Patent Publication Number: US-2023145214-A1

Title: Opportunistic vehicle air brake system pressurization systems and methods

Description:
BACKGROUND 
     Often, vehicles with air brake systems may experience small air leaks which can, over time, depressurize the air tanks carried by the vehicle. This is particularly the case when such vehicles are parked for an extended period of time. Recharging of air tanks is typically performed using an air compressor that is carried by such vehicles. However, if air tanks are significantly depressurized, it may take one to three minutes to recharge those air tanks before the air brake system may be reliably used. 
     This leads to a number of disadvantages. For example, a driver may arrive at a vehicle and may be required to initiate the re-pressurization of air tanks prior to departure. This can unnecessarily delay the driver&#39;s departure time. Furthermore, in vehicles having internal combustion engines, often accessory devices such as air compressors are powered by energy generated by the internal combustion engine, and therefore require the vehicle to be running for use of such accessory devices. In battery electric vehicles (BEVs) and plug-in hybrid vehicles (PHEVs), operation of an air compressor may occur at any time, but has the effect of draining the battery, thereby potentially reducing the effective operating range of the vehicle on a given charge. Electric air compressors can be similarly used with vehicles that use internal combustion engines, but the low voltage battery charge, not impacting range, will be reduced instead. 
     SUMMARY 
     The disclosure generally relates to systems and methods for providing opportunistic vehicle air brake system pressurization. For example, in some instances, vehicle air tanks used within an air brake system are pressurized during a time at which the vehicle is electrically connected to a power source other than or in addition to a battery of the vehicle. An example of such a power source may include an electrical charger, such as an electric vehicle charging station, or an electric generator during a regenerative braking event. 
     In a first aspect, a method includes determining that a battery recharging event is in progress for a battery of a vehicle. The method further includes, based on a determination that the battery recharging event is in progress, determining a pressure of an air brake system of the vehicle. The method also includes, upon determining that the pressure of the air brake system is below a predetermined threshold during the battery recharging event, activating an air compressor included in the air brake system during the battery recharging event. Activating the air compressor during the battery recharging event is performed by powering the air compressor from an electrical power source other than the battery. 
     In a second aspect, a vehicle is disclosed that includes a battery operable to power an at least partially electric drivetrain, as well as an air brake system including an air compressor. The vehicle includes a control circuit including a processor and a memory. The memory stores instructions executable by the processor to: determine that a battery recharging event is in progress for the battery; based on a determination that the battery recharging event is in progress, determine a pressure of an air brake system of the vehicle; upon determining that the pressure of the air brake system is below a predetermined threshold during the battery recharging event, activate the air compressor during the battery recharging event. Activating the air compressor during the battery recharging event is performed by powering the air compressor from an electrical power source other than the battery. 
     In a third aspect, a non-transitory computer-readable storage medium storing computer-executable instructions is disclosed. When executed by a control circuit of a vehicle, the instructions cause the control circuit to perform a method of pre-conditioning an air brake system of a vehicle. The method includes assessing a battery charging status for a battery of a vehicle, and, upon determining that a battery charging status meets a predetermined condition determining a pressure of an air tank within the air brake system of the vehicle. The method also includes, upon determining that the pressure of the air brake system is below a predetermined threshold, activating an air compressor included in the air brake system. Determining the battery charging status includes at least one of (1) determining that the battery is connected to an external power source, (2) determining that electrical power is being generated by regenerative braking that is in excess of a battery recharging capacity, or (3) determining that the battery has a charge level above a predetermined threshold. 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive examples are described with reference to the following figures: 
         FIG.  1    is an illustration depicting a side view of a vehicle implementing an opportunistic vehicle air brake system repressurization; 
         FIG.  2    is a schematic block diagram of a portion of an air brake system of a vehicle, such as the vehicle of  FIG.  1   . 
         FIG.  3    is a block diagram of a portion of an electrical system of a vehicle, such as the vehicles of  FIGS.  1 - 2   . 
         FIG.  4    is a timing diagram illustrating example opportunistic vehicle air brake system repressurization when a vehicle is not in operation, according to an example embodiment. 
         FIG.  5    is a timing diagram illustrating example opportunistic vehicle air brake system repressurization when a vehicle is in operation, according to an example embodiment. 
         FIG.  6    is a further timing diagram illustrating example opportunistic vehicle air brake system repressurization based on a vehicle battery status, according to an example embodiment. 
         FIG.  7    is a flow diagram depicting general stages of an example process for assessing and performing opportunistic vehicle air brake system repressurization, in accordance with an example embodiment. 
         FIG.  8    is a flow diagram depicting general stages of an example process for assessing and performing opportunistic vehicle air brake system repressurization, in accordance with a further example embodiment. 
         FIG.  9    is a block diagram of an example physical components of a computing device or system with which embodiments may be practiced. 
