Patent Publication Number: US-2023155395-A1

Title: System and method with a direct current to direct current (dc/dc) converter

Description:
TECHNICAL FIELD 
     Embodiments of this disclosure relate to a battery system and, more particularly, to a battery system that includes a direct current to direct current (DC/DC) converter. 
     BACKGROUND 
     High voltage batteries (e.g., batteries of 100 volts of direct current (Vdc) potential or more that are used to supply direct current (DC)) are used to recharge and sustain low voltage operations (e.g., operations that rely on about 50 Vdc or less) while a vehicle is in operation. For instance, these types of systems may use a standalone DC/DC converter that is external to the high voltage battery system to convert the high voltage DC to low voltage DC. While the vehicle is in an idle state (high voltage deactivated), the high voltage bus bar is not energized, and the contactors within the battery packs are disengaged for safety. This can help prevent accidental exposure to high voltage, as well as reduce the risk of a high energy short. This practice, however, does prevent the energy in the high voltage batteries from being used to sustain various important devices and systems that are activated even when the vehicle is not being used (e.g., “keep alive” systems, such as a fire suppression system or an anti-theft system). By using an external DC/DC converter, the pack contactors have to be energized to deliver power, nullifying the safety of the previously described practice. This also increases an electric load of an idle vehicle by the amount of energy needed to keep the battery contactors activated, thereby causing the vehicle to consume excess power. 
     In addition, battery systems that include strings of multiple battery packs connected in series can experience issues where battery pack voltages within a string are not equal. This can produce operational problems because the individual battery cell voltages are closer to the upper and lower extremes than the rest of the battery system, which can limit charging and discharging power. Such battery pack imbalance problems can be addressed in the maintenance shop, typically by connecting an external equipment to one or more of the battery packs in the battery system, and discharging the excess energy from those battery packs to reduce their stored energy and bring their stored energy into balance with the rest of the battery packs in the string. The whole string of battery packs may then be charged up independently before it is reconnected to another parallel string on the vehicle. Thus, balancing stored energy levels among multiple battery packs of a vehicle requires external equipment and can waste stored energy in the form of discharged energy. Embodiments of the current disclosure may address these limitations and/or other problems in the art. 
     SUMMARY 
     Embodiments of the present disclosure relate to, among other things, battery systems for electric vehicles. Each of the embodiments disclosed herein may include one or more of the features described in connection with any of the other disclosed embodiments. 
     In one embodiment, a battery system may include at least one battery pack including a direct current to direct current (DC/DC) converter and at least one battery cell. A positive terminal and a negative terminal of the at least one battery cell may be electrically connected to a positive terminal and a negative terminal, respectively, associated with the DC/DC converter. The battery system may further include a high voltage bus bar electrically connected to the positive terminal and the negative terminal of the at least one battery cell and a low voltage bus bar electrically connected to the DC/DC converter. The DC/DC converter may be configured to at least one of import power to the at least one battery cell from the low voltage bus bar or export the power from the at least one battery cell to the low voltage bus bar. The battery system may additionally include a communication bus bar electrically connected to the DC/DC converter and at least one computing system configured to communicate with the DC/DC converter via the communication bus bar. 
     In another embodiment, a method of using a direct current to direct current (DC/DC) converter located within a battery pack of a battery system, where the DC/DC converter may be electrically connected to a low voltage bar and to one or more battery cells of the battery pack, may include receiving, by a computing system, an instruction to activate the DC/DC converter. The method may further include sending one or more instructions to the DC/DC converter. The one or more instructions may be associated with configuring the DC/DC converter at least to operate based on a set of parameters including a direction or an amount of power flow import to or export from the battery pack. 
     In another embodiment, a method for balancing stored energy levels among a plurality of battery packs of a battery system may include receiving, by a computing system, one or more first instructions to activate a plurality of direct current to direct current (DC/DC) converters. Each of the plurality of battery packs may include at least one of the plurality of DC/DC converters. The method may additionally include receiving information related to the stored energy levels of the plurality of battery packs. The method may further include determining, for each of the plurality of battery packs, a direction of power flow and an amount of the power flow to balance the stored energy levels among the plurality of battery packs and sending one or more second instructions to each of the plurality of DC/DC converters. The one or more second instructions may be associated with configuring the each of the plurality of DC/DC converters to operate based on a set of parameters including the direction and the amount of the power flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the present disclosure and together with the description, serve to explain the principles of the disclosure. 
         FIGS.  1 A and  1 B  illustrate an exemplary electric bus having a battery system, according to the present disclosure; 
         FIG.  2    is a schematic illustration of an exemplary battery system of the bus of  FIGS.  1 A and  1 B , according to the present disclosure; 
         FIG.  3    is a schematic illustration of an exemplary battery module of the battery system of  FIG.  2   , according to the present disclosure; 
         FIG.  4    is a schematic illustration of connections between the battery pack of  FIG.  2    and peripheral devices or systems of the bus of  FIGS.  1 A and  1 B , according to the present disclosure; 
         FIG.  5    is a schematic illustration of a battery pack of the battery system of  FIG.  2    that includes a DC/DC converter, according to the present disclosure; 
         FIG.  6    is another schematic illustration of a battery pack of the battery system of  FIG.  2    that includes a DC/DC converter, according to the present disclosure; 
         FIG.  7    is a schematic illustration of power export from battery packs of  FIG.  6    with balanced stored energy levels, according to the present disclosure; 
         FIG.  8    is a schematic illustration of power import to, and power export from, battery packs of  FIG.  6    with imbalanced stored energy levels, according to the present disclosure; 
         FIG.  9    is another schematic illustration of power import to, and power export from, battery packs of  FIG.  6    with imbalanced stored energy levels, according to the present disclosure; 
         FIG.  10    illustrates an exemplary method of enabling a DC/DC converter included in a battery pack of  FIG.  6   , according to the present disclosure; 
         FIG.  11    illustrates an exemplary method of balancing stored energy levels among multiple battery packs of  FIG.  6   , according to the present disclosure; and 
         FIG.  12    illustrates example components of a computing device, according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure describes a system and method for a battery system including a DC/DC converter. While principles of the current disclosure are described with reference to an electric bus, it should be understood that the disclosure is not limited thereto. Rather, the systems and methods of the present disclosure may be used in any vehicle having a battery system (e.g., electric vehicle, electric machine, electric tool, electric appliance, etc.). As used herein, the term “electric vehicle” includes any vehicle or transport machine that is driven at least in part by electricity (e.g., hybrid vehicles, all-electric vehicles, etc.). Heavy duty electric vehicles (e.g., electric buses, electric trucks, electric airplanes, electric boats, etc.) may store and/or consume a large amount of energy compared to smaller electric vehicles (e.g., electric cars, electric bicycles or motorcycles, electric carts, etc.). 
     In this disclosure, relative terms, such as “about,” “substantially,” or “approximately” are used to indicate a possible variation of ±10% of a stated value. 
     Any implementation described herein as exemplary is not to be construed as preferred or advantageous over other implementations. Rather, the term “exemplary” is used in the sense of example or illustrative. 
       FIGS.  1 A and  1 B  illustrate an electric vehicle in the form of a bus  10 .  FIG.  1 A  shows the bus  10  with its roof visible, and  FIG.  1 B  shows the bus  10  with its undercarriage visible. In the discussion below, reference will be made to both  FIGS.  1 A and  1 B . The bus  10  may include a body  12  enclosing a space for passengers. In some embodiments, some (or substantially all) parts of the body  12  may be fabricated using one or more composite materials to reduce the weight of the bus  10 . Without limitation, the body  12  of the bus  10  may have any size, shape, and configuration. In some embodiments, the bus  10  may be a low-floor electric bus. In a low-floor electric bus, there may be no stairs at the front and/or the back doors of the bus  10 . In such a bus  10 , the floor may be positioned close to the road surface to ease entry and exit into the bus  10 . In some embodiments, the floor height of the low-floor bus may be about 30-45 centimeters from the road surface. In this disclosure, the term “about” is used to indicate a possible variation of ±10% in a stated numeric value, however embodiments described herein are not limited to such variation. 
