Patent Publication Number: US-2020303792-A1

Title: Thermal regulation of batteries

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 62/821,297, filed Mar. 20, 2019, the contents of which are herein incorporated by reference in their entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under contract number NNC14CA32C awarded by the National Aeronautics and Space Administration. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to thermal management and more particularly to thermal management for aircraft batteries. 
     2. Description of Related Art 
     Aircraft batteries need thermal management to avoid damage due to overheating and overcooling. Future aircraft may use batteries to feed motors for electric or hybrid-electric propulsion, airfoil stall margin management, and other applications. Thermal management of battery banks is required in order to avoid overheating batteries resulting in premature aging that reduces their storage capacity with repeated cycles (“capacity fade”). In addition, some battery chemistries may suffer from thermal runaway at elevated temperature leading to damage of the batteries and potentially of surrounding equipment. 
     The conventional techniques have been considered satisfactory for their intended purpose. However, there is an ever present need for improved thermal management for batteries. This disclosure provides a solution for this need. 
     SUMMARY OF THE INVENTION 
     A battery thermal management system for an air vehicle includes a liquid heat exchange circuit. The system includes at least one battery in thermal communication with the liquid heat exchange circuit. The system includes a controller operatively connected to the liquid heat exchange circuit. The controller is configured and adapted to variably select whether heat will be rejected to the liquid heat exchange circuit. The system includes at least one heat exchanger positioned on the liquid heat exchange circuit. The at least one heat exchanger is a liquid-air heat exchanger or a liquid-liquid heat exchanger. 
     In certain embodiments, the liquid-liquid heat exchanger is configured and adapted to be operatively connected to a movable second liquid heat exchange circuit. The liquid-air heat exchanger can be positioned on an air heat exchange circuit in fluid communication with an air scoop. The system can include a temperature sensor upstream from the liquid-air heat exchanger positioned to measure a total air temperature of air entering the air scoop. The system can include a pump operatively connected to the liquid heat exchange circuit. The system can include a controller operatively connected to the ram air door. The controller can be operatively connected to the pump. If an expected battery temperature exceeds a pre-determined threshold, the controller is configured and adapted to command the pump ON to circulate heat transfer fluid through the liquid heat exchange circuit and pre-cool the at least one battery prior to the at least one battery being charged or discharged. The temperature sensor can be positioned upstream from the air heat exchange circuit to measure a total air temperature of air entering the air scoop. The controller can be operatively connected to the temperature sensor to receive temperature data therefrom. 
     In some embodiments, the controller can be configured and adapted to at least one of activate or deactivate a coolant pump depending on the temperature data from the temperature sensor. The controller can be configured and adapted to close a flow restrictor if the total air temperature of air entering the air scoop is greater than a desired battery temperature. The controller can be operatively connected to the liquid heat exchange circuit. If the expected battery temperature exceeds a pre-determined threshold, the controller can be configured and adapted to command the liquid heat exchange circuit “ON” to pre-cool the at least one battery prior to the at least one battery being charged or discharged. The system can include a battery temperature sensor positioned to measure a temperature of the at least one battery. 
     In accordance with another aspect, a method for controlling a thermal management system for an air vehicle includes determining an expected temperature of at least one battery, and cooling the at least one battery with a cooling device if the expected temperature exceeds a pre-determined expected temperature threshold. 
     In certain embodiments, determining the expected temperature includes measuring a temperature of the at least one battery. The cooling device can be a liquid-air heat exchanger positioned on a liquid heat exchange circuit and an air heat exchange circuit to exchange heat therebetween. The air heat exchange circuit can be in fluid communication with an air scoop. The method can include measuring a total air temperature of air entering the air scoop with a temperature sensor upstream from the air heat exchange circuit. Cooling the at least one battery includes measuring a total air temperature of air entering an air scoop. If the total air temperature of air entering the air scoop is below a pre-determined total air temperature threshold, cooling the at least one battery can include opening a flow restrictor to cool the at least one battery. The cooling device can be a liquid-liquid exchanger positioned on a first liquid heat exchange circuit and a movable second liquid heat exchange circuit to exchange heat therebetween. The method can include charging and/or discharging the at least one battery if the expected temperature of the at least one battery does not exceed the pre-determined expected temperature threshold. 
     In accordance with another aspect, a method for controlling a thermal management system for an air vehicle includes determining an expected temperature of at least one battery, and charging and/or discharging the at least one battery if the expected temperature of the at least one battery does not exceed a pre-determined temperature threshold. Charging and/or discharging can include selectively allowing the at least one battery to heat up depending on whether a heat sink is available. 