     
    
    
     DETAILED DESCRIPTION 
     As briefly described above, embodiments of the present invention are directed to systems and methods for opportunistic vehicle air brake system pressurization. In some aspects, an air compressor associated with a vehicle air tank automatically monitors air pressure within the air tank, and automatically runs on an as-needed basis while the vehicle is connected to a source of electricity other than a battery of the vehicle, in particular in instances where the battery of the vehicle is not currently being charged. For example, on an as needed basis, the air compressor may be actuated to re-pressurize air tanks for a vehicle&#39;s air brake system during a period in which regenerative braking occurs, or in which some other external electricity source (non-vehicle battery source) is provided. 
     In accordance with the present disclosure, the selective use of an air compressor during appropriate times provides a number of benefits to vehicle operators, as well as improvements in vehicle operation. Regarding the vehicle operator, by re-pressurizing air tanks when an electric air compressor is electrically connected to an external power source, time may be saved during truck startup. Specifically, a vehicle operator may be required to wait for 1 to 3 minutes for air tanks to pressurize after a vehicle has been started. By preemptively recharging air tanks, this delay may be avoided. Additionally, the vehicle operator would not be exposed to the noise, vibration, or harshness that is introduced by operation of the air compressor, since the vehicle operator would not need to be nearby during air compressor operation. 
     The methods described herein provide advantages to vehicle operation itself. For example, in some instances a vehicle battery is incapable of recharging at a rate equal to the rate of energy recaptured via regenerative braking. That is, regenerative braking generates more energy than may be received by the vehicle battery. Using the air compressor when the vehicle battery is incapable of keeping up with the regenerative braking power input will improve the regenerative braking power capabilities. This additional energy may be used to power an electric air compressor to recharge the air brake system of the vehicle, thereby providing more efficient vehicle operation by avoiding loss of the excess regenerative energy occurring during regenerative braking. Still further, providing such energy directly to an air compressor, rather than first to a battery from regenerative braking and then to the air compressor improves efficiency of power delivery, since some efficiency loss would otherwise be experienced in charging and discharging the vehicle battery. 
     Additionally, by powering the air compressor from a non-battery power source (e.g., an external power connection or from regenerative braking events), avoidance of using the battery may result in improved range in electric or hybrid vehicles, since stored energy in the battery may be used for vehicle propulsion rather than to power and air compressor. Still further, because use of the battery is avoided, a decrease in the number of charging and discharging events from the battery may reduce the wear rate of the battery, and therefore increase battery life. 
     In example embodiments, a threshold may be set for a pressure of the vehicle air brake system, and in particular one or more pressurization tanks used in such an air brake system. In examples, a low pressure threshold requires air pressure above 65 pounds per square inch (psi). However, in other examples, a higher threshold air pressure may be utilized. 
     In still further example embodiments, air brake system re-pressurization may be performed based on a battery status other than its current status as being recharged. For example, a battery charge status of being charged above a predetermined threshold (e.g., 85%-90%, or some other predetermined charge level) may be a condition that, if satisfied, would allow for actuation of an air compressor to perform air brake system re-pressurization, regardless of whether that battery is currently being charged. 
     In some instances, to ensure that air brake system re-pressurization occurs shortly before a vehicle is operated (and is not unnecessarily operated at a time significantly before a next scheduled vehicle operation), a re-pressurization operation may be performed based on a combination of observed events. For example, re-pressurization may be based on detection of both an external electrical connection provided to the vehicle (e.g. at an electric vehicle recharging station) or battery charge status, as well as initiation of another vehicle preconditioning operation. For example, air brake system re-pressurization may be scheduled to occur at the same time as or in close temporal proximity to vehicle cabin climate conditioning which may occur shortly before scheduled operation of the vehicle. Other indications that a vehicle is about to be operated may also be used to initiate air brake system re-pressurization, such as detecting that a door is opened, detecting that a check light switch is actuated, determining, from vehicle telematics messages received at the vehicle, that the vehicle is scheduled to be operated, or detecting that a propulsion battery pack is being preconditioned for operation (e.g., being heated in a cold environment to improve battery performance). In still further examples, a vehicle controller may be configured to predict when the vehicle will next be operated and can initiate an air brake system re-pressurization process in advance of that predicted operation timing. For example, the vehicle controller can assess prior periods of operation, or may be programmed to receive a particular vehicle operation schedule. In some instances, predicting impending operation of the vehicle may be performed based on a model of past vehicle operation. 
     In the case of electric vehicles, it is often the case that cabin climate conditioning (or pre-conditioning) may be performed in conjunction with battery charging. In particular, while an electric vehicle is electrically connected to a charging station, the electric vehicle may have one or more controllers that detect the charging condition, and in response, may pre-condition the interior environment of the vehicle, for example through use of an air conditioner or heater. Accordingly, the cab of a vehicle is already at an appropriate temperature at the time the driver enters the vehicle. The driver does not need to either (1) wait for vehicle air conditioning to occur before departing the charger, or (2) draw battery energy for purposes of air conditioning the cab. 
     In example implementations, a scheduled cabin climate conditioning program may be used to turn on an electric air compressor to charge air tanks used for the air brakes of an electric vehicle. The electric air compressor may then optionally use charger power, rather than depleting the battery of the vehicle. 