     The bus  10  may include a powertrain  24  that propels the bus  10  along a road surface. The powertrain  24  may include one or more electric motors  22  that generate power, and a transmission that transmits the power to a pair of drive wheels (e.g., wheels  18 ) of the bus  10 . A battery system  14  may store electrical energy to power the electric motors  22  of the powertrain  24 . In some embodiments, the batteries of the battery system  14  may be configured as a plurality of battery packs  20  positioned in cavities located under the floor of the bus  10 . In some embodiments, some or all of the battery packs  20  may be positioned elsewhere (e.g., roof) on the bus  10 . The batteries of the battery system  14  may have any chemistry and construction. The battery chemistry and construction may activate fast charging of the batteries. In some embodiments, the batteries may be lithium titanate oxide (LTO) batteries. In some embodiments, the batteries may be nickel metal cobalt oxide (NMC) batteries. It is also contemplated that, in some embodiments, the batteries may include multiple different chemistries. 
     The bus  10  may include a charging interface. For example, the bus  10  may include a charge port (e.g., an electric socket) that is configured to receive a charging plug and charge the bus  10  using power from a utility grid. In such embodiments, the bus  10  may be charged by connecting the plug to the socket. In some embodiments, the charge port may be a standardized charge port (e.g., a Society of Automotive Engineers (SAE) J1772 charge port) that is configured to receive a corresponding standardized connector (e.g., a SAE J1772 connector). However, in general, the charge port and the mating connector may be of any type and form (custom design or standardized). As illustrated in  FIG.  1 A , to protect the charge port from the environment (rain, snow, debris, etc.), a hinged lid  16  may cover the charge port when not in use. Additionally, or alternatively, a charging interface may be provided on the roof of the bus  10  (not illustrated in  FIGS.  1 A and  1 B ) to charge the batteries of the battery system  14 . For example, the charging interface may include components that interface with a charging head (e.g., an inverted pantograph that interfaces with a set of rails mounted on the forward rooftop of the bus  10 ) of an external charging station to charge the batteries. 
       FIG.  2    is a schematic illustration of an exemplary battery system  14  of the bus  10  of  FIGS.  1 A and  1 B , according to the present disclosure. The battery system  14  may include a plurality of battery packs  20 . Each battery pack  20  may include a plurality of battery modules  34 , and each battery module  34  may include a plurality of battery cells  38  arranged therein. In  FIG.  2   , the inside structure of one of the battery packs  20 , and the inside structure of one of the battery modules  34  of the battery pack  20 , are shown to aid in the discussion below. The battery cells  38  may have any chemistry and construction. In some embodiments, the battery cells  38  may have a lithium-ion chemistry. Lithium-ion chemistry comprises a family of battery chemistries that employ various combinations of anode and cathode materials. In automotive applications, these chemistries may include lithium-nickel-cobalt-aluminum (NCA), lithium-nickel-manganese-cobalt (NMC), lithium-manganese-spinel (LMO), lithium titanate (LTO), and lithium-iron phosphate (LFP), for example. In consumer applications, the battery chemistry may also include lithium-cobalt oxide (LCO), for example. 
     The plurality of battery packs  20  of the battery system  14  may be connected together in series or in parallel. In some embodiments, these battery packs  20  may also be arranged in strings. For example, the battery system  14  may include multiple strings connected in parallel, with each string including multiple battery packs  20  connected together in series. Configuring the battery system  14  as parallel-connected strings may allow the bus  10  to continue operating with one or more strings disconnected if a battery pack  20  in a string fails or experiences a problem. The plurality of battery modules  34  in each battery pack  20 , and the plurality of battery cells  38  in each battery module  34 , may also be electrically connected together in series or parallel. In some embodiments, some of the battery modules  34  in a battery pack  20  may be connected together in series, and groups of the series-connected battery modules  34  connected together in parallel. Similarly, in some embodiments, a group of battery cells  38  in each battery module  34  may be connected together in series to form multiple series-connected groups of battery cells  38 , and these series-connected groups may be connected together in parallel. That is, some or all battery packs  20  in the battery system  14  may include both series-connected and parallel-connected battery modules  34 , and some or all battery modules  34  in each battery pack  20  may include both series-connected and parallel-connected battery cells  38 . In some embodiments, each battery pack  20  of the battery system  14  may be substantially identical (in terms of number of battery modules  34 , number of battery cells  38  in each battery module  34 , how the battery modules  34  are connected, etc.) to each other. In other embodiments, one or more of the battery packs  20  of the battery system  14  may have a different configuration than one or more other battery packs  20  of the battery system  14 . 
     In general, the battery packs  20  of the battery system  14  may be physically arranged in any manner. In some embodiments, the battery packs  20  may be arranged in a single layer on a common horizontal plane to decrease the height of the battery system  14 , so that it may be positioned under the floor of the low-floor bus  10 . For example, the battery packs  20  may have a height less than or equal to about 18 centimeters, to allow the battery system  14  to be accommodated under the floor of the low-floor bus  10 . The low height profile of the battery system  14  may allow the battery system  14  to be more aerodynamic, and may increase its surface area relative to the number of battery cells  38 , which may increase heat dissipation and improve temperature regulation. In general, the battery system  14  may be configured to store any amount of energy and to export or import electrical power (in terms of Watts (W)) at a voltage (V). Increasing the amount of energy stored in the battery system  14  may increase the distance that the bus  10  can travel between recharges. In some embodiments, the number of the battery packs  20 , the battery modules  34 , the battery cells  38 , and the chemistry of the battery cells  38 , etc. may be configured such that the total energy capacity of the battery system  14  may be between, for example, about 200-700 kilowatt hours (KWh). 
     In general, the battery system  14  may have any number (e.g., 1, 2, 3, 4, 6, 8, 10, etc.) of battery packs  20 . In some embodiments, the number of battery packs  20  in the battery system  14  may be between about 2 and 6. Each battery pack  20  may have a protective housing  28  that encloses the plurality of battery modules  34  (and other components of the battery pack  20 ) therein. Although the battery pack  20  of  FIG.  2    is illustrated as including six battery modules  34  arranged in two columns, this is merely an example. In general, any number (e.g., 1, 4, 5, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, etc.) of battery modules  34  may be provided in a battery pack  20 , and each battery module  34  may include any number of battery cells  38  (e.g., 1, 100, 101, 200, 300, 400, 500, 600, 800, etc.) arranged in any manner. In some embodiments, the number of battery modules  34  in each battery pack  20  may be between about 10 and 20, and the number of battery cells  38  housed in each battery module  34  may be between about 400 and 700. In some embodiments, the battery modules  34  housed in the housing  28  of a battery pack  20  may be separated from each other with dividers (not shown) that provide electrical and thermal insulation. The dividers may protect the other battery modules  34  if any battery module  34  fails (e.g., experiences a high temperature event). The dividers may be made of a material that does not oxidize or otherwise become damaged when exposed to electrical arcs and/or high temperatures. 
