Patent Application
20080280192
The invention relates generally to energy storage, and particularly to thermal management of batteries.
Electrochemical batteries generate heat due to electrical resistance and internal electrochemical processes. In most cases, battery packs do not need active thermal management as the heat is dissipated by natural convection and radiation.
An extreme application for electrochemical batteries is an engine-dominant hybrid electric vehicle powertrain. The engine-dominant hybrid system, also known as a self-sustaining hybrid system or full hybrid system, consists of a relatively large internal combustion engine, one or more electric machines that can operate as motors or generators, and a relatively small battery pack. Because the battery pack in such a system should be as small as possible due to cost, weight and packaging constraints, however, it experiences repetitive high loads relative to its size. For instance, typical peak loads for a 6 Ah hybrid car battery may exceed 150 A during acceleration and 60 A during regenerative braking charge. That represents repetitive 25C discharge and 10 C charge loads, where C is an industry measure of discharge/charge rate equivalent to the current required to completely discharge/charge the battery in one hour.
Depending on the duty cycle, the heat from a charge-discharge cycle of the battery can build up and cause the battery to overheat. Certain types of batteries exhibit a thermal runaway phenomenon where the heat generation increases rapidly with the temperature leading to battery failure or even destruction. Thus batteries and battery packs without adequate cooling will fail to maintain the temperature of cells within the optimum operating limits. With the exception of high temperature battery technologies, the batteries used in electric and hybrid electric vehicle drives should be operated in moderate temperatures, ideally within the 10-30° C. range to ensure performance, efficiency and durability. Additionally, in extremely cold environments, battery heating must be provided.
A particularly important aspect of the hybrid vehicle application is the battery life requirement of 8 to 10 years. Typically, in consumer applications, the battery life is up to 4 years for products such as cellular phones and computers. One of the factors known to affect battery life is temperature. Operation in elevated temperatures, for example above 40° C., significantly reduces the lifetime number of charge-discharge cycles a battery is capable of. Another critical factor in achieving long battery life is a uniform temperature distribution across the battery pack. In a battery pack comprising a large number of cells, all cells should be maintained at a substantially uniform temperature. If the battery pack temperature is not uniform, the cells with higher temperature deteriorate faster and ultimately fail. In pack configurations with a large number of cells arranged in series, failure of a single cell results in the whole pack failing.
As hybrid vehicles can operate in a variety of climates, low temperature performance is an important requirement for traction and energy losses during operation are sufficient to maintain the battery within the desired temperature range. However, in extremely cold climates, battery heating must be provided, particularly if the battery is exposed to long periods in a low temperature environment, for example in a parked vehicle.
Hybrid vehicles utilize air cooling to prevent the battery pack from overheating. Air cooling is a simple and cost effective solution that is adequate for series-parallel configurations where the battery load can be controlled by increasing or decreasing the engine contribution to the motive power demand. The disadvantages of air cooling are its limited effectiveness at higher thermal loads and difficulty in achieving uniform temperature distribution.
In series hybrid configurations, the battery load can be higher, particularly for heavier vehicles, and liquid cooling may be used. Liquid cooling is more effective and can provide a more uniform battery pack temperature than air cooling. However, liquid cooling systems are more complex and much heavier than air cooling systems and therefore consume more energy in transit.
A need therefore exists to provide a battery thermal management system, method or device that provides a solution to at least some of the deficiencies in the prior art.
According to one aspect of the invention, there is provided a battery thermal management system for managing the temperature of a battery pack in a hybrid vehicle. The system comprises a battery pack; a fan having an air outlet; and a plenum having an inlet in fluid communication with the fan air outlet and a supply air diffuser in fluid communication with the battery pack. The supply air diffuser has openings configured to deliver a substantially uniform air flow across the battery pack from an air flow supplied by the fan to the plenum.
The plenum can also have at least one baffle extending from the fan air outlet and across the supply air diffuser in such a configuration that the plenum is divided into multiple sections receiving substantially equal air flux from the fan. The diffuser openings can be slots of increasing size with increasing distance from the fan air outlet. Alternatively, the diffuser openings can be holes with one or both of increasing density and increasing size with increasing distance from the fan air outlet. The fan outlet can have a rectangular cross-section, and there can be multiple baffles in which case the leading edges of the baffles are concentrated around the centre of the fan air outlet. In particular, the system can comprise three baffles per fan air outlet, wherein a centre baffle is straight and has a leading edge at the centre of the fan outlet and the other two baffles are respectively located on either side of the centre baffle and each have a trailing edge that curves away from the centre baffle.