     These and other features of the systems and methods of the subject disclosure will become more readily apparent to those skilled in the art from the following detailed description of the preferred embodiments taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that those skilled in the art to which the subject disclosure appertains will readily understand how to make and use the devices and methods of the subject disclosure without undue experimentation, preferred embodiments thereof will be described in detail herein below with reference to certain figures, wherein: 
         FIG. 1  is a schematic representation of an exemplary embodiment of a battery thermal management system constructed in accordance with the present disclosure, showing the battery operatively connected to a battery heat exchanger; 
         FIG. 2  is a plot representing temperature versus mission time, showing battery temperature across the mission with pre-cooling versus no-precooling; and 
         FIG. 3  is a plot representing temperature versus altitude, showing a maximum battery temperature. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made to the drawings wherein like reference numerals identify similar structural features or aspects of the subject disclosure. For purposes of explanation and illustration, and not limitation, a schematic view of an exemplary embodiment of the battery thermal management system in accordance with the disclosure is shown in  FIG. 1  and is designated generally by reference character  100 . Other embodiments of the battery thermal management system in accordance with the disclosure, or aspects thereof, are provided in  FIG. 2  as will be described. The systems and methods described herein can be used to pre-cool idle batteries and cool batteries during charge/discharge in order to avoid overheating and capacity fade for batteries in hybrid-electric propulsion, all-electric propulsion, airfoil stall margin management, and other applications. 
     As shown in  FIG. 1 , a battery thermal management system  100  for an air vehicle includes a controller  102 , a liquid heat exchange circuit  116 , an air heat exchange circuit  114 , a liquid-air heat exchanger  112  and a liquid-liquid heat exchanger  108 . Signals to and from the controller  102 , indicated schematically by solid and/or dashed lines extending therefrom, can be routed to and from dedicated ports on the controller  102 , or the sensors and actuators (described below) can be networked together with each device having its own address. In some embodiments, the components may communicate with the controller via a standard communication protocol involving the transfer of digital information. Those skilled in the art will readily appreciate that while certain inputs/outputs are shown controller could include a variety of other inputs/outputs. A liquid heat transfer fluid, coolant, circulates through said liquid heat exchange circuit  116 , to transfer heat between batteries  104  and the coolant. The system  100  includes a battery heat exchanger  106  positioned on the liquid heat exchange circuit  116  in thermal communication with a bank of batteries  104 . The liquid-air heat exchanger  112  is positioned on the air heat exchange circuit  114  in fluid communication with an air scoop (not shown) by way of a ram air door  115 . Ram air is air outside of the fuselage of an aircraft. While the heat sink for air heat exchange circuit  114  is described herein as ram air, those skilled in the art will readily appreciate that heat exchange circuit  114  can be operatively connected to other ducts to receive fan duct bypass air, cabin outflow air, conditioned air from an environmental control system, or the like. The bank of batteries  104  are in thermal communication with the liquid heat exchange circuit  116  by way of the battery heat exchanger  106 . For example, these can be reserve batteries that are inoperative during cruise, or batteries of a hybrid electric aircraft propulsion system where the batteries and electric motors assist with takeoff and climb, but are inoperative during cruise. The batteries  104  could be positioned in a variety of positions throughout the aircraft, for example, in the wings (e.g. in wing roots). In some embodiments, batteries  104  can furnish energy to a propulsion system for propulsion power or for transient operation (e.g. to avoid stall) or for other applications. The system  100  assists in avoiding battery overheating and damage during charging and discharging due to internal resistances. 