     The housing  28  of each battery pack  20  may have a box-like structure, and may be shaped to allow the battery modules  34  of the battery pack  20  to be arranged in a single layer on a common horizontal plane to decrease the height of the battery pack  20 . In some embodiments, the housing  28  may be watertight (e.g., to about 1 meter) and may have a rating for dust and water resistance (e.g., an International Protection (IP)  67  rating). The housing  28  may be configured to contain any failures (e.g., electric arcs, fires, etc.) within the battery pack  20  in order to prevent damage to other battery packs  20  or other portions of the bus  10  if a component inside a battery pack  20  fails. In some embodiments, the housing  28  may be constructed of corrosion and puncture resistant materials. For example, the materials of which the housing  28  may be constructed may include composite materials, Kevlar, stainless steel, aluminum, high strength plastics, etc. 
     In addition to the battery modules  34 , the housing  28  may also enclose a battery management system (BMS)  30  that monitors or controls the operation of the battery modules  34  and a thermal management system  32  that assists in managing the temperature of the battery modules  34  of the battery pack  20  (i.e., heat, cool, etc.). As described in more detail elsewhere herein, the BMS  30  and/or one or more other pack controllers may monitor the state (e.g., humidity, state of charge (SOC), current, temperature, etc.) of the battery modules  34  and the battery cells  38  in the battery pack  20 , and may control the operations of the battery pack  20  to ensure that power is safely and efficiently directed into and out of the battery pack  20 . The thermal management system  32  may include components that circulate air and/or a liquid coolant to the battery modules  34  to heat or cool the battery modules  34 . These components may include, for example, circulating fans, coolant conduits, heat exchangers, etc. that assist in circulating air and/or a coolant through the battery modules  34  packaged in the housing  28  to manage the temperature of the battery pack  20 . 
     The battery system  14  may include an energy storage management (ESM) system  26  that communicates with the BMS  30  included in the battery pack  20  to control the operation of the battery system  14  on a per-battery pack  20  basis. The ESM system  26  may include circuit boards, electronic components, sensors, and controllers that monitor the performance of the components (e.g., the battery packs  20 , the battery modules  34 , and the battery cells  38 ) of the battery system  14  based on sensor input (e.g., voltage, current, temperature, humidity, etc.), provide feedback (e.g., alarms, alerts, etc.), and control the operation of the battery system  14  for safe and efficient operation of the bus  10 . In some embodiments, the ESM system  26  may perform charge balancing between different battery packs  20 , battery modules  34  and/or battery cells  38  during recharging or during operation of the bus  10 . The ESM system  26  may also thermally and/or electrically isolate sections (e.g., battery cells  38 , battery modules  34 , battery packs  20 , etc.) of the battery system  14  when one or more sensor readings (e.g., temperature, etc.) exceed a threshold value. As will be described in more detail elsewhere herein, in some embodiments, the ESM system  26  may initiate or control energy discharge from all or selected battery packs  20 , battery modules  34 , or battery cells  38  based on the occurrence of predefined trigger events. 
       FIG.  3    is a schematic illustration of an exemplary battery module  34  of the battery system  14  of  FIG.  2   , according to the present disclosure. The battery module  34  includes a casing  36  that encloses the plurality of battery cells  38  of the battery module  34  therein. Similar to the housing  28  of the battery pack  20 , the casing  36  may be configured to contain any failures (e.g., electric arcs, fires, etc.) of the battery cells  38  of the battery module  34  within the casing  36  in order to prevent the damage from spreading to other battery modules  34  of the battery pack  20 . The casing  36  may be made of any material suitable for this purpose. In some embodiments, the casing  36  may be constructed of one or more of materials such as, for example, Kevlar, aluminum, stainless steel, composite materials, etc. In some embodiments, the casing  36  may be substantially air-tight to hermetically seal the battery cells  38  of the battery module  34  therein. 
     In general, the battery cells  38  may have any shape and structure (e.g., a cylindrical cell, a prismatic cell, a pouch cell, etc.). Typically, all the battery cells  38  of a battery module  34  may have the same shape. However, it is also contemplated that different shaped battery cells  38  may be packed together in the casing  36  of a battery module  34 . In addition to the battery cells  38 , the casing  36  may also include sensors (e.g., a temperature sensor, a voltage sensor, a humidity sensor, etc.) and controllers (e.g., a battery module controller  44 ) that monitor and control the operation of the battery cells  38 . Although not illustrated, the casing  36  also may include electrical circuits (e.g., voltage and current sense lines, low voltage lines, high voltage lines, etc.), and related accessories (e.g., fuses, switches, etc.), that direct electrical current to and from the battery cells  38  during recharging and discharging. 
     As explained previously, the battery cells  38  of the battery module  34  may be electrically connected together in any manner (e.g., in parallel, in series, or in groups of series-connected battery cells  38  connected together in parallel). These battery cells  38  may also be physically arranged in any manner. In some embodiments, the battery cells  38  of a battery module  34  may be packed together tightly to fill the available volume within the casing  36 . In some embodiments, the battery cells  38  may be arranged together to form multiple groups (e.g., bricks) of battery cells  38  electrically connected together in series. The multiple bricks (each comprising multiple battery cells  38  electrically connected together) may then be electrically connected together (e.g., in series or parallel) and packaged together in the casing  36 . In some embodiments, one or more sensors may be associated with each brick of the battery module  34 . Terminals (e.g., positive and negative terminals) electrically connected to the battery cells  38  of the battery module  34  may be provided on an external surface of the casing  36 . 
     The casing  36  may also include a coolant loop  46  configured to circulate a coolant through the battery module  34 . The coolant loop  46  may comprise fluid conduits arranged to pass through, or meander (e.g., zigzag) through, the volume enclosed by the casing  36 . An inlet port  40  and an outlet port  42  of the casing  36  may fluidly couple the coolant loop  46  to a coolant circuit of the battery system  14 . The coolant may enter the coolant loop  46  through the inlet port  40  and may exit the casing  36  through the outlet port  42 . In some embodiments, where the battery module  34  is air cooled, the casing  36  may also include inlet and outlet vents configured to direct cooling air into and out of the casing  36 . In some embodiments, the coolant may cool all the battery modules  34  of a battery pack  20  before exiting the battery pack  20 . That is, the coolant loops  46  of the battery modules  34  of the battery pack  20  may be connected in series such that the coolant exiting one battery module  34  enters the coolant loop  46  of another battery module  34 . In some embodiments, coolant may be directed into each battery module  34  individually (for e.g., from a common coolant gallery of the battery pack  20 ). In some embodiments, groups of battery modules  34  within a battery pack  20  may be fluidly connected in series and multiple series-connected battery modules  34  may be connected together in parallel. 
     During operation of the battery system  14 , the battery cells  38  of the battery module  34  release heat. This released heat may be transferred to the coolant circulating through the coolant loop  46  and then removed from the casing  36  along with the coolant. In general, any known fluid may be used as the coolant. In some embodiments, water (with suitable additives such as antifreeze, etc.) or another suitable liquid may be used as the coolant. The battery cells  38  of the battery module  34  may be arranged to enhance heat dissipation into the coolant circulating through the battery module  34 . For example, in some embodiments, the battery cells  38  may be in close thermal contact with the coolant loop  46 . In some embodiments, the battery cells  38  may be placed in close thermal contact with metal plates that serve as heat conducting pathways to the coolant loop  46 . 
     The battery module  34  may also include one or more heaters  48  positioned within the casing  36  (or in close thermal contact with the casing  36 ). In general, any type of heating device (e.g., a resistance heater, a positive temperature coefficient (PTC) heater, etc.) may be used as the heater  48 . In some embodiments, the heater  48  may be a PTC cartridge heater. Unlike a resistance heater which generates heat at a constant rate, a PTC heater may use PTC resistive elements which generate heat at a lower rate at higher temperatures. Therefore, a PTC heater is self-regulating to a fixed working temperature 
     In some embodiments, the heater  48  (or the multiple heaters  48 ) of each battery module  34  may be powered solely by the battery cells  38  of that battery module  34 . The heater  48  may be activated by the battery module controller  44  and/or by another controller (e.g., the ESM system  26 , the BMS  30 , etc.) of the battery system  14 . When the heater  48  is activated, it generates heat using the energy stored in the battery cells  38  of that battery module  34 . Consequently, the stored energy (or SOC) of the battery cells  38  in the battery module  34  decrease as a result of activation of the heater  48 . The heat dissipated by the heater  48  may be removed from the battery module  34  by the circulating coolant (or by conduction). A temperature sensor (or thermistor) of the battery module  34  may monitor the heat dissipated by the heater  48 . 