       FIG.  1    is an illustration depicting a side view of a vehicle  101  implementing an opportunistic vehicle air brake system repressurization, according to an example embodiment of the present disclosure. The vehicle  101  is one example of a type of vehicle that may be used in accordance with the opportunistic vehicle air brake system repressurization processes described herein. 
     In some examples, the vehicle  101  may be a heavy-duty truck such as a part of a tractor-trailer combination. The vehicle  101  may have what is sometimes referred to as, a fifth wheel by which a box-like, flat-bed, or tanker semi-trailer  103  (among other examples). may be attached for transporting cargo or the like. While the vehicle  101  is depicted as a truck in  FIG.  1   , it should be appreciated that the present technology is applicable to any type of vehicle where automated throttle filtering may be desired. 
     In the example shown, the vehicle  101  may be operated by an operator  10 , along a driving surface  12 . The vehicle  101 , in the example shown, includes an air brake system  110 , a vehicle controller  120 , a battery subsystem  130 , a propulsion motor  140  (e.g. sometimes operable as a generator and referred to in some instances as a motor/generator), a powertrain  150 , and one or more other vehicle subsystems. 
     In the example shown, the air brake system  110  may include air brakes  112 , one or more air tanks  114 , and lines there between, as well as an air compressor  116 . The air brake system  110  is described in greater detail in conjunction with an example schematic view seen in  FIG.  2   ; generally, the air brake system is operable by a vehicle operator  10  who presses on a brake pedal  118  to release air from air tanks  114  to actuate the air brakes  112 . On an as needed basis, and air compressor  116  may be operated to re-pressurize the air tanks  114  and lines connecting the air tanks with air brakes  112 . The air compressor  116  may be powered, for example as selected by vehicle controller  120 , via battery subsystem  130 . 
     As noted above, over time, a pneumatic pressure within an air brake system  110 , in particular within air tanks or lines  114  may gradually decrease. Accordingly, it may be necessary to operate the air compressor  116  periodically, or shortly before operation of the vehicle  101 . As further described herein, methods of selecting opportunistic times for operating the air compressor  116  allow for improved efficiency and reduced usage of battery capacity of the battery subsystem  130 , thereby improving overall efficiency in operation of the vehicle  101 , in particular in instances where the vehicle  101  corresponds to an electric vehicle. 
     The vehicle controller  120  includes a programmable circuit, such as a computing device, which may be operable to control one or more subsystems of the vehicle  101 . For example, the vehicle controller  120  may receive one or more sensor signals associated with the air brake system  110 , battery subsystem  130 , motor  140 , powertrain  150 , or other vehicle subsystems  180 , and may provide control signals, for example via a control bus within the vehicle, for actuating one or more subsystems in response to sensed conditions and/or user inputs. In some example embodiments, the vehicle controller  120  may include instructions for opportunistic recharging of the air brake system  110  in response to sensed or predicted vehicle conditions or operational statuses, as described in further detail below. 
     The battery subsystem  130  includes one or more batteries that are usable to power accessory subsystems within the vehicle  101 , as well as optionally batteries used to power the motor  140  and associated drivetrain  150  (e.g., in the case of the electric or plug-in hybrid electric vehicle). In some example instances, the battery subsystem  130  may include a connector configured to receive a connection from an external electrical source, such as a vehicle charging station  20 . 
     The propulsion motor  140  and associated powertrain  150  may operate to generate power and to convert the power into movement. For example, the propulsion motor  140  may include a power source, such as an engine, and the powertrain  150  various components that operate to convert the engine&#39;s power into movement of the vehicle (e.g. the transmission, driveshafts, differential, and axles). The powertrain  150  may be one of various types of powertrains (e.g., diesel, hydrogen fuel cell, battery electric). In some examples, the powertrain  150  may be operable with the propulsion motor  140  to selectively operate as a generator, for example in the case of a regenerative braking arrangement. In an example implementation and as will be described in further detail below, one or more criteria for operating the air compressor  116  may include that the propulsion motor  140  is generating energy in excess of a recharging capacity of the battery subsystem  130  in response to a vehicle operator&#39;s  10  engagement of a brake pedal  118 . Accordingly, in such circumstances, the propulsion motor  140  may generate and supply electrical power to the vehicle from a location external to the battery subsystem  130  which may be provided to other vehicle systems, such as the air brake system  110  and in particular the air compressor  116 . 
     In an example where the powertrain  150  comprises a battery electric powertrain operable with an electric motor implementing the propulsion motor  140  and battery subsystem  130  (or, in the alternative, a plug-in hybrid drivetrain that uses, in part, electrical power from battery subsystem  130  for power to the propulsion motor  140  and in part uses an internal combustion engine to drive the powertrain  150 ), the vehicle  101  may be operatively connectable to a vehicle charging station  20 . The vehicle charging station  20  may be a home or commercial vehicle charging station capable of supplying external electrical power to the vehicle, in particular for recharging battery subsystem  130 . Supply of electrical power to vehicle subsystems from a vehicle charging station may also correspond to a criteria for operating the air compressor  116  in an opportunistic manner. 