     With continued reference to  FIG. 1 , a coolant pump  109  is upstream from the battery heat exchanger  106 . Pump  109  is fluidically connected to the liquid heat exchange circuit  114  to circulate heat transfer fluid to heat or cool the batteries  104  downstream. The heat transfer fluid can be any of a number of fluids, including but not limited to water and other aqueous fluids, as well as polar and non-polar organic fluids. In some embodiments, the heat transfer fluid can be in liquid form, but can also be in gaseous form as well as including gas or liquid form such as in a vapor compression heat transfer loop. Examples of heat transfer fluids include but are not limited to glycols such as ethylene glycol or propylene glycol, alcohols such as methanol or ethanol, water and aqueous solutions such as heat transfer brines, and other organic fluids such as propane, butane, and substituted hydrocarbons (e.g., fluoro-substituted) and other organic compounds such as 2,2,3,3-tetrafluoropropene or 1,1,1,2-tetrafluoroethane. Flight-critical components of a propulsion system  100 , such as a coolant circulation pump  109 , may be present in duplicate on an aircraft for redundancy. Moreover, it is contemplated that more than two pumps  109  can be used in system  100 . Those skilled in the art will readily appreciate that pump  109  can be a centrifugal pump, reciprocating pump, screw pump, diaphragm pump, or any other suitable mechanical pump. A respective motor  111  is operatively connected to the coolant pump  109  to drive the coolant pump  109 . In general, the batteries  104  reject heat to the air heat exchange circuit  114  by way of the liquid heat exchange circuit  116 . In some embodiments, the liquid-air heat exchanger  112  can be sized to reject waste heat generated by the batteries  104  in full at a particular altitude, which is associated with temperature and pressure of the air heat sink (ram air). Below that altitude, only part of the heat generated by the batteries may be rejected. Above that altitude, not only the heat load from battery operation, but also the stored thermal energy can be rejected. Thermal energy storage allows excess thermal energy to be stored and used hours, days, or months later. Operation of system  100  is controlled by controller  102  at various altitudes, as described in more detail below. 
     As shown in  FIG. 1 , controller  102  is in operative communication with the pump  109  and motor  111 . A temperature sensor  107  is operatively connected to the batteries  104  and controller  102 . Those skilled in the art will readily appreciate that, due to the complexities involved in obtaining a battery temperature, the temperature sensor  107  may be operatively connected to the heat transfer fluid exiting the battery heat exchanger  106  in liquid heat exchange circuit  116  as a proxy for battery temperature. Controller  102  is configured to receive information about battery temperature from temperature sensor  107  and relay a command, if needed, to one or more of motor  111 , pump  109 , or valves  110  and  120  (described below). In some embodiments, it is also contemplated that controller could send a command to temperature sensor  107 . The system  100  includes a bypass valve  120  positioned in the liquid heat exchange circuit  116  upstream from the liquid-air heat exchanger  112 . The system  100  includes a bypass line  118  branching from the liquid heat exchange circuit  116  upstream from the liquid-air heat exchanger  112  and reconnecting to the liquid heat exchange circuit  116  downstream from the liquid-air heat exchanger  112 . The system  100  includes a bypass line valve  110  positioned on the bypass line  118 . 
     With continued reference to  FIG. 1 , bypass line valve  110  and bypass valve  120  include respective motors  105  in operative communication with controller  102  to receive open/close commands therefrom. In some embodiments, valves  110  and  120  can be combined into a three-way valve that directs flow either to liquid-air-heat exchanger  112  or to bypass line  118 . Bypass line  118  assists in diverting around the liquid-air heat exchanger  112  to avoid heating the fluid during conditions where the total air temperature of ram air is greater than the maximum battery temperature operating limit. If the total air temperature of the ram air is above a pre-determined total air temperature threshold, the controller  102  operates to command valve  110  open and  120  close (e.g. by way of their respective motors  105 ). Motors  105  receive a command from controller and, in turn, operate to open or close their respective valves  110  or  120 . This bypass command can accompany the command that closes ram air door  115  or can be independent thereof. The reverse command can be executed once the total air temperature lowers below the pre-determined total air temperature threshold. Valves  120  and/or  110  can be a gate valve, a globe valve, a needle valve, or any other proportional valve. 