     The heater  48  may be positioned at any location within the casing  36 . In general, the location of the heater  48  may be selected such that the maximum energy discharged by the heater  48  does not damage (or jeopardize the safety of) the battery cells  38  of the battery module  34 . Therefore, in some embodiments, the heater  48  may be spaced away from (i.e., not directly in contact with) the battery cells  38  such that the heater  48  is thermally isolated from the battery cells  38 . The location of the heater  48  may also be selected such that the dissipated heat can be easily transferred to the body of the battery pack  20  (thus allowing the heater  48  to dissipate more heat without a resulting increase in temperature). Therefore, in some embodiments, the heater  48  may be positioned in direct contact with the metal frame of the battery pack  20  to enhance heat conduction. In some embodiments, the heater  48  may be positioned close to (as illustrated in  FIG.  3   ) the coolant loop  46  of the battery module  34  so that the dissipated heat may be easily transferred to the coolant circulating through the coolant loop  46 . It is also contemplated that, in some embodiments, the heater  48  may be positioned within the coolant loop  46  (i.e., submerged in the coolant of the coolant loop  46 ). In some embodiments, as illustrated in  FIG.  3   , the heater  48  may be positioned about midway of the coolant loop  46  in the battery module  34 . That is, the heater  48  may be positioned proximate to (on within) the coolant loop  46 , and substantially equidistant from the inlet port  40  and the outlet port  42 . 
     Although a single heater  48  is illustrated in  FIG.  3   , in some embodiments, multiple heaters (similar to the heater  48 ) may be positioned within the casing  36  of each battery module  34 . Each of these multiple heaters  48  may be powered by the battery cells  38  of that battery module  34  so that activating these multiple heaters  48  may discharge energy from all the battery cells  38  at a faster rate as compared to a case when a single heater  48  is used. In some embodiments, a first group of battery cells  38  of the battery module  34  (e.g., a brick) may power a first heater  48 , and a second group of battery cells  38  of the battery module  34  may power a second heater  48 . In such an embodiment, activating the first heater  48  may selectively discharge energy from the first group of battery cells  38 , and activating the second heater  48  may selectively discharge energy from the second group of battery cells  38 . The multiple heaters  48  may be positioned adjacent to each other or spaced apart from each other in the casing  36 . In some embodiments, the multiple heaters  48  may be positioned such that desired regions of the battery module  34  can be selectively discharged by activating different heaters  48 . 
     As explained previously, the heater  48  may be activated by the BMS  30  alone or in cooperation with the battery module controller  44  and/or the ESM system  26 . In some embodiments, the BMS  30  may simultaneously activate the heaters  48  embedded in (inserted in, positioned in, included in, etc.) each battery module  34  of the battery system  14  to discharge energy from the battery cells  38  of every battery module  34 , and thereby, reduce the SOC of the entire battery system  14 . In some embodiments, the BMS  30  may selectively activate the heaters  48  embedded in selected battery modules  34  to preferentially discharge energy from (and thereby reduce the SOC of) the selected battery modules  34 . For example, if sensors detect that one battery module  34  of a battery pack  20  includes a damaged battery cell  38 , the BMS  30  may selectively activate the heaters  48  embedded in all the other battery modules  34  of the battery pack  20  (i.e., except the battery module  34  with the damaged battery cell  38 ) to safely decrease the SOC of the battery pack  20 . In embodiments where multiple heaters  48  are embedded in a battery module  34 , the BMS  30  may also be configured to selectively activate some heaters  48  of the battery module  34  to preferentially discharge energy from selected battery cells  38  (e.g., bricks) of the battery module  34 . 
     The BMS  30  may activate the heaters  48  embedded in the battery modules  34  to discharge energy from (and thus decrease the SOC of) the battery system  14  of a stranded (or otherwise incapacitated) bus  10  before service personnel operate on (repair, remove the batteries from, etc.) the bus  10 . The battery system  14  of the bus  10  may store a relatively large amount of energy (e.g., between about 200-700 KWh). Operating on a bus  10  with such a large amount of stored energy may be undesirable. Dissipating the stored energy from the battery system  14  by activating the heaters  48  lowers the SOC of the battery system  14 . After the SOC of the battery system  14  has been lowered to a suitable level, the heaters  48  may be deactivated. Although the discussion above describes embedding a heater  48  in a battery module  34  of a battery pack  20 , this is merely exemplary. In general, any electric load may be embedded in a battery module  34  to selectively dissipate energy from the battery cells  38  of the battery module  34   
     In general, the heat produced by the heaters  48  may be dissipated from the battery system  14  by conduction, convection, or radiation. The heaters  48  may be positioned in the battery modules  34  such that the heat produced by them can be removed without overheating the battery cells  38  of the battery module  34 . In some embodiments, the heat produced by the heaters  48  of a battery module  34  may be used to increase the temperature of the battery cells  38  of the battery module  34 . In some embodiments, the inlet port  40  and/or the outlet port  42  of the coolant loop  46  may be selectively opened and closed (e.g., using adjustable valves  41  and  43  shown by the dashed lines in  FIG.  3   ) by the BMS  30 , based on sensor readings (e.g., humidity, temperature, etc.) from within the battery module  34 . The BMS  30  may use these adjustable valves to redirect the coolant flow within the battery system  14  based on the local conditions within the battery modules  34 . 
     The implementation of a heater  48  in every battery module  34  of the battery system  14  (as opposed to providing a coolant heater external to the battery system  14 ) may activate the battery cells  38  of the battery system  14  to be heated more quickly and efficiently. Further, locating the heater  48  to be substantially in the middle of the coolant loop  46  may activate the heat dissipated by the heater  48  to be distributed throughout the coolant loop  46  which may result in improved heating performance in a short amount of time. 
     The BMS  30  (and/or other controllers of the battery system  14 ) may selectively activate the heaters  48  of a battery module  34  in response to any triggering event. In some embodiments, the triggering event may include input from a human operator or one or more sensors of the bus  10 . In response to the triggering signal, the BMS  30  may selectively activate one or more of the heaters  48  embedded in selected battery modules  34  (i.e., all or some of the battery modules  34 ). 
       FIG.  4    is a schematic illustration of connections between the battery pack  20  of  FIG.  2    and peripheral devices or systems of the bus  10  of  FIGS.  1 A and  1 B , according to the present disclosure. As illustrated, the schematic in  FIG.  4    includes the battery pack  20 , a high voltage bus bar  50 , a high voltage peripheral device or system  52 , a low voltage bus bar  54 , and a low voltage peripheral device or system  56 . The battery pack  20  may be electrically connected (e.g., through one or more electrical terminals  62 ,  66  not illustrated in  FIG.  4   ) to the high voltage bus bar  50 . The high voltage bus bar  50  may provide one or more electrical connections between the battery pack  20  and the high voltage peripheral device or system  52  for carrying high voltage power (e.g., at greater than or equal to approximately 100 V) from the battery pack  20  to the high voltage peripheral device or system  52 . The high voltage peripheral device or system  52  may include, for example, devices or systems of the bus  10  used during operation of the bus  10 , such as the powertrain  24 , an heating, ventilation, and air conditioning (HVAC) system, an external DC/DC system, or the like. 
