Patent Publication Number: US-10759303-B2

Title: Autonomous vehicle route planning

Description:
TECHNICAL FIELD 
     This disclosure relates to route planning for autonomous vehicles, and more specifically to planning vehicle speeds based on battery thermal-management constraints so that battery temperatures remain below a threshold for the route. 
     BACKGROUND 
     Vehicles may include autonomous driving systems configured to drive the vehicle with minimal user input. Autonomous driving systems receive data from on-board vehicle systems, such as cameras, Radar, etc., as well as from external sources. This data is used to generate commands, e.g., steering, braking, and acceleration, for autonomously driving the vehicle. 
     An autonomous vehicle may include an electric powertrain having an electric machine(s) powered by a traction battery. The traction battery produces heat when providing power to the electric machine, and require a thermal-management system to thermally regulate the temperature of the battery cells. Example thermal-management systems include air and liquid cooling systems. Multiple types of liquid cooling systems are available such as radiator cooling, chiller cooling (which utilizes a heat pump), or combinations thereof. 
     Traction batteries are operated within upper and lower temperature limits to prevent battery degradation and optimize performance. Hybrid vehicles are typically programmed to power limit the battery in response to these upper and lower temperature limits being exceeded. Thus, the electric range of the vehicle may be reduced due to deficiencies in battery cooling. 
     SUMMARY 
     According one embodiment, an autonomous vehicle includes an electric powertrain and a controller. The controller is programmed to autonomously operate the powertrain to maintain constant vehicle speed along a segment of a route responsive to predicted battery temperatures for the segment not exceeding a threshold, and autonomously operate the powertrain to vary vehicle speed along the segment responsive to the predicted battery temperatures exceeding the threshold such that actual battery temperatures remain below the threshold for the segment. 
     According to another embodiment, an autonomous vehicle includes an electric powertrain and a controller. The controller is programmed to autonomously operate the powertrain to propel the vehicle along a segment of a route according to a predetermined first speed profile that has a constant vehicle speed responsive to predicted battery temperatures for the segment not exceeding a threshold. The controller is further programmed to autonomously operate the powertrain to propel the vehicle along the segment according to a predetermined second speed profile that has multiple vehicle speeds responsive to the predicted battery temperatures exceeding the threshold such that actual battery temperatures remain below the threshold for the segment. 
     According to yet another embodiment, an autonomous vehicle includes an electric powertrain having an electric machine and a traction battery. A vehicle controller is programmed to command power to the electric machine to propel the vehicle along a segment of a route according to a predetermined speed profile that is derived from a predicted heat generation of the battery for the segment such that actual temperatures of the battery remain below a temperature threshold for the segment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a representative electrified vehicle. 
         FIG. 2  is a plot showing the power consumption for battery heat convection coefficients. 
         FIG. 3  is a series of plots showing vehicle operating conditions for a segment of a route according to prior-art designs. 
         FIG. 4  is a flow chart illustrating an algorithm for generating a plurality of possible vehicle speed profiles for a route. 
         FIG. 5  is flow chart illustrating an algorithm for selecting one of the possible vehicle speed profiles for use while autonomously driving a vehicle. 
         FIG. 6  is a series of plots showing vehicle operating conditions for a segment of a route according to this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments are disclosed herein; however, it is to be understood that the disclosed embodiments are merely representative of the claimed subject matter and may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the embodiments. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
     The embodiments of the present disclosure may include various internal and external circuits or other electrical devices. All references to the circuits and other electrical devices and the functionality provided by each, are not intended to be limited to encompassing only what is illustrated and described herein. While particular labels may be assigned to the various circuits or other electrical devices disclosed, such labels are not intended to limit the scope of operation for the circuits and the other electrical devices. Such circuits and other electrical devices may be combined with each other and/or separated in any manner based on the particular type of electrical implementation that is desired. It is recognized that any circuit or other electrical device disclosed herein may include any number of discrete passive and active components such as resistors, capacitors, transistors, amplifiers, analog/digital converters (ADC or A/D converters), microprocessors, integrated circuits, non-transitory memory devices (e.g., FLASH, random access memory (RAM), read-only memory (ROM), electrically programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), or other suitable variants thereof) and software which cooperate with one another to perform operation(s) disclosed herein. In addition, any one or more of the electric devices may be configured to execute a computer program that is embodied in a non-transitory computer-readable storage medium that includes instructions to program a computer or controller to perform any number of the functions as disclosed. 