     With continued reference to  FIG. 1 , the liquid-air heat exchanger  112  is positioned on the liquid heat exchange circuit  116  and the air heat exchange circuit  114  to exchange heat therebetween. The air heat exchange circuit  114  includes a flow restrictor, e.g. a ram air door  115 , to impede or allow flow of a heat sink fluid (e.g. ram air) to the liquid-air heat exchanger  112 , described in more detail below. Ram air is air outside of the fuselage of an aircraft. Ram air door  115 , when at least partially opened, provides during conditions where the batteries are idle and non-idle). The system  100  includes a temperature sensor  113  upstream from the liquid-air heat exchanger  112  positioned to measure a total air temperature of air entering the air scoop via ram air door  115 . In some embodiments, the total air temperature may be reported to controller  102  from another source on the aircraft such as an air data computer which may be in operative communication with a thermometer that measure total air temperature. Air heat exchange circuit  114  can also include a fan or the like for conditions where the aircraft is idle, e.g. on ground. In the systems and embodiments of the present invention, the flow of air through ram air door  115  can be curtailed or eliminated during hot conditions, e.g. when the total air temperature is above a pre-determined total air temperature threshold, by closing the ram air door  115  either partially or fully. The controller  102  is operatively connected to the flow restrictor, e.g. ram air door  115 , and to temperature sensor  113  to send/receive data therefrom. Generally, controller  102  is configured and adapted to open and/or close the ram air door  115  depending on the temperature data from the temperature sensor  113  and/or measured at the temperature sensor  107 . In embodiments where several heat exchangers share a common ram air duct, the air heat exchange circuit  114  may be optionally outfitted with an air splitter (not shown) to branch the flow of ram air to individual heat exchangers. In this case, the flow restrictor can be a flow control valve such as a butterfly valve can be located upstream of liquid-air heat exchanger  112  to restrict or allow flow as needed. Those skilled in the art will readily appreciate that, in some embodiments, fluid in the heat exchange circuit  114  can vent outside of the aircraft after going through the liquid-air heat exchanger  112 . 
     In some embodiments, if the total air temperature of the ram air is low enough, e.g. if the stagnation temperature of the ram air (T ra ) heat sink is less than the battery temperature (T B ) plus a delta T (ΔT), the controller  102  keeps ram air door open and circuits  114  and  116  operational so that batteries may be cooled. In other words, when T ra &lt;(T B +ΔT), then cooling the batteries  104  using the ram air can be initiated or continued. Delta T (ΔT) is to ensure that there is enough of a temperature difference between T ra  and T B  ensure good heat transfer and to accommodate any heat generated from operation of pump  109 . Delta T can be 5-10° C. (41-50° F.), for example. In some embodiments, aircraft may encounter a condition wherein heat rejection from batteries  104  continues during cruise in excess of battery heat loads. In other words, instead of just maintaining a battery temperature during periods of time when the batteries  104  are generating heat, such as charging or discharging, the batteries  104  can be cooled such that the temperature reduces while the batteries  104  are charging/discharging, or the batteries  104  can be cooled while the batteries  104  are idle (e.g. not charging/discharging). As a result, the batteries are cooled overall (for example to 5° C.). This is desirable because the charging process causes the batteries to heat up, and this gives an option to pre-cool them to reduce the cooling load later on. Since it is common for several heat exchangers to share a common ram air duct, the air heat exchange circuit  114  may be optionally outfitted with an air splitter (not shown) to branch the flow of ram air to individual heat exchangers. In this case, a flow control valve such as a butterfly valve can be located upstream of liquid-air heat exchanger  112  to restrict flow. Instead of commanding a ram air door  115  to close, as described throughout, controller  102  can command the flow control valve upstream from liquid-air heat exchanger  112  to open or close. 
     With continued reference to  FIG. 1 , the liquid-liquid heat exchanger  108  is positioned on a movable second liquid heat exchange circuit  117 . Movable liquid heat exchange circuit  117  is operatively connected to a ground cart  121  that is used when the aircraft is landed and is permitted to be connected/disconnected from heat exchanger  108  at will depending on whether cooling on-ground is needed. The ground cart  121  can include a pump, motor and/or other hydraulic system components that can be used to drive liquid heat exchange circuit  117 . 
     If an expected battery temperature exceeds a pre-determined threshold, and the total air temperature measured at sensor  113  is below a predetermined ram air threshold, it may be desired to pre-cool the batteries  104  with ram air to avoid overheating when thermal loads are imposed. Battery temperature is a function of several variables including battery mass, battery heat capacity, initial battery temperature, heat sink temperature, and thermal loads. The expected battery temperature can be predicted (calculated) from the heat loads, the specific heat of the battery (which can be characterized), the mass of the battery, and the initial battery temperature. Alternatively, the expected battery temperature can be predicted from empirical battery operating data to simplify the control of the thermal management system. Those skilled in the art will readily appreciate that the expected temperature of a battery  104  (or group of batteries  104 ) can be calculated in real time or a look-up table can be used. Pre-cooling is a form of thermal storage in which heat has been removed and the lower temperature battery reflects the absence of heat. For example, on a hot day, aircraft batteries can be pre-cooled prior to takeoff in order to avoid high temperatures during discharging. If ram air is not available or is not cool enough, a cooling device such as a vapor cycle machine, ground cart cooling (as described above) or ice, etc. may be used for pre-cooling, or just cooling in general. 