     Similarly, the battery pack  20  may be electrically connected to the low voltage bus bar  54 . The low voltage bus bar  54  may provide one or more electrical connections between the battery pack  20  and the low voltage peripheral device or system  56  for carrying low voltage power (e.g., at less than 100 V) from the battery pack  20  to the low voltage peripheral device or system  56 . The low voltage device or system  56  may include, for example, devices or systems that are operational when the bus  10  is not in use or is in an idle state, such as a fire suppression system, a security system, a lighting system, an indicator, a cooling pump, or the like. In some implementations, the low voltage device or system  56  may include any device or system of the bus  10  that does not operate on a high voltage energy storage system. 
     Although  FIG.  4    illustrates a single battery pack  20 , there may be multiple battery packs  20  electrically connected to the high voltage bus bar  50  or the low voltage bus bar  54 , and the multiple battery packs  20  may be organized into electrically parallel strings of battery packs  20  (with the battery packs  20  included in a string connected in series). In addition, the illustration of a single high voltage peripheral device or system  52  and a single low voltage peripheral device or system  56  is merely exemplary and some embodiments may include multiple high voltage peripheral devices or systems  52  and/or multiple low voltage peripheral devices or systems  56 . 
       FIG.  5    is a schematic illustration of a battery pack  20  of the battery system  14  of  FIG.  2    that includes a DC/DC converter  58 , according to the present disclosure. The schematic illustrated in  FIG.  5    includes, a battery pack  20 , a BMS  30 , a battery module  34 , a high voltage bus bar  50 , a low voltage bus bar  54 , a DC/DC converter  58 , positive electrical connections  60 , positive electrical terminals  62 , negative electrical connections  64 , negative electrical terminals  66 , software layer communication lines  68 , a hardware layer communication line  70 , and electrical connections  72 . 
     The DC/DC converter  58  may include one or more electrical circuits or electromechanical devices that receives an input of a direct current of electrical power at one voltage and outputs a direct current of electrical power at another voltage (a higher or lower voltage than the input voltage). For example, the DC/DC converter  58  may perform such a conversion by storing the input power (e.g., using magnetic field storage components, such as inductors or transformers, or using electric field storage components, such as capacitors) and then releasing the power to the output of the DC/DC converter  58 . 
     As illustrated in  FIG.  5   , the DC/DC converter  58  may be electrically connected to the battery module  34  via a positive electrical terminal  62  and a negative electrical terminal  66 , which may be separate components from the DC/DC converter  58 , and thus separately controllable (e.g., control of on/off states) from the DC/DC converter  58 . For example, the DC/DC converter  58  may be directly connected to a high voltage DC bus bar included in the battery pack  20 . In addition, the DC/DC converter  58  may be electrically connected to the low voltage bus bar  54  via the electrical connections  72  (e.g., separate positive and negative electrical connections). The DC/DC converter  58  may include one or more relays and fuses for protection from the high voltage connections to the battery module  34  (e.g., the electrical connections  60  and  64  via the electrical terminals  62  and  66 ) and one or more fuses for protection from the low voltage connections to the low voltage bus bar  54  (e.g., the electrical connections  72 ). 
     In some embodiments, the DC/DC converter  58  may be located in an ancillary bay of the battery pack  20 . Additionally, or alternatively, the DC/DC converter  58  may be included in the coolant system of the battery pack  20 . For example, the DC/DC converter  58  may have one or more mechanical connections to the coolant loop  46 . This may reduce or eliminate a need for a DC/DC converter  58  external to the battery pack  20  or for independent cooling channels, heat sinks, or fans for cooling the DC/DC converter  58 . 
     The DC/DC converter  58  may be bidirectional. For example, the DC/DC converter  58  may receive electrical power from the low voltage bus bar  54 , may increase the voltage of the electrical power, and may then output the electrical power to the battery module  34 . Alternatively, the DC/DC converter  58  may receive electrical power from the battery module  34 , may decrease the voltage of the electrical power, and may then output the electrical power to the low voltage bus bar  54 . As a specific example, the DC/DC converter  58  may receive an input of about 330 Vdc power and may output about 24-28 Vdc power, or vice versa. 
     The amount of power available to the bus  10  via the DC/DC converter  58  may be based on the output power of the DC/DC converter  58  and the quantity of DC/DC converters  58  (e.g., power may equal converter power multiplied by converter quantity). For example, the amount of power available to the bus  10  may be equal to the quantity of DC/DC converters  58  times the wattage of the DC/DC converter  58 . As a specific example, if the bus  10  includes four battery packs  20  with a 500 W DC/DC converter  58  per battery pack  20 , there may be about 2 kilowatts (kW) of DC power available to the bus  10  without an externally connected DC/DC converter. If the bus  10  includes a lower quantity of battery packs  20 , e.g., two battery packs  20 , the battery packs  20  may include higher output power DC/DC converters  58  or may provide the bus  10  with less than approximately 2 kW of power. 
     The software layer communication lines  68  may include wired or wireless connections for bidirectional communication between the DC/DC converter  58  and the BMS  30 . For example, the software layer communication lines  68  may include a controller area network (CAN) bus, a serial communication line, and/or the like. As described in more detail elsewhere herein, the BMS  30  may send instructions to the DC/DC converter  58  to configure the DC/DC converter  58  to operate in a particular manner and/or may receive data related to the operation of the DC/DC converter  58  via the software layer communication lines  68 . The hardware communication line  70  may include an electrical connection for logic and/or voltage signaling from the BMS  30  to the DC/DC converter  58 , or vice versa. As described in more detail elsewhere herein, the BMS  30  may provide enabling/disabling signaling to the DC/DC converter  58  via the hardware communication line  70 . The software layer communication lines  68  and/or the hardware communication lines  70  may form a communication bus bar and the BMS  30  may be controlled by the ESM system  26  (see  FIG.  6   ). 
     The battery pack  20  may include one or more additional components not illustrated in  FIG.  5    (or elsewhere herein). For example, the battery pack  20  may include a high voltage interlock loop (HVIL), which may be configured to protect people from electrical power stored in the battery pack  20  during maintenance, assembly, etc. In some embodiments, the BMS  30  may activate or deactivate the HVIL, such as when the bus  10  is in a maintenance facility. In some embodiments, an activate signal for enabling the DC/DC converter  58  via the software layer communication lines  68  and/or the hardware layer communication line  70  may be part of the enabling signal for the HVIL. For example, when the DC/DC converter  58  is activated to import power to or export power from a battery pack  20 , low voltage DC terminal pins for the bus  10  may be deactivated for safety. 
       FIG.  6    is another schematic illustration of a battery pack  20  of the battery system  14  of  FIG.  2    that includes a DC/DC converter  58 , according to the present disclosure. The battery pack  20  illustrated in  FIG.  6    may include some of the same components as the battery pack  20  illustrated in  FIG.  5   . However, rather than being connected to the BMS  30  via the software layer communication lines  68  and the hardware layer communication line  70 , the DC/DC converter  58  of  FIG.  6    may be connected directly to the ESM system  26  (not illustrated in  FIG.  6   ) via the software layer communication lines  68 . Thus, in some embodiments, the ESM system  26 , rather than the BMS  30 , may directly control the operation of the DC/DC converter  58 . 
     In some embodiments, a positive electrical terminal  62  and a negative electrical terminal  66  may be included in the DC/DC converter  58  and may be controlled in conjunction with the DC/DC converter  58 . For example, the positive electrical terminal  62  and the negative electrical terminal  66  may be controlled by the same enabling/disabling signals as the DC/DC converter  58  and/or may be controlled directly by the DC/DC converter  58  based on signaling received from the ESM system  26 . Although the schematics of  FIGS.  5  and  6    have been described separately, the schematics may be combined in some embodiments. For example, the battery pack  20  of  FIG.  5    may be modified in some embodiments such that the DC/DC converter  58  is directly connected to the ESM system  26 , as in the schematic of  FIG.  6   . 