       FIG. 1  is a block diagram of a representative electrified vehicle embodiment having at least one controller programmed to autonomously drive the vehicle. While a fully electric vehicle is illustrated in this representative embodiment, those of ordinary skill in the art will recognize that the disclosed embodiments may also be utilized in other types of electrified vehicles, such as hybrid vehicles. 
     In the representative implementation illustrated in  FIG. 1 , a vehicle  20  may include one or more electric machines  22  mechanically connected to a transmission  24 , such as a one-speed gearbox. The electric machine  22  may be capable of operating as a motor or a generator. The transmission  24  is mechanically connected to a drive shaft  26  that is mechanically connected to the wheels  28 . The electric machine  22  can provide propulsion and deceleration capability. The electric machine  22  may also act as a generator to provide fuel economy benefits by recovering energy that would normally be lost as heat in the friction braking system during regenerative braking. 
     A traction battery or traction battery pack  30  stores energy in a plurality of individual battery cells connected together to provide a desired voltage and charge capacity for the electric machine  22 . In one embodiment, battery pack  30  includes an array of lithium-ion battery cells. The vehicle battery pack  30  typically provides a high-voltage direct current (DC) output to a high-voltage bus  32 , although the voltage and current may vary depending on particular operating conditions and loads. The traction battery  30  is electrically connected to one or more external circuits  34 , which may include a power electronics or inverter circuit  36 , and a DC/DC converter circuit  38 , for example. One or more contactors may isolate the traction battery  30  from other components when opened, and connect the traction battery  30  to the other components when closed. The traction battery  30  may include various internal circuits for measuring and monitoring various operating parameters including cell current and individual cell voltage. Parameters such as voltage, current and resistance for a battery cell or a group of battery cells (sometimes referred to as a block or brick) may be monitored and/or controlled. The battery  30 , the electric machine  22 , the transmission  24 , and other components make up an electric powertrain  23 . 
     In addition to providing energy for propulsion, the traction battery  30  may provide energy for other external circuits  34  connected to the high-voltage bus  32 . The power-distribution system of vehicle  20  may also include a DC/DC converter module or circuit  38  that converts the high-voltage DC output of the traction battery  30  to a low-voltage DC supply that is compatible with other vehicle loads that may be directly connected. Other external high-voltage circuits or loads, such as those for cabin or component heaters, may be connected directly to the high-voltage bus  32  without the use of the DC/DC converter circuit  38 . 
     Vehicle  20  may also include an auxiliary battery  42  having a relatively lower nominal voltage (such as 24V or 48V, for example) and may be implemented using different battery chemistries than the traction battery  30 . The auxiliary battery  42  may also be referred to as a low-voltage battery, starter battery, or simply the vehicle battery for various applications. The auxiliary battery  42  may be used to power various low-voltage components, controllers, modules, motors, actuators, sensors, etc. generally represented by electric loads  44 . One or more relay/voltage converters  46  may be used to power vehicle electrical load(s)  44 . 
     The traction battery  30  may be recharged by an external power source. The external power source may include an electrical outlet connected to the power grid. The external power source may be electrically connected to electric vehicle supply equipment (EVSE). The EVSE may provide circuitry and controls to regulate and manage the transfer of energy between the power source and the vehicle  20 . The external power source may provide DC or AC electric power to the EVSE. The EVSE may have a charge connector for plugging into a charge port of the vehicle  20 . The charge port may be electrically connected to a charger or on-board power conversion module. Alternatively, various components described as being electrically connected may transfer power using a wireless inductive coupling. 