     The vehicle  101  may include one or more other vehicle subsystems  180 , such as accessory power systems, lighting systems, vehicle cabin temperature conditioning systems, communication systems, and various other types of equipment. Each of the other vehicle subsystems  180  may also be powered via the battery subsystem  130 . 
       FIG.  2    is a schematic block diagram of a portion of an air brake system  110  of a vehicle, such as the vehicle  101  of  FIG.  1   . In the example shown a vehicle operator may actuate a brake pedal  118 , which will release air stored in pressurized air reservoirs  114  toward brake chambers  112   a - n . The pressurized air may then be used to actuate breaks at wheels  105 . Additionally, in some embodiments, a parking brake control valve  117  may be pneumatically connected to trailer couplings  119 , thereby providing an air supply from the air reservoir  114  to trailer couplings, for example for connection to breaks chambers of a braking system included within semi-trailer  103 . 
     As noted above, an air compressor  116  may provide an air supply to the air reservoir is  114 , and may be actuated by signals from a vehicle control unit, such as vehicle controller  120 . For example, vehicle controller  120  may monitor a pressure within the air reservoirs  114 , or within lines between the reservoirs  114  and either the brake pedal  118  or parking brake control valve  117 , to ensure adequate air pressure within the reservoirs and/or lines to be able to effectively actuate braking systems of the vehicle  101 . In example embodiments, adequate air pressure corresponds to an air pressure above approximately 65 psi, and preferably up to or exceeding 100 psi. That is, at pressures below about 65 psi, and certainly below about 60 psi, air pressure within the air reservoirs  114  may be inadequate, when provided to break chambers  112   a - n , to provide adequate braking power to slow the vehicle  101  or otherwise maintain the vehicle in a stopped position. 
     As noted in  FIG.  2   , when a vehicle  101  is in operation, and in motion, the wheels  105  may be in rotation, and brakes may be actuated (e.g., via brake chambers  112   a - n ). In further embodiments, other manners of applying braking may be provided as well, for example through use of engine braking, or maintaining an engaged relationship between motor  140  and the powertrain  150  leading to the wheels  105 . If the motor  140  remains engaged with the powertrain  150  and wheels  105 , in some examples, in particular where the vehicle  101  is a battery electric vehicle or plug-in hybrid vehicle, the rotation of wheels  105  may result in operation of the motor  140  as a generator, thereby generating electrical power which may be provided back to the battery subsystem  130 . Such a regenerative braking action may cause the vehicle to slow, and also may, during the regenerative braking action, generate and deliver electrical power to the battery subsystem  130 . In some instances, the electrical power provided by regenerative braking may be greater than is usable to recharge the battery subsystem  130 . That is, in such instances, the electrical power may be provided at a rate higher than a recharging rate of a battery subsystem  130 . Accordingly, in some embodiments, an indication of excess electrical power available to the vehicle  101  may be one of the conditions in which the air compressor  116  may be actuated to re-pressurize air reservoirs  114  without requiring use of electrical power directly from the battery subsystem  130 . 
       FIG.  3    is a block diagram of a portion of an electrical system  130  of a vehicle, such as the vehicles of  FIGS.  1 - 2   . The electrical system  130  may be operatively connected to controller  120 , as well as may be used to provide electrical power to one or more external systems as described below. In the example shown, the electrical system  130  can include one or more high voltage battery packs  131  managed by a battery management system  132 , and connected to inverter  133  and a charger  134 . 
     In the example shown, the high voltage battery packs  131  may be connected to a control and data bus  302  via the battery management system  132 , which communicates with the controller  120 , as well as various other vehicle subsystems, regarding battery levels and discharge rate. The battery management system  132  further provides voltage regulation and current control output from the battery packs  131  to control degradation of the battery packs due to rapid charging/output. 
     In the example shown, the inverter  133  is electrically connected to the high voltage battery packs  131  and the battery management system via both the control and data bus  302 , as well as a high voltage bus  304 . The inverter  133  provides electrical power to motor  140  to power drivetrain  150 , for example by converting direct current (DC) energy to alternating current (AC) energy for use by an electric motor. The inverter  133  may also act to provide electrical energy back to the battery packs  131  in a regenerative braking situation, as previously described, via the high voltage bus  304 . The charger  134  provides external charging capabilities for the battery packs  131 , for example by providing an external electrical connection for use in connection to an external energy sources such as a vehicle charging station  20  as previously described, via the high voltage bus  304 . 
     In the example shown, the controller  120  may monitor a state of the inverter  133  and/or charger  134 , for example to determine an appropriate time to operate an auxiliary system, such as air compressor  116  as described above. 
     In the example shown, the battery packs  131  may provide electrical power to the air compressor  116 , as well as a plurality of external systems, including a DC-DC converter  135 , a cabin heater circuit  136 , an air conditioner refrigerant compressor  137 , a propulsion battery pack heater  138 , and a propulsion battery pack chiller  139  via the high voltage bus  304 . The air compressor  116  is generally an electrically-powered air compressor operatively integrated within an air brake system of the vehicle, such as is seen in  FIG.  2   . The DC-DC converter  135  may change a voltage level of output voltage from the battery packs  131  and battery management system  132 , for example to power external systems, such as the air brake system  110  and or other vehicle subsystems  180 . The cabin heater circuit  136 , air conditioner refrigerant compressor, propulsion battery pack heater  138 , and propulsion battery pack chiller  139  are each connected to an output of the battery packs  131  to receive electrical power via a high voltage bus  302 , and to communicate with controller over a controller area network  304 . 