       FIG.  7    is a schematic illustration of power export from battery packs  20  of  FIG.  6    with balanced stored energy levels, according to the present disclosure. As illustrated, the schematic in  FIG.  7    includes, for example, 4 battery packs  20  (battery pack  20 - 1 , battery pack  20 - 2 , battery pack  20 - 3 , and battery pack  20 - 4 ) similar to the battery pack  20  illustrated in  FIG.  6   . The battery packs  20 - 1  and  20 - 2  may form a first string of battery packs  20  and may be electrically connected in series with each other via corresponding positive electrical connections  74  and negative electrical connections  76  (where electrical connections  74  and  76  may form the high voltage bus bar  50 ). The battery packs  20 - 3  and  20 - 4  may form a second string of battery packs  20  and may be electrically connected in series with each other via corresponding positive electrical connections  74  and negative electrical connections  76 . The first string and the second string may be electrically connected in parallel with each other. As used herein, “Vess” is an acronym for voltage-energy storage system. 
     Assume for the example of  FIG.  7    that the battery packs  20  have balanced energy storage (or about the same amount of energy storage). In this case, the DC/DC converters  58  may each be configured to export an equal amount of power at a target current from their respective battery modules  34  to the low voltage bus bar  54  on a per-string basis. For example, as illustrated by power flow paths  78  and  80 , the DC/DC converters  58  of the battery packs  20 - 1  and  20 - 2  may each be configured to export half of the power output for the first string at half the target current for the first string (where “½ Vess” indicates that each high voltage battery  20 - 1  and  20 - 2  contributes half of the overall string voltage and that the two battery packs on the first string have balanced energy levels) and, as illustrated by power flow paths  82  and  84 , the DC/DC converters  58  of the battery packs  20 - 3  and  20 - 4  may each be configured to export half of the power output for the second string at half the target current for the second string. As a specific example and without limitation, if each string of battery packs  20  have to output a maximum of 1000 watts (W) of power at 24 V, then the battery packs  20 - 1  and  20 - 2  may be each configured to export 500 W of power at 24 V to the low voltage bus bar  54  and the battery packs  20 - 3  and  20 - 4  may be each configured to export 500 W of power at 24 V to the low voltage bus bar  54 . In this way, the DC/DC converters  58  may be configured to evenly deplete battery packs  20  when the battery packs  20  have equal energy storage levels within and between each string of battery packs  20 . 
       FIG.  8    is a schematic illustration of power import to, and power export from, battery packs  20  of  FIG.  6    with imbalanced stored energy levels, according to the present disclosure. In contrast to the schematic illustrated in  FIG.  7   , assume for example that the battery packs  20  in the schematic illustrated in  FIG.  8    do not store equal amounts of energy and represent different portions of a target current of a string of battery packs  20 . In addition, assume for the example schematic illustration of  FIG.  8    that different strings of battery packs  20  (e.g., a first string of battery packs  20  that includes battery packs  20 - 1  and  20 - 2  and a second string of battery packs  20  that includes battery packs  20 - 3  and  20 - 4 ) are capable of operating at a target voltage. In this case, some of the DC/DC converters  58  may be configured to export power (the same or different amounts of power), and some of the DC/DC converters  58  may be configured to import power. For example, as illustrated by power flow path  86 , the battery pack  20 - 1  may be configured to export 1000 W (“+1000 W”) of power to the low voltage bus bar  54  for the first string of battery packs  20 . As illustrated by power flow path  88 , the battery pack  20 - 2  may be configured to import 500 W (“−500 W”) of power from the low voltage bus bar  54  for the first string of battery packs  20 . The “¾ Vess” and the “¼ Vess” may indicate that the battery packs  20 - 1  and  20 - 2  contribute 75% and 25% of the string voltage, respectively, and that the battery packs  20 - 1  and  20 - 2  have imbalanced energy levels. As illustrated by power flow paths  90  and  92 , the battery packs  20 - 3  and  20 - 4  may both be configured to export 250 W (“+250 W”) of power to the low voltage bus bar  54  each at 50 percent of the target current of the second string of battery packs  20 . Thus, the first string of battery packs  20  (battery packs  20 - 1  and  20 - 2 ) and the second string of battery packs  20  (battery packs  20 - 3  and  20 - 4 ) may each export a net of 500 W of power at a target voltage while charging the battery pack  20 - 2 . In addition, the battery packs  20 - 3  and  20 - 4  may contribute half of the voltage for the second string and may have balanced energy levels. 
     In this way, energy may be transferred from the higher charge level battery packs  20  (battery packs  20 - 1 ,  20 - 3 , and  20 - 4 ) to the lowest charge level battery pack (battery pack  20 - 2 ), thus rebalancing the charge levels of the battery packs  20 . Furthermore, this can advantageously reduce and/or eliminate a need to remove the bus  10  from service or deactivate a string of battery packs  20  of the bus  10  (e.g., the string of battery packs  20  that includes the lowest charge battery pack  20 ) while charge level imbalances are corrected. 
       FIG.  9    is another schematic illustration of power import to, and power export from, battery packs  20  of  FIG.  6    with imbalanced stored energy levels, according to the present disclosure. For example, in the schematic illustrated in  FIG.  9   , the battery pack  20 - 2  may have a very low state of charge (e.g., a 25 percent or less charge), such as due to being a replacement battery pack  20 . In this case, the first string of battery packs  20  (battery packs  20 - 1  and  20 - 2 ) may be deactivated by the ESM  26  for high voltage use but, as illustrated by power flow path  94 , the battery pack  20 - 1  may be configured by the ESM  26  to export power to the low voltage bus bar  54  (e.g., 1000 W export, shown as “+1000 W”). As illustrated by power flow path  96 , the battery pack  20 - 2  may be configured to import power from the low voltage bus bar  54  (e.g., 1000 W import, shown as “−1000 W”). For the first string, the battery packs  20 - 1  and  20 - 2  may have imbalanced stored energy levels and the battery pack  20 - 2  may contribute less than half of the voltage for the first string (“&lt;½ Vess”). In addition, as illustrated by power flow paths  98  and  100 , the battery packs  20  of the second string (battery packs  20 - 3  and  20 - 4 ) may be each configured to export power to the low voltage bus bar  54  (e.g., 1000 W export each, shown as “+1000 W”). This configuration of power export and import may help to quickly charge the battery pack  20 - 2  from a state of very low charge without needing to connect the bus  10  to a charging port. Because the first string of battery packs  20  is deactivated in this example, there may not be a need to balance the net power output between the first string and the second string, in contrast to the example illustrated in  FIG.  8   , as the first string may not be able to affect operations of the high voltage bus bar  50  in a deactivated state. 
     In this way, power may be transferred from the battery packs  20 - 1 ,  20 - 3 , and  20 - 4  to the battery pack  20 - 2  during operation of the bus  10  or while the bus  10  is in an idle state. Furthermore, this may result in rapid charging of the battery pack  20 - 2  because the first string of battery packs  20  is deactivated. In this configuration, battery packs  20 - 3  and  20 - 4  may supply all of the available low voltage power from the battery packs  20 , while battery pack  20 - 1  may supply the imported energy for battery pack  20 - 2 . 