     The various components illustrated in  FIG. 1  may have one or more associated controllers, control modules, and/or processors such as controller  50  to control vehicle and traction battery operation. The controller  50  may be one or more controllers that cooperate with each other to control the vehicle. Any reference herein to “a controller” means one or more controllers. The controllers may communicate via a serial peripheral interface (SPI) bus (e.g., Controller Area Network (CAN)) or via discrete conductors. Various operating parameters or variables may be broadcast or published using the CAN or other conductors for use by vehicle control modules or sub-modules in controlling the vehicle or vehicle components. One or more controllers may operate in a stand-alone manner without communication with one or more other controllers. The controller  50  is programmed to control various charging and discharging functions, battery cell charge balancing, battery pack voltage measurements, individual battery cell voltage measurements, battery over-charge protection, battery over-discharge protection, battery end-of-life determination, battery current polarity or direction (charging and discharging), etc. 
     The controller(s)  50  may include and/or communicate with various types of non-transitory computer-readable storage media including persistent and temporary storage devices to store control logic, algorithms, programs, operating variables, and the like. In one embodiment, the controller  50  may communicate with memory for storing values associated with battery cell desired open circuit voltage values, thresholds, or patterns. Similarly, controller  50  may communicate with memory having values stored in lookup tables or arrays associated with battery cell internal resistance based on battery parameters such as temperature, state of charge (SOC), age, etc. In one embodiment, controller  50  communicates with memory having a battery power vs. temperature lookup table. The controller  50  may also communicate with memory storing battery charge and discharge power limits, and/or battery minimum and maximum temperature limits. 
     As an autonomous vehicle, the controller  50  is configured to drive the vehicle  20  along a route, which may be user-selected, with no or minimal input from vehicle occupants. In order to accomplish autonomous driving, the vehicle may be equipped with a vision system that includes radar, lidar, ultrasonic sensors, cameras, etc. The vehicle  20  may also be equipped with communication systems that allow the vehicle  20  to communicate with other vehicles, central stations, and the like, and navigation systems such as global positioning systems (GPS). The vehicle may include one or more user interfaces, such as touch screens, voice systems, etc., allowing communication between the vehicle  20  and the vehicle occupants. The controller  50  is configured to receive signals from relevant systems and issue commands to drive the vehicle autonomously. The commands may include propulsion commands, e.g. acceleration and speed, braking commands, e.g. friction braking and regenerative braking, steering commands, and the like. Baseline profiles for these commands may be generated during route calculation. But of course, the vehicle  20  is capable of deviating from these profiles in real-time based on actual operating conditions. 
     Driving of the vehicle  20  requires the traction battery  30  to supply voltage and current to the electric machine  22 , which generates heat. The vehicle  20  includes a battery-cooling system  54  for thermally regulating the battery  30 . The battery-cooling system  54  may take various forms in different embodiments. For example, the battery-cooling system  54  may be an air-cooled system that circulates air across the cells of the battery  30  to cool the battery. Alternatively, the battery-cooling system  54  may be a liquid-cooled system that circulates coolant through or around the traction battery  30  to cool the cells. Multiple types of liquid-cooled systems can be utilized in the vehicle  20 . In one embodiment, the coolant circulating through the battery  30  may be routed to an external radiator to exchange heat with an outside airstream. In another embodiment, the coolant circulating through the battery may be routed to a battery chiller that is associated with a heat pump of the vehicle  20 , such as the vehicle air-conditioning system. In yet another embodiment, the cooling system  54  may be capable of circulating coolant to a radiator and to a chiller. 
     The battery-cooling system  54  may be powered by the traction battery  30  and may consume power otherwise available for vehicle propulsion. Excessive use of the cooling system  54  can significantly reduce electric range. Thus, it is advantageous to utilize the battery-cooling system  54  efficiently to maximize range of the vehicle  20  while also maintaining the battery  30  within a desired range of temperatures. 
     Temperature change of the traction battery  30  can be calculate by equation 1, where I is battery current, R is battery resistance, SOC is battery state of charge, T bat  is battery temperature, h is battery heat convection coefficient, P_cooling is consumed power of battery-cooling system, T enviroment  is inlet coolant temperature of the cooling system, and α is battery heat capacity. 
     