     As noted above, based on various operational modes and circumstances in which electrical power may be available from sources other than the battery subsystem  130  (and in particular, from sources other than the battery packs  131 ), there are circumstances in which an air compressor may be allowed to operate using electrical power from such non-battery sources. For example, either during operation, or shortly before operation, of the vehicle, the air compressor  116  may be activated to repressurize air tanks  114  to an appropriate pressure for use. Air tanks may be depressurized over time (e.g., experiencing a reduction in pressure of 1-2 psi per minute or less) and therefore may need repressurization before vehicle  101  may be operated (e.g., after a long period of non-use) or may need repressurization mid-use. 
     Referring now to  FIGS.  4 - 5   , example timing diagrams illustrating example situations in which opportunistic vehicle air brake system repressurization may occur are shown.  FIG.  4    illustrates a first timing diagram  400  in which vehicle air brake system repressurization may occur when a vehicle is not in operation, according to a possible implementation. 
     In the timing diagram  400 , a series of time periods T 1 -T 5  are illustrated, representing different possible phases of operation of a vehicle and its subsystems. In time period T 1 , the vehicle is not in operation. Additionally, the vehicle may be connected to an external electrical connection, for example, because it is connected to an external vehicle charging station  20 , as described above. In time period T 1 , however, a control system of the vehicle may assess a system pressure of an air brake system  110 , for example, by determining a pressure within air tanks  114 . If the pressure is not below a threshold (e.g., below 60-65 psi, or in some instances below a higher threshold such as 80-100 psi), as seen in time period T 1 , the air compressor is not in operation. 
     In time period T 2  within the timing diagram  400 , the vehicle remains charging and not in operation, but the pressure sensed within the air brake system is below the predetermined threshold. In this instance, in some cases, an additional condition may be considered to be satisfied—that is, external electrical power is available, and the air brake system is below an ideal pressure. However the control system  120  may determine that the vehicle will not be in operation in the near future. This may be based on, for example, a programmed schedule of vehicle operation input by an operator, a learned schedule managed by the controller based on historical operation timing, or other input, such as a remote signal from the user. Accordingly, in time period T 2 , the air compressor will remain not in operation. 
     In time period T 3  within the timing diagram, the control system  120  may have determined that a time has been reached that is within a predetermined amount of time before expected vehicle operation. The time reached may be variable, selected by the vehicle operator, or chosen based on historical information about how long the vehicle requires to repressurize its air brake system. In example embodiments, the time before expected vehicle operation may be 5-10 minutes or less, or may be a time after receipt of a user indication of impending vehicle operation. 
     In some instances, the indication of impending vehicle operation may be tied to one or more external events to the air brake system. For example, in some electric vehicles, a vehicle cabin preconditioning operation may be programmed to begin a predetermined or estimated period of time before expected vehicle operation; in such instances, air brake system repressurization may occur during the same or a similar time before operation. 
     In alternative embodiments, in time period T 2 , the vehicle will not consider whether operation is near in time, and will instead of waiting for a time closer to operation, will operate the air compressor  116  to repressurize the air brake system. This may be preferably when no operator input has been received regarding an intended schedule, or where historical operation timeframes are either highly variable or unknown. 
     In time period T 4  within the timing diagram  400 , the electrical connection is disconnected, indicating that the vehicle  101  is no longer connected to an external power source. In this instance, assuming the air brake system pressure is above a threshold, the air compressor  116  may cease operation. Nevertheless, because the air brake system was repressurized during time period T 3 , at time period T 5  when operation of the vehicle  101  is begun, the vehicle operator  10  does not need to initiate repressurization at the time he or she wishes to begin operating the vehicle, does not need to wait for repressurization of the air brake tanks  114 , and does not need to either rely on battery power to supply power to the air compressor  116  or otherwise delay disconnection of an external electrical connection to wait for repressurization to complete. 
       FIG.  5    is a second example timing diagram  500  illustrating example opportunistic vehicle air brake system repressurization when a vehicle is in operation. The opportunistic vehicle air brake system repressurization reflected in timing diagram  500  may be used in addition to, or in place of, the repressurization arrangement seen in  FIG.  4   . 
     In the example shown, the vehicle  101  is illustrated as being in operation in all timing stages T 6 -T 9 . In time period T 6 , no braking is occurring, and the air compressor  116  is not being operated. The control system  120  may continually monitor air brake system pressure, and, in period T 6 , the air pressure is above the predetermined threshold that is selected for safe brake operation. However, as noted above, air pressure within the air brake system  110  may fall over time, e.g., due to use or leakage. 