     It is also contemplated that pre-cooling be done during flight for anticipated charging on the ground. In this embodiment, if T ra &lt;(T B +ΔT), controller  102  is configured and adapted to command the ram air door  115  to open in order to pre-cool the batteries  104  prior to the batteries  104  being charged or discharged. For example, at a cruise altitude prior to landing, the ram air door  115  is opened to pre-cool the batteries  104  prior to landing where it is expected that the batteries  104  will need to be recharged. It is also contemplated that batteries may be recharged in flight and the available ram air through ram air door  115  can be used as a heat sink via air heat exchange circuit  114 . This tends to allow the batteries  104  to take advantage of the excess power that engines typically have during cruise when they are no longer accelerating, thin air, and due to fuel weight loss. Batteries  104  can be charged during cruise to take advantage of the available power combined with the available heat sink, e.g. ram air. 
     As shown in  FIG. 1 , the controller  102  is configured and adapted to close the ram air door  115  if a total air temperature of air entering the air scoop is greater than a desired battery temperature or actual battery temperature (e.g. if T ra &gt;(T B +ΔT)). Those skilled in the art will readily appreciate that referring to “battery temperature,” “battery heat load” or other singular reference to battery, could mean the temperature/heat load of an individual battery  104  or group of batteries  104 . The controller  102  is operatively connected to the liquid heat exchange circuit  116  by way of its respective pump  109  and motor  111 . If an expected heat load for batteries  104  exceeds a pre-determined threshold, and the total air temperature measured at sensor  113  is below a predetermined ram air threshold, controller is configured and adapted to command the liquid heat exchange circuit “ON” and the ram air door  115  open to pre-cool the batteries  104  with air heat exchange circuit  114  and liquid heat exchange circuit  116  prior to the batteries  104  being charged or discharged. 
     As shown in  FIG. 3 , in some embodiments, system  100  is used on a hot day to provide cooling during charging/discharging or pre-cooling before any charging/discharging. On a hot day, at low altitude and high velocity, the total air temperature of the ram air, represented by line  304  may exceed the desired maximum battery temperature, represented by line  302 , e.g. 113° F. or 45° C. As the altitude increases, the total air temperature and the air temperature of the atmospheric air will decrease. Line  306  represents the ambient air temperature. In this case, because T ra &gt;(T B +ΔT) at low altitudes, controller  102  will command pump  109  to turn off and instead will allow the temperature in the batteries  104  to rise, also described below with respect to  FIG. 2 . Once T ra &lt;(T B +ΔT), controller  102  (e.g. between an altitude of 5,000-10,000 feet) controller  102  will command ram air door  115  to open (if not already open) and pump  109 , motor  111  and circuit  116  “ON” to begin to remove heat from batteries  104  via air heat exchange circuit  114 . Those skilled in the art will readily appreciate that the temperatures shown herein may vary depending on battery chemistry, weather, time of day, or the like. The pre-determined threshold for the battery temperature, described above, can be set based on the maximum battery temperature for a given battery chemistry. 
     A method for controlling a thermal management system, e.g. system  100 , for an air vehicle includes determining an expected temperature of batteries, e.g. batteries  104 , cooling the batteries with a cooling device, e.g. liquid-air heat exchanger  112  or liquid-liquid heat exchanger  108 , if the expected temperature exceeds a pre-determined expected temperature threshold. Determining the expected temperature includes measuring a temperature of the batteries. Cooling the batteries includes cooling the batteries with the liquid-air heat exchanger positioned on a liquid heat exchange circuit, e.g. liquid heat exchange circuit  116 , and an air heat exchange circuit, e.g. air heat exchange circuit  114 , to exchange heat therebetween. 
     The method includes measuring a total air temperature of air entering the air scoop with a temperature sensor, e.g. temperature sensor  113 , upstream from the air heat exchange circuit. Cooling the batteries includes measuring a total air temperature of air entering an air scoop. If the total air temperature of air entering the air scoop is below a pre-determined total air temperature threshold, cooling the batteries includes opening a ram air door, e.g. ram air door  115 , to cool the batteries. This cooling can be pre-cooling and can be used where a scenario like that of  FIG. 2  is expected. Line  202  represents a ram air temperature and line  204  represents the battery temperature at an adiabatic state (e.g. if there were not any cooling). By pre-cooling and starting from approximately 20° C. (68° F.), as indicated by line  208 , battery overheating can be prevented without having to cool the batteries during the actual charge/discharge. 