       
         
           
             
               
                 
                   
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     The first term, ∫ 0   t  I 2 R(SoC, T bat )dt, represents heat generated by the battery and is largely dependent upon current. The second term, ∫ 0   t  h(P_cooling)(T bat −T enviroment )dt, represents the cooling ability of the battery-cooling system  54 . 
       FIG. 2  illustrates a graph of heat convection coefficients (h) (y-axis) and the amount of power consumed (x-axis) by the cooling system  54  to achieve those heat coefficients (h). As shown by the graph, the amount of power (P) required to increase the convection coefficient (h) follows a nonlinear curve and the gains in h for each unit of power (P) greatly diminish as h increases. Thus, it is more efficient to operate the cooling system  54  with lower values of h. 
     Operating the cooling system  54  with lower values of h, however, is not always possible due to power demands from the electric machine to maintain a desired driving speed. The desired driving speed may force the cooling system  54  to operate at maximum capacity in order to avoid overheating of the traction battery  30 . The controls and methods of this disclosure account for battery thermal management while route planning so that the cooling system  54  may be operated in a most efficient range of h while also not exceeding the upper temperature limit of the battery  30 . This will be described in more detail below. 
     Before turning to the controls and methods of this disclosure, common problems associated with the current state-of-the-art are highlighted by the example of  FIG. 3 . Referring to  FIG. 3 , the plot  100  illustrates the road grade for a segment. This particular segment  106  includes sections  108  of increasing road grade and decreasing road grade. The vehicle speed profile  110  for this segment  106  is constant. Maintaining a constant speed over variable road grade requires the battery to discharge when traveling uphill and charge (regeneratively brake) while traveling downhill to maintain the constant speed as shown by plot  103 . Battery temperature is highly dependent upon current output of the traction battery. As shown by plot  104 , attempting to maintain a constant speed over the varying road grade of segment  106  generates a significant amount of heat  112 . As explained above, battery-cooling systems have a maximum capacity and operate more efficiently when operating in a lower range of that capacity. In this example, the heavy charging and discharging of the battery to maintain the vehicle speed profile  110  is generating heat  112  in excess of the cooling capacity  114 . Thus, the temperature of the battery  116  is continuing to increase as shown by plot  105 . To prevent battery damage, traction batteries have a maximum temperature limit  118 , and vehicle controllers typically prevent the battery from exceeding that limit  118  by power limiting the battery in response to the battery temperature exceeding the upper limit  118 . In this example, the battery temperature exceeds the upper limit  118  at time T 1 . In response, the battery is power limited causing the actual vehicle speed  120  drop below the speed profile  110  at time T 1 . Thus, after time T 1 , the vehicle will be operated at a speed slower than that desired by the occupant of the vehicle until the battery temperature is reduced below the upper limit  118 . Most occupants find it highly dissatisfying to operate the vehicle in a power-limited mode, and thus conditions such as those described in  FIG. 3  should be avoided. 
     The scenario of  FIG. 3  can be avoided by considering battery-heat generation and cooling-system capacity during route planning, and more specifically during generation of the vehicle speed profile(s) for the route. In one embodiment of the controls, a two-phase methodology is used to optimize the speed profile(s) for a route. In the first phase, a plurality of possible speed profiles are generated based on battery power capability, e.g., battery charging and discharging limits, and speed profiles that pass the battery power capability are saved for further screening in phase two. In the second phase, predicted battery heat generation and cooling system energy consumption are determined for each of the saved profiles, and optimum one of the saved profiles is chosen to be used for the route. 
       FIG. 4  is a flow chart  150  illustrating a method/controls for calculating one or more vehicle speed profiles for use during autonomous driving of the vehicle  20 . The processes, systems, methods, heuristics, etc. described herein may be described as occurring in an ordered sequence although such processes could be performed with the described steps completed in an order other than the order described or illustrated. It should be understood that certain steps could be performed simultaneously, that other steps could be added, or that certain steps described herein could be omitted while keeping with the teachings of this disclosure and being encompassed by the claimed subject matter. The descriptions of methods or processes are provided for the purpose of illustrating certain embodiments, and should be understood to be representative of one of many variations and not limited to only those shown or described. As generally understood by those of ordinary skill in the art, the system or method may be implemented through a computer algorithm, machine-executable code, or software instructions programmed into one or more suitable programmable devices associated with the vehicle. 
     Referring to  FIG. 4 , the flowchart begins at operation  152  where the controller receives a trip request from a vehicle occupant. The trip request may be an end destination, e.g., address, cross roads, business, etc., input by the occupant. In response to receiving the address, the controller may utilize a mapping system to calculate a route between the current destination and the end destination. 
     At operation  154 , the controller generates a plurality of speed profiles V(i,t) for the route. The speed profiles may be for the entire route or for a segment of the route. Each speed profile includes one or more propulsion commands for the electric powertrain. The speed profiles may include a constant speed, such as in the example of  FIG. 3 , or variable speeds. Governmental minimum and maximum speed limits for the route may be considered in generating baseline speed profiles, and additional speed profiles derived therefrom may be generated to create a spectrum of possible speed profiles at operation  154 . 
     At operation  156 , the controller calculates vehicle power demand P(i,t) for each of the speed profiles. The power demand represents the amount of power that must be supplied by the traction battery in order to execute the speed profile. Vehicle accessory load power can be estimated and included in the vehicle power demand. At operation  158 , the controller receives current operating conditions of the traction battery and of the battery-cooling system. 
     The outputs of operations  156  and  158  are passed to operation  160  where the controller determines battery capabilities. The battery capabilities may include the battery state of charge, predicted battery temperature, the battery power charge limit (positive value), and the battery power discharge limit (negative value). 
     At operation  162 , the controller tests the speed profiles of  154  to determine if any violate the power capabilities of operation  160 . For example, the controller may determine if the vehicle power demand P(i,t) is greater than the charge limit or less than the discharge limit. If yes, that profile violates the power capabilities and is discarded at operation  164 . The controls of operation  162  are looped until all of the speed profiles have been tested. Profiles that pass the test of operation  162  are saved in a set for later use and those that do not are discarded. 
     At operation  166 , the controller determines if the set has any entries. If yes, control passes to operation  168  and the set is stored for later use in phase  2 . If the set is determined to be empty at operation  166 , control passes operation  170  and the flowchart  150  is exited. Once exited, the controller may initiate other algorithms to operate the vehicle without violating the power capabilities. For example, control may loop back to generate a new speed profile that limits battery power. 
     Referring to  FIG. 5 , a flow chart  180  is used to optimize the set of profile from operation  168 . In the method illustrated in flow chart  180 , each of the speed profiles are studied to determine the amount of battery heat generated and the battery cooling efficiencies. An optimum speed profile is then output based on the battery heat generated and the battery cooling efficiencies. 
     At operation  182 , the controller receives the set of speed profiles output by the algorithm  150 . At operation  184 , the controller calculates, for each profile, the current (I) required from the battery in order to deliver the speed profile. Using the current calculations from operation  184 , the controller determines the heat generated for each speed profile at operation  186 . The heat generated may be calculated using equation 2. As can be seen, the battery current is a predominate factor for heat generated. Thus, speed profiles that reduce the amount of charging and discharging generate less heat and demand less on the vehicle cooling system  54 .
 