     In time period T 7 , a regenerative braking process has begun during operation of the vehicle. In this instance, the motor  140  may be generating electrical power which may be returned to the battery subsystem  130  for recharging the battery. Depending on the rate at which energy is generated by the regenerative braking operation, it may be that more energy is generated than may be used to recharge the battery subsystem  130 , for example because a rate of recharging the battery subsystem is limited. Optionally, either regardless of the among of electrical energy, or based at least in part on the energy generated by regenerative braking exceeding a rate of battery subsystem recharge, energy generated by regenerative braking may be used to power the air compressor  116 , such that it may be used to re-pressurize the air brake system (e.g., air tanks  114 ) during time period T 7 . 
     In time period T 8 , regenerative braking has continued, but an air brake system pressure has increased to the point that it is above a pressure threshold. In this instance, operation of the air compressor  116  may be discontinued, since at least the below-pressure condition is no longer satisfied. In time period T 9 , the regenerative braking event has also discontinued, and operation may continue as in time period T 6 . 
     It is noted that there may be additional time periods during operation of a vehicle in which the air compressor  116  should be operated, but where no external power source (such as regenerative braking of an external electrical connection) are available. For example, if, in time period T 6  the air brake system pressure fell below a safe threshold, the air compressor  116  may be actuated, despite the fact that no regenerative braking event has occurred/is occurring. Additionally, the time during which regenerative braking occurs may be shorter than a time required to repressurize regenerative brakes, rather than longer (as is seen in time periods T 7 -T 8 ); in some instances, the air compressor will be allowed to continue operation after a regenerative braking event has ended (e.g., into time period T 8 ) if an air pressure within the air brake system has not yet returned to above the predetermined threshold. Other alterations and variations of the timing diagrams  400 ,  500  are possible as well, consistent with the present disclosure. 
       FIG.  6    is a third example timing diagram  600  illustrating example opportunistic vehicle air brake system repressurization based on a battery charging status. The opportunistic vehicle air brake system repressurization reflected in timing diagram  700  may be used in addition to, or in place of, the repressurization arrangement seen in  FIGS.  4 - 5   . 
     In time period T 10 , a battery is below a threshold capacity. The threshold capacity may be a programmable threshold, for example 85-90%, at which the battery is considered to be adequately charged to allow for initiation of a repressurization process. In this example, the air brake system pressure is also above a predetermined threshold at which repressurization would be required. Both of these conditions indicate that repressurization during this time period would be suboptimal. 
     In time period T 11 , the battery has been charged to a battery charge level above the predetermined threshold. This may be due to the battery being recharged via an external power source, such as either an external power connection or via regenerative braking. In this time period, the air brake system pressure remains above the predetermined threshold at which repressurization would be required, and therefore a repressurization process need not be invoked. 
     At time period T 12 , the air brake system pressure has fallen below a predetermined pressure threshold. Additionally, the battery capacity of a vehicle battery (e.g., a battery used for propulsion or an auxiliary battery, such as 12V auxiliary battery) is above the threshold at which the battery is considered adequately charged. Accordingly, during time period T 12 , a condition for activation of an air compressor  116  may be satisfied. That is, assuming any other conditions that might apply are satisfied, the air compressor will be activated (shown as “ON”), thereby causing an increase in air brake system pressure. 
     At time period T 13 , the air brake system pressure will reach or exceed a predetermined threshold, and as such, the air compressor  116  may cease operation. In some instances, operation of the air compressor  116  may cease at a time the battery capacity decreases to the programmed threshold. In other instances, operation of the air compressor may cease upon the air pressure within the air brake system exceeding a pressure threshold, or reaching a target pressure level above the pressure threshold. 
     At time period T 14 , the vehicle may begin operation. As illustrated in timing diagram  600 , time periods T 10 -T 13  preferably occur when the vehicle is not in operation, since, at least during time period T 12 , the air brake system pressure would be below a preferred pressure to be maintained during vehicle operation. However, in some instances, one or more of those time periods may include operation of the vehicle. Preferably, time T 12  occurs shortly before beginning operation of a vehicle (e.g., prior to T 14 ), such that the air brake system pressure does not have time to depressurize before intended vehicle operation. For example, time period T 12  may be selected, for example based not only on battery charge level, but also based on historical operating timing of the vehicle, based on cabin or battery preconditioning operations, or based on vehicle telematics messages received that indicate impending operation of the vehicle. 
     Referring to  FIGS.  7 - 8   , flow diagrams of general stages of example processes for assessing and performing opportunistic vehicle air brake system repressurization are shown. The processes described in  FIGS.  7 - 8    are generally able to be performed using a combination of an air brake system  110 , controller  120 , and optionally some other portions of a vehicle, such as the battery subsystem  130  and motor  140 /powertrain  150  of  FIG.  1   . 
     Referring first to  FIG.  7   , an example method  700  includes monitoring for a vehicle battery charging status, at operation  702 . Monitoring for a vehicle battery charging status may include, for example, determining whether a connection to an external electrical power source, such as a vehicle charging station  20 , is in place. 