       FIG.  10    illustrates an exemplary method  200  of enabling a DC/DC converter  58  included in a battery pack  20  of  FIG.  6   , according to the present disclosure. The method  200  may be performed by the BMS  30 , the ESM system  26 , and/or one or more other controllers associated with the battery system  14 . The method  200  may include, at operation  202 , receiving one or more first instructions to activate a DC/DC converter  58 . For example, when the battery pack  20  is configured in the manner illustrated in  FIG.  5   , the BMS  30  may receive the one or more first instructions from another system associated with the bus  10  (e.g., the ESM system  26 ) based on the bus  10  being connected to a charging station, based on entering an idle state, or based on input from a control panel associated with the bus  10  or a maintenance facility. When the battery pack  20  is configured in the manner illustrated in  FIG.  6   , the ESM system  26  may receive the one or more first instructions to activate the DC/DC converter  58  from another system associated with the bus  10 , such as a from a diagnostic system of the bus  10 , or may determine to activate the DC/DC converter  58  based on detecting an imbalance of stored energy across multiple battery packs  20 . 
     The method  200  may further include, at operation  204 , sending one or more second instructions to the DC/DC converter  58  to cause the DC/DC converter  58  to operate based on a set of parameters. For example, the BMS  30  (when the battery pack  20  is configured as illustrated in  FIG.  5   ) or the ESM system  26  (when the battery pack  20  is configured as illustrated in  FIG.  6   ) may send the one or more second instructions via the software layer communication lines  68 . Additionally, or alternatively, the BMS  30  and/or the ESM system  26  may send logic or voltage signaling via the hardware layer communication line  70  as the one or more second instructions, depending on the configuration of the battery pack  20 . The set of parameters may include, for example, a direction of power flow (import or export) relative to the battery cells  38  of the battery pack  20 , an operating voltage for the battery pack  20 , an operating current for the battery pack  20 , an amount of power to be imported to, or exported from, the battery pack  20  (e.g., a power limit), and/or the like. 
     Additionally, or alternatively, the set of parameters may include an indication of whether the battery pack  20  is to operate in a particular mode, such as a soft start mode or a low voltage battery charging mode. For example, a soft start mode may include a mode where the ESM system  26  sets a target voltage limit (e.g., approximately 28 V) and the DC/DC converter  58  increases the operating voltage from a starting voltage limit (e.g., approximately 24 V) to the target voltage limit over time rather than starting operation at the target voltage limit set by the ESM system  26 . As another example, a low voltage battery charging mode may include a mode where the DC/DC converter  58  operates at a lower voltage. For example, the battery system may operate below a nominal voltage range (e.g., a 24V nominal system (a 24V-28V system) may start at 18V to account for a severely depleted low voltage battery). This operation mode may be needed if the power supplies for the DC/DC converter  58  are disabled, or if the standby load has exceeded their ability to supply the needed power and the balance was drawn from the low voltage battery pack  20 . 
     In some embodiments, the method  200  may include sending control signaling for enabling and/or disabling various electrical contacts of the battery pack  20  (e.g., electrical contacts of terminals  62  or  66 ). The configuration of activated and deactivated electrical contacts may depend on whether the battery pack is to import power or export power. The control signaling may be included in the one or more second instructions, such as when the DC/DC converter  58  includes the electrical contacts, or as separate signaling, such as when the electrical contacts are separate components from the DC/DC converter  58 . 
     The operation at  204  may be performed when the bus  10  is in an active (e.g., in operation on a route) or an idle state (e.g., powered off in a maintenance facility or storage yard). For example, the ESM system  26  may send the instructions to multiple DC/DC converters  58  for power import or export when a charge level imbalance is detected during operation of the bus  10 , and the DC/DC converters  58  may operate according to the instructions at a later time when the bus  10  enters an idle state at a maintenance facility or storage yard. Additionally, or alternatively, the ESM system  26  may send the instructions after the bus  10  enters the idle state, such as during an idle state diagnostic test that checks the charge level of battery packs  20  of the bus  10 . 
     The method  200  may further include, at operation  206 , performing one or more actions. For example, after the DC/DC converter  58  has been operating for some amount of time according to the set of parameters sent in connection with the operations at  204 , the method  200  may include sending one or more third instructions to deactivate the operation of the DC/DC converter  58 , to modify the set of parameters, and/or the like. In addition, the method  200  may include, receiving, from the DC/DC converter  58 , metrics related to the operation of the DC/DC converter  58  and sending the third instructions to modify the operation of the DC/DC converter  58  based on the received metrics (e.g., increase or decrease the operation, stop the operation, etc.). The metrics may include, for example, an operating voltage and/or current of the DC/DC converter  58 , a charge level of the battery pack  20 , an amount power import to or export from the battery pack  20 , one or more fault indicators, and/or the like. 
       FIG.  11    illustrates an exemplary method  300  of balancing stored energy levels among multiple battery packs  20  of  FIG.  6   , according to the present disclosure. The method  300  may be performed by the ESM system  26 , the BMS  30 , and/or one or more other controllers associated with the battery system  14 . The method  300  may include, at operation  302 , receiving one or more first instructions to activate a plurality of DC/DC converters  58 . For example, the ESM system  26  may receive the one or more first instructions from another system associated with the bus  10 , based on the bus  10  being connected to a charging station, based on the bus  10  entering an idle state, or based on input from a control panel associated with the bus  10  or a maintenance facility. The method  300  may further include, at operation  304 , receiving information related to stored energy levels of a plurality of battery packs  20  associated with the plurality of DC/DC converters  58 . For example, the ESM system  26  may query the BMSs  30  in the battery packs  20  for information related to the stored energy levels of the battery packs  20 , and the BMSs  30  may provide that information based on receiving the query. In some embodiments, the operations at  302  and  304  may be different. For example, the ESM system  26  may first query the BMSs  30  for the information related to the energy storage levels (similar to operation  304 ), and may then determine to activate the DC/DC converters  58 , rather than receiving the first instructions at  302 . 
     The method  300  may include, at operation  306 , determining a direction of power flow and an amount of the power flow for each of the plurality of battery packs  20 . For example, the ESM system  26  may determine the direction of power flow and the amount of the power flow based on the information received in connection with the operation at  304 . The direction of power flow for each battery pack  20  may be determined based on whether a battery pack  20  has more or less stored energy relative to the other battery packs  20  and the power flow directions and amounts needed to balance stored energy among the battery packs  20  of the battery system  14 . For example, the ESM system  26  may determine that a battery pack  20  is to export power to the low voltage bus bar  54  if that battery pack  20  has the most stored energy relative to other battery packs  20  in the battery system  14 , has at least a threshold amount of stored energy, has more than the average stored energy among multiple battery packs  20 , and/or the like. Conversely, the ESM system  26  may determine that a battery pack  20  is to import power from the low voltage bus bar  54  if the battery pack  20  has the least amount of stored energy relative to other battery packs  20  in the battery system  14 , has less than a threshold amount of stored energy, has less than the average stored energy among multiple battery packs  20 , and/or the like. 
     The ESM system  26  may determine the direction and amount of power flow for the battery packs  20  on a per-string basis. For example, the ESM system  26  may determine directions and amounts of power flow for each battery pack  20  of a string of battery packs  20  such that the direction and amount of an aggregate power flow for the string of battery packs  20  is equal to the direction and amount of an aggregate power flow for one or more other strings of battery packs  20 , results in an aggregated direction and amount of power flow into a particular string of battery packs  20  and/or particular battery pack  20  of the string, and/or the like. In some embodiments, the directions and amounts of power flow may be the same or different for different battery packs  20  in the same string or in different strings. Additionally, or alternatively, the direction and amount of an aggregate power flow for a string of battery packs  20  may be the same as or different from another string of battery packs  20 . 
     In some embodiments, the ESM system  26  may determine to activate or deactivate high voltage output of one or more strings of battery packs  20  and/or one or more battery packs  20  of a string in connection with determining the direction and amount of power flow for each of the battery packs  20 . For example, the ESM system  26  may determine to deactivate a battery pack  20  or a string of battery packs  20  if a battery pack  20  is to import power from the low voltage bus bar  54  and if the amount of power to be imported satisfies a threshold (e.g., in the case where the battery pack  20  is a replacement battery pack with a very low factory-provided SOC, such as a 10 percent SOC). 