heat generated=∫ 0   t   I   2   R (SoC, T   bat ) dt   (eq. 2)
 
     The output of operation  184  is also passed to operation  188  where the controller defines a battery temperature range. The battery temperature may range includes an upper limit and a lower limit. Batteries are maintained between the upper and lower limits to prevent damage to the battery cells and to prolong life of the battery. As discussed above, the vehicle may be programmed to power limit the battery if the battery temperature falls outside of this defined range. 
     At operation  190 , the controller determines one or more battery-cooling profiles for each of the vehicle speed profiles. The battery-cooling profiles are generated such that the battery temperature remains within the upper and lower limits. Multiple cooling profiles, that maintain the battery within the limits, can be generated for each vehicle speed profile by modifying the heat convection coefficients (h) and other factors. For example, the cooling profiles may include time-varied convection coefficients (h) to generate multiple cooling profiles. As explained above, the energy associated with providing the different coefficients varies. As such, the different cooling profiles will require a different amount of energy to execute. 
     At operation  192  the controller determines the energy-consumption value for each of the cooling profiles, and identified the cooling profiles with the minimum energy-consumption value for each vehicle speed profile. The energy consumption for identified cooling profiles may be calculated using equation 3.
 
energy=∫ 0   t   h   i ( T   bat   −T   enviroment ) dt   (eq. 3)
 
     The output of operation  186  and the output of operation  192  are fed into operation  194  where the controller chooses an optimum speed profile. Equation 4 may be used to determine the optimal speed profile, where w is a weighting factor, heat generated is the output of operation  186 , and energy is the output of operation  192 . The weighting factor can be adjusted according to the preference of the design and the specific components of the vehicle.
 