     Continuing at decision operation  704 , it is determined whether a charging event is occurring. If a charging event is not occurring, the method  700  may continue at operation  702  to continue monitoring for such a charging event. However, if a charging event is occurring, at operation  706 , an air brake system pressure may be assessed. If, at decision operation  708 , it is determined that an air brake system pressure is not below a threshold, in some embodiments, flow will return to operation  702  to continue monitoring for future vehicle battery charging events without initiating air compressor operation. In alternative embodiments, even if above a preset threshold, flow may proceed with decision operation  710 . 
     At decision operation  710 , it is determined whether the charging event is occurring within a predetermined time before operation of the vehicle. That is, decision operation  710  may determine that expected operation of the vehicle is within a predetermined amount of time, for example based on a preset operation schedule or observed operation and predicted future operation schedule. For example, a control system may be configured to predict impending operation of the vehicle based on a model generated from stored records of past vehicle operation. In still further examples, determining whether the charging event is occurring within a predetermined time includes determining that another vehicle preconditioning operation is occurring, such as a vehicle cabin temperature conditioning operation, a battery temperature conditioning operation, or other preconditioning operations. In still further examples, the predetermined time may be triggered based on a signal from a vehicle operator  10  that operation is impending, for example by the vehicle operator  10  opening a door to a cab of the vehicle, turning on a check lights switch, or other operation typically performed in advance of operation. 
     In some embodiments, if not within the predetermined operation schedule, it may be deemed unnecessary to activate the air compressor  116 , since repressurization of the air brake system would be unnecessary, and that system would likely discharge before operation in any event. Accordingly, flow would return to operation  702 . However, in some other embodiments, regardless of whether within a predetermined amount of time before vehicle operation, or if it is determined that vehicle operation is pending (i.e., within the predetermined period), flow proceeds to operation  712 , in which an air compressor  116  is powered. In example embodiments, the air compressor may be powered using the external power source used for the vehicle charging event, thereby avoiding use of the battery subsystem to power the air compressor  116 . 
     Referring to  FIG.  8   , a further example method  800  includes monitoring for a vehicle battery charging status, at operation  802 . Monitoring for a vehicle battery charging status, in this instance, may include detecting a regenerative braking operation being performed by the vehicle while the vehicle is in operation. Monitoring for a vehicle battery charging status may also include, in some examples, detecting that a battery charging status is above a predetermined threshold (e.g., above 85-90%, or above some other programmable threshold). 
     At decision operation  804 , it is determined whether a charging status satisfies a particular condition. In some examples, this determination includes determining whether a charging event is occurring that can provide an adequate power supply to deliver electrical power to the air compressor  116 . In some instances, operation  804  may include determining whether a charging event provides adequate power without relying on electrical power from the battery subsystem  130 , and optionally while concurrently prioritizing electrical power delivery for recharge of the battery subsystem  130 . This may include, for example, determining that power obtained by regenerative braking is in excess of the power intake of a battery subsystem  130 , and therefore, such excess electrical power may be provided to an air compressor. In alternative instances, operation  804  may include determining whether a battery charging status indicates that the battery is charged above a predetermined charge level, regardless of whether the vehicle is connected to an external charger or whether a regenerative braking operation is occurring. 
     If this charging status is not satisfied (e.g., inadequate power is provided, no charging event occurs, or battery power is inadequate), flow may return to operation  802 . However, if adequate power is provided by way of the charging status, flow proceeds to operation  806 , in which an air brake system pressure is assessed. If, at decision operation  808 , it is determined that a pressure of the air brake system is below a threshold, at operation  810 , the air compressor  116  may be powered. In some circumstances, the air compressor may be powered using power obtained from somewhere other than a battery subsystem (e.g., the source of the battery charging event, such as excess regenerative braking energy). If the air brake system pressure is not below a threshold, in some examples, flow may return to operation  802  to continue to monitor for battery charging events. 
     Referring generally to  FIGS.  7 - 8   , it is noted that certain assessments may be performed in different orders or may be excluded entirely, in some embodiments. 
       FIG.  9    is a block diagram of an illustrative computing device  900  appropriate for use in accordance with embodiments of the present disclosure. The description below is applicable to the control system  120 , servers, personal computers, mobile phones, smart phones, tablet computers, embedded computing devices, and other currently available or yet-to-be-developed devices that may be used in accordance with embodiments of the present disclosure. 
     In its most basic configuration, the computing device  900  includes at least one processor  902  and a system memory  904  connected by a communication bus  906 . Depending on the exact configuration and type of device, the system memory  904  may be volatile or nonvolatile memory, such as read-only memory (“ROM”), random access memory (“RAM”), EEPROM, flash memory, or other memory technology. Those of ordinary skill in the art and others will recognize that system memory  804  typically stores data or program modules that are immediately accessible to or currently being operated on by the processor  902 . In this regard, the processor  902  may serve as a computational center of the computing device  900  by supporting the execution of instructions. According to one example, the system memory  904  may store one or more instructions  950  for opportunistic air brake repressurization (e.g., to perform the methods of  FIGS.  7 - 8   , above). 