     During ground operations, embodiments of the method include cooling the batteries with a liquid-liquid heat exchanger, e.g. liquid-liquid heat exchanger  108 , positioned on a first liquid heat exchange circuit, e.g. liquid heat exchange circuit  116 , and a movable second liquid heat exchange circuit, e.g. liquid heat exchange circuit  117 , to exchange heat therebetween. Cooling with the movable liquid heat exchange circuit may be desirable where ram air is not available. Additionally, if the batteries themselves have thermal capacity, the method includes charging and/or discharging the batteries without cooling if the expected temperature of the batteries does not exceed the pre-determined expected temperature threshold. If so desired, the batteries can be cooled once a heat sink, e.g. ram air is available. This scenario is described in more detail below with reference to  FIG. 2 . 
     In accordance with some embodiments, the ability to reject heat to an on-ground liquid heat exchange circuit, e.g. liquid heat exchange circuit  117 , or to ambient ram air via air heat exchange circuit, e.g. air heat exchange circuit  114 , may not be available. Accordingly, a method for controlling a thermal management system, e.g. system  100 , for an air vehicle includes determining an expected temperature of batteries, e.g. batteries  104 , and charging and/or discharging the batteries without pre-cooling or cooling if the expected temperature of the batteries does not exceed a pre-determined temperature threshold. In one embodiment, the stagnation temperature of ram air for an aircraft at low altitude on a hot day (40° C.) during takeoff, climb out, and initial climb, may be higher than the maximum desired battery temperature, so cooling with ram air may not be desired. In this case, the batteries are allowed to heat up optionally with monitoring by a sensor, e.g. sensor  107 , in operative communication with a controller, e.g. controller  102 . In other words, if the expected temperature of the batteries during charge or discharge is not expected to rise above its maximum temperature, pre-cooling or cooling during discharge may not be required. 
     As shown in  FIGS. 2-3 , where the total ram air temperature, indicated schematically by line  304 , exceeds a battery temperature, the batteries, e.g. batteries  104 , are allowed to heat up by way of charge or discharge and then are cooled once the ram air temperature decreases. In other words, the method can include turning the cooling system, e.g. circuits  116  and  114 , to an “OFF” state with the controller. The temperature of the batteries in this scenario is represented schematically by line  204  in  FIG. 2 . An adiabatic temperature of the batteries (e.g. without any cooling) is also indicated schematically by line  204  to show a potential outcome without the later cooling. After discharge, and/or once the aircraft reaches an altitude where the total air temperature is lower than the battery temperature, the method includes turning the cooling circuit, e.g. liquid heat exchange circuit  116  and air heat exchange circuit  114 , “ON” by the controller, during, before, or after charge or discharge, when a heat sink, e.g. cooling ram air, is readily available. For example, at an altitude between 5,000 and 10,000 ft, the ambient air is sufficiently cool even on a hot day, as defined by MIL-STD-210C, to serve a heat sink for rejected heat from the batteries. 
     At the altitude at which the cooling system is actuated, the batteries may still continue to store some heat as well as reject some to the air heat exchange circuit, e.g. air heat exchange circuit  114 . An aircraft may encounter a condition wherein heat rejection from batteries continues during cruise in excess of battery heat loads. As a result, in this embodiment, the method includes cooling the batteries (e.g. to 5° C.) at a higher altitude when the cooling system is actuated (e.g. circuits  116  and  114  are actuated by controller  102 ). This is desirable because the charging process causes the batteries to heat up, and pre-cooling them helps reduce the cooling load. Moreover, for an aircraft application, the power required for climb steadily diminishes until cruising altitude is reached due in part to the reduced drag in thinner atmosphere. As a result, an aircraft that derives propulsive energy from batteries, e.g. batteries  104 , during climb will experience steadily decreasing heat loads. The decreases in battery heat load and in ambient temperature both facilitate battery-cooling. 
     The methods and systems of the present disclosure, as described above and shown in the drawings, provide for thermal battery control with superior properties including smaller heat exchangers, avoiding overheating and capacity fade, and avoiding thermal runaway. The systems and methods of the present invention can apply to automotive batteries, aircraft batteries, terrestrial batteries, or the like. While the apparatus and methods of the subject disclosure have been shown and described with reference to preferred embodiments, those skilled in the art will readily appreciate that changes and/or modifications may be made thereto without departing from the scope of the subject disclosure.