     In connection with the operation  306 , the method  300  may include determining a configuration of activated electrical contacts and deactivated electrical contacts. For example, the ESM  26  may determine a first configuration of electrical contacts that allows for import of power to the battery pack  20  from the low voltage bus bar  54  or a second configuration of electrical contacts that allows for export of from the battery pack  20  to the low voltage bus bar  54 . The method  300  may then include sending control signaling that indicates the determined configuration, such as in connection with the operation  308  below or as separate signaling. 
     The method  300  may include, at operation  308 , sending one or more second instructions to the plurality of DC/DC converters  58  to cause the plurality of DC/DC converters  58  to operate based on a set of parameters that includes the direction and the amount of the power flow. For example, the ESM system  26  may provide the one or more second instructions directly to the DC/DC converters  58  via the software layer communication lines  68 , or the ESM system  26  may provide the one or more second instructions to the BMSs  30  in each battery pack  20  and the BMSs  30  may provide the one or more instructions to the DC/DC converters  58 . 
     The method  300  may include, at operation  310 , performing one or more actions related to controlling the plurality of DC/DC converters  58 . For example, the ESM system  26  may monitor the energy levels of the battery packs  20  (e.g., by querying the BMSs  30  for information related to the energy levels). The ESM system  26  may then determine to modify the direction and amount of the power flow for one or more battery packs  20 , may determine to stop the rebalancing of power among battery packs  20 , and/or may bring one or more battery packs  20  (or strings) back online or take one or more additional battery packs  20  (or strings) offline. The ESM system  26  may then provide one or more third instructions related to these operations to the DC/DC converter  58  and/or to the BMS  30 . 
       FIG.  12    illustrates example components of a computing device  400 , according to the present disclosure. In particular,  FIG.  12    is a simplified functional block diagram of a computing device  400  that may be configured as a device for executing methods of this disclosure, such as  FIGS.  10  and  11   . For example, the computing device may be configured as the ESM system  26 , the BMS  30 , a battery pack controller, the high voltage peripheral device or system  52 , the low voltage peripheral device or system  56 , and/or another device or system according to exemplary embodiments of the present disclosure. In various embodiments, any of the devices or systems described herein may be the computing device  400  illustrated in  FIG.  12    and/or may include one or more of the computing devices  400 . 
     As illustrated in  FIG.  12   , the computing device  400  may include a processor  402 , a memory  404 , an output component  406 , a communication bus  408 , an input component  410 , and a communication interface  412 . The processor  402  may include a central processing unit (CPU), a graphics processing unit (GPU), an accelerated processing unit (APU), a microprocessor, a microcontroller, a digital signal processor (DSP), a field-programmable gate array (FPGA), an application-specific integrated circuit (ASIC), or another type of processing component. In some embodiments, the processor  402  includes one or more processors capable of being programmed to perform a function. The memory  404  may include a random access memory (RAM), a read only memory (ROM), and/or another type of dynamic or static storage device (e.g., a flash memory, a magnetic memory, and/or an optical memory) that stores information and/or instructions for use by the processor  402 . 
     The output component  406  may include a component that provides output information from the computing device  400  (e.g., a display, a speaker, and/or one or more light-emitting diodes (LEDs)). The communication bus  408  may include a component that permits communication among the components of the computing device  400 . The input component  410  may include a component that permits the computing device  400  to receive information, such as via user input (e.g., a touch screen display, a keyboard, a keypad, a mouse, a button, a switch, and/or a microphone). Additionally, or alternatively, the input component  410  may include a sensor for sensing information (e.g., a global positioning system (GPS) component, an accelerometer, a gyroscope, and/or an actuator). The communication interface  412  may include a transceiver-like component (e.g., a transceiver and/or a separate receiver and transmitter) that activates device  400  to communicate with other devices, such as via a wired connection, a wireless connection, or a combination of wired and wireless connections. The communication interface  412  may permit the computing device  400  to receive information from another device and/or provide information to another device. For example, the communication interface  412  may include a controller area network (CAN) bus, an Ethernet interface, an optical interface, a coaxial interface, an infrared interface, a radio frequency (RF) interface, a universal serial bus (USB) interface, a wireless local area network interface, a cellular network interface, and/or the like. 
     As noted above, the computing device  400  illustrated in  FIG.  12    may perform one or more processes described herein. The computing device  400  may perform these processes based on the processor  402  executing software instructions stored by a non-transitory computer-readable medium, such as the memory  404  and/or another storage component. For example, the storage component may include a hard disk (e.g., a magnetic disk, an optical disk, a magneto-optic disk, and/or a solid state disk), a compact disc (CD), a digital versatile disc (DVD), a floppy disk, a cartridge, a magnetic tape, and/or another type of non-transitory computer-readable medium, along with a corresponding drive. A computer-readable medium is defined herein as a non-transitory memory device. A memory device includes memory space within a single physical storage device or memory space spread across multiple physical storage devices. 
     Software instructions may be read into the memory  404  and/or a storage component from another computer-readable medium or from another device via the communication interface  412 . When executed, software instructions stored in the memory  404  and/or the storage component may cause the processor  402  to perform one or more processes described herein. Additionally, or alternatively, hardwired circuitry may be used in place of or in combination with software instructions to perform one or more processes described herein. Thus, embodiments described herein are not limited to any specific combination of hardware circuitry and software. 
     Certain embodiments described herein may provide various technological advantages or improvements. For instance, by including a DC/DC converter in a battery pack, certain embodiments may allow for power to be imported to, or exported from, a battery pack to rebalance stored energy levels across multiple battery packs without needing to connect an electric bus to external equipment. Thus, the re-balancing can be performed while the bus in in operation and/or automatically when the bus enters an idle state, which improves an efficiency of maintenance of a bus by reducing the need for certain types of maintenance efforts and/or reducing an amount of time that a bus has to be taken out of service for maintenance. In addition, this improves safety with respect to maintenance of the bus by reducing a need for maintenance personnel to interact with the battery system of the bus. Similarly, by actively monitoring and rebalancing stored energy levels of battery packs, certain embodiments described herein may provide for faster detection and correction of energy level imbalances, which can reduce or eliminate severe imbalances that might deactivate an electric bus or may allow for the bus to be returned to service faster as charge imbalances can be corrected while the bus is in operation. 
     Certain embodiments may provide for a system where batteries and charging can operate without external input from the electric bus. For example, with the batteries able to supply sufficient DC voltage to run pumps, keep contactors closed, and keep the low voltage battery system at a healthy level, the bus may not need to be powered on to charge or condition the high voltage batteries. Control of the charge port contactors may be provided by a charge controller and a separate path that may not need to go through the main DC load contactors in the electric bus. In addition, certain embodiments may provide vehicle-off operation efficiency gains in charging, and may satisfy operator needs for having, e.g., the interior lights and destination signs of the bus off while charging without needing specialty software to determine this activity (the operator may just turn the vehicle off before plugging it in at a charging station). 
     While principles of the present disclosure are described herein with reference to a battery pack that includes a DC/DC converter for electric buses, it should be understood that the disclosure is not limited thereto. Rather, the systems and methods described herein may be employed in any type of electric vehicle. Also, those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, embodiments, and substitution of equivalents all fall within the scope of the embodiments described herein. Accordingly, the invention is not to be considered as limited by the foregoing description. For example, while certain features have been described in connection with various embodiments, it is to be understood that any feature described in conjunction with any embodiment disclosed herein may be used with any other embodiment disclosed herein.