     As further illustrated in  FIG.  9   , the computing device  900  may include a network interface  910  comprising one or more components for communicating with other devices over a network. Embodiments of the present disclosure may access basic services that utilize the network interface  910  to perform communications using common network protocols. The network interface  910  may also include a wireless network interface configured to communicate via one or more wireless communication protocols, such as WiFi, 2G, 3G, 4G, 5G, LTE, WiMAX, Bluetooth, or the like. 
     In the illustrative embodiment depicted in  FIG.  9   , the computing device  800  also includes a storage medium  908 . However, services may be accessed using a computing device that does not include means for persisting data to a local storage medium. Therefore, the storage medium  908  depicted in  FIG.  9    is optional. In any event, the storage medium  908  may be volatile or nonvolatile, removable or non-removable, implemented using any technology capable of storing information such as, but not limited to, a hard drive, solid state drive, CD-ROM, DVD, or other disk storage, magnetic tape, magnetic disk storage, or the like. 
     As used herein, the term “computer-readable medium” includes volatile and nonvolatile and removable and non-removable media implemented in any method or technology capable of storing information, such as computer-readable instructions, data structures, program modules, or other data. In this regard, the system memory  904  and storage medium  908  depicted in  FIG.  9    are examples of computer-readable media. 
     For ease of illustration and because it is not important for an understanding of the claimed subject matter,  FIG.  9    does not show some of the typical components of many computing devices. In this regard, the computing device  900  may include input devices, such as a keyboard, keypad, mouse, trackball, microphone, video camera, touchpad, touchscreen, electronic pen, stylus, or the like. Such input devices may be coupled to the computing device  900  by wired or wireless connections including RF, infrared, serial, parallel, Bluetooth, USB, or other suitable connection protocols using wireless or physical connections. 
     In any of the described examples, data can be captured by input devices and transmitted or stored for future processing. The processing may include encoding data streams, which can be subsequently decoded for presentation by output devices. Media data can be captured by multimedia input devices and stored by saving media data streams as files on a computer-readable storage medium (e.g., in memory or persistent storage on a client device, server, administrator device, or some other device). Input devices can be separate from and communicatively coupled to computing device  900  (e.g., a client device), or can be integral components of the computing device  900 . In some embodiments, multiple input devices may be combined into a single, multifunction input device (e.g., a video camera with an integrated microphone). The computing device  900  may also include output devices such as a display, speakers, printer, etc. The output devices may include video output devices such as a display or touchscreen. The output devices also may include audio output devices such as external speakers or earphones. The output devices can be separate from and communicatively coupled to the computing device  900 , or can be integral components of the computing device  900 . Input functionality and output functionality may be integrated into the same input/output device (e.g., a touchscreen). Any suitable input device, output device, or combined input/output device either currently known or developed in the future may be used with described systems. 
     In general, functionality of computing devices described herein may be implemented in computing logic embodied in hardware or software instructions, which can be written in a programming language, such as C, C++, COBOL, JAVA™, PHP, Perl, HTML, CSS, JavaScript, VBScript, ASPX, Microsoft .NET™ languages such as C#, or the like. Computing logic may be compiled into executable programs or written in interpreted programming languages. Generally, functionality described herein can be implemented as logic modules that can be duplicated to provide greater processing capability, merged with other modules, or divided into sub-modules. The computing logic can be stored in any type of computer-readable medium (e.g., a non-transitory medium such as a memory or storage medium) or computer storage device and be stored on and executed by one or more general-purpose or special-purpose processors, thus creating a special-purpose computing device configured to provide functionality described herein. 
     Many alternatives to the systems and devices described herein are possible. For example, individual modules or subsystems can be separated into additional modules or subsystems or combined into fewer modules or subsystems. As another example, modules or subsystems can be omitted or supplemented with other modules or subsystems. As another example, functions that are indicated as being performed by a particular device, module, or subsystem may instead be performed by one or more other devices, modules, or subsystems. Although some examples in the present disclosure include descriptions of devices comprising specific hardware components in specific arrangements, techniques and tools described herein can be modified to accommodate different hardware components, combinations, or arrangements. Further, although some examples in the present disclosure include descriptions of specific usage scenarios, techniques and tools described herein can be modified to accommodate different usage scenarios. Functionality that is described as being implemented in software can instead be implemented in hardware, or vice versa. 
     Many alternatives to the techniques described herein are possible. For example, processing stages in the various techniques can be separated into additional stages or combined into fewer stages. As another example, processing stages in the various techniques can be omitted or supplemented with other techniques or processing stages. As another example, processing stages that are described as occurring in a particular order can instead occur in a different order. As another example, processing stages that are described as being performed in a series of steps may instead be handled in a parallel fashion, with multiple modules or software processes concurrently handling one or more of the illustrated processing stages. As another example, processing stages that are indicated as being performed by a particular device or module may instead be performed by one or more other devices or modules. 
     The principles, representative embodiments, and modes of operation of the present disclosure have been described in the foregoing description. However, aspects of the present disclosure which are intended to be protected are not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. It will be appreciated that variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present disclosure. Accordingly, it is expressly intended that all such variations, changes, and equivalents fall within the spirit and scope of the claimed subject matter.