltage) to provide power through the tap location (e.g., at the closed second tap  115 C). In this way, the control system  120  can increase the voltage associated with the first string  112 A and balance the open circuit voltage among the plurality of strings. 
     In addition, and/or in the alternative, the method  800  can include adjusting the tap location for the second string  112 B to decrease the number of cells  114  that provide power through the tap location for the second string  112 B. For instance, prior to the change in the voltage associated with the first string  112 A, the second string  112 B can have a tap location at a first tap  116 B, as shown in  FIG. 6 . When the voltage associated with the first string  112 A changes (e.g., reduces), the control system  120  can adjust the tap location associated with the second string  112 B by adjusting the first tap  116 B from a closed position to an open position and adjusting a second tap  116 A from an open position to a closed position, as shown in  FIGS. 6 and 7 . The tap location can now be associated with the closed second tap  116 A and can prevent cell  114 B (that was previously providing power prior to the change in voltage of first string  112 A) from providing power through the tap location (e.g., at second tap  116 A). In this way, the control system  120  can decrease the voltage associated with the second string  112 B and balance the open circuit voltages of the first and second string(s)  112 A,  112 B. 
     Although specific features of various embodiments can be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the present disclosure, any feature of a drawing can be referenced and/or claimed in combination with any feature of any other drawing. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and can include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.