min=( w ×heat generated)+((1− w )×energy)  (eq. 4)
 
     The optimum speed profile is the profile that minimizes equation 4. The optimum speed profile is saved and sent to control modules associated with autonomous driving. The controller  50 , responsive to the occupant requesting departure, then generates commands to various vehicle systems to autonomously drive the vehicle at operation  196 . For example, the controller may command the traction battery  30  to provide voltage and current to the electric machine(s)  22  according to the selected speed profile. The controller may also command steering commands and the like to autonomously drive the vehicle  20 . 
     Operating the vehicle using the methods of  FIGS. 4 and 5  may prolong battery life, increase electric range, and reduce power limiting of the traction battery.  FIG. 6  illustrates the same driving cycle as  FIG. 3  but with the vehicle  20  according this disclosure. As will be explained below in more detail, the teachings of this disclosure avoid power limiting of the battery by significantly reducing heat generation of the traction battery and increases electric range by operating the battery-cooling system more efficiently. 
     Referring to  FIG. 6 , the plot  210  illustrates the road grade for a segment  212  of the route. This particular segment  212  includes sections of increasing road grade  214  (uphill sections) and decreasing road grade  216  (downhill sections). 
     The vehicle speed profile  218 , which was selected using the method/controls of this disclosure, for the segment  212  is variable as shown in plot  220 . The vehicle speed profile  218  is varied to reduce the workload on the electric machine  22 , which reduces the amount of current the battery  30  must supply to the electric machine  22 . The vehicle speed profile  218  has reduced values corresponding to the uphill sections  214  and increased values corresponding to downhill sections  216 . The vehicle speed profile  218 , while varied, remains within a range between maximum and minimum speed limits  222 ,  224 . 
     In comparing the battery current plot  230  with the battery current plot  103 , the variable speed profile  218  reduces the charging  234  and discharging currents  232 . Reducing vehicle speeds for the uphill sections  214  reduces the required battery discharge current  232 , and increasing vehicle speeds for the downhill sections  216  allows the vehicle  20  to reduce the charge currents  234  by coasting for portions of the downhill sections  216  until regenerative breaking is required to prevent the vehicle from exceeding the upper speed limit  222 . 
     During coasting, the battery  30  is not supplying current and thus is not generating heat as shown in plot  240  at portions  242  and  244 , for example. During the downhill sections  216  the generated heat  246  is substantially below the capacity  248  of the battery-cooling system  54 . The controller can command the battery-cooling system  54  to operate at a relatively high duty cycle at least during the portions  242  and  244  to reduce the battery temperature and precondition the battery  30  from the next discharge cycle so that the battery temperature  252  remains below the upper temperature limit  254  as shown in plot  250 . In this example, unlike  FIG. 3 , the battery  30  is not power limited and the vehicle  20  can be operated as planned throughout the route. The battery  30  does not have to be power limited because current commanded to execute the speed profile  218  has an aggregate value of heat generated, e.g., integral of line  246 , over the segment  212  is less than an aggregate value of cooling capacity, e.g., integral of line  248 , of the battery-cooling system  54  over the segment  212 . 
     While the speed profile  218  includes variable speed, not all speed profiles selected by the method/controls of  FIGS. 4 and 5  will have variable speeds within a segment of a route. The controller may be programmed to operate at a constant speed, e.g., the speed limit of the road, whenever possible as most occupants prefer to drive at the speed limit. The speed profile may be constant for at least one segment of the route responsive to predicted battery temperatures for that segment not exceeding a threshold, e.g., upper temperature limit. The route segment  212 , however, do to elevations changes, cannot have a constant speed profile that maintains the traction battery  30  below the upper temperature limit  254 . In response to the controller determining that predicted battery temperatures for a constant speed profile would exceed the upper temperature limit, a variable speed profile, such as profile  218 , is selected such that actual battery temperatures for the segment remain below the upper temperature limit. 
     While representative embodiments are described above, it is not intended that these embodiments describe all possible forms of the claimed subject matter. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the disclosure. Additionally, the features of various implementing embodiments may be combined to form further embodiments that are not explicitly described or illustrated. While various embodiments may have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, as one of ordinary skill in the art is aware, one or more features or characteristics may be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. Embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not necessarily outside the scope of the disclosure and may be desirable for particular applications.