The system can further comprise a discharge air diffuser having openings in fluid communication with the battery pack. The supply air diffuser and discharge air diffuser are spaced from each other to form a battery compartment therebetween, and the battery pack is located inside the battery compartment. The discharge air diffuser can have openings which are slots of increasing size with increasing distance away from the fan air outlet. Alternatively, the discharge air diffuser openings can be holes with one or both of increasing density and increasing size with increasing distance away from the fan air outlet.
The battery pack can comprise of a plurality of modules and temperature and current sensors. Each module comprises a plurality of electronically linked cells wherein the temperature of at least one cell from each module is monitored by the sensors. The battery thermal management system can further comprise a cell management board (CMB) communicative with the sensors, and an electronic control unit communicative with the CMB and the fan and operable to control the operation of the fan based on temperature and current data received from the CMB. The electronic control unit can have a memory having code recorded thereon for execution by the control unit. The code comprises a map of a typical cell temperature distribution encoded thereon, a step for comparing temperatures measured by the sensors with temperatures in the map, and a step for generating a warning message when the measured temperatures differ by a selected threshold from the temperatures in the map.
The system can further comprise power contactors electrically connected to the battery pack and for electrically connecting to a vehicle motor controller. In such case, the electronic control unit is communicative with the power contactors and the code includes a step for opening the power contactors when temperature measured by the sensors exceeds a selected threshold.
The system can further comprise a heater in communication with the electronic control unit. In such case, the code includes a minimum battery pack temperature threshold, and a step for activating the heater to maintain the temperature of the battery pack above the minimum battery pack temperature threshold based on the temperature signals received from the sensors.
According to another aspect of the invention, there is provided a computer readable memory having recorded thereon code for execution by an electronic control unit of a battery thermal management system, to carry out a method comprising: measuring the temperature of at least one module of a battery pack; and when the measured temperature is within a predefined range and increasing, reducing charge and discharging power limit of the battery pack by a prescribed amount. The method can also comprise: when the measured temperature is steady or decreasing, comparing the measured temperature of multiple modules of the battery pack with temperatures in a map of a typical cell temperature distribution, and generating a warning message when the measured temperatures differ by a selected threshold from the temperatures in the map. Also, the method can further comprise opening a power contactor electrically connected to the battery pack and when also electrically connected to a vehicle motor controller when closed, when the measured temperature exceeds the predefined range. Also, the method can further comprise activating a battery heater when the measured temperature falls below the predefined range.
Also, the method can further comprise: reading a state of charge and battery current of a battery cell in the battery pack; determining a battery internal resistance and heat generated or absorbed by the battery cell due to enthalpy change, calculating heat Q generated by the battery cell according to formula:
Q=I
2
*R+Q
e,
wherein: I=battery current;
R=battery internal resistance; and
measuring ambient temperature; calculating battery heat loss at the ambient temperature; and integrating the battery heat loss at periodic intervals and setting a fan at a selected speed when the integrated battery heat loss is greater than 0, and turning off the fan when the integrated battery heat loss is less than 0, the fan being in air communication with the battery pack.
Other features and advantages of the present disclosure will be set forth, in part, in the descriptions which follow and the accompanying drawings, wherein preferred embodiments and some exemplary implementations of the present invention are described and shown, and in part, will become apparent to those skilled in the art upon examination of the following detailed description taken in conjunction with the accompanying drawings or may be learned by practice of the present invention. The advantages of the present invention may be realized and attained by means of the instrumentalities and combinations of elements and instrumentalities particularly pointed out in the appended claims. dr
a is an illustrative isometric rear view of the battery module having a printed circuit board (PCB) in partially exploded view;
b is an illustrative top plan view of the battery module with its top panel removed;
a is an illustrative isometric rear view of a fan mounting of the battery thermal management system according to one embodiment of the invention;
b is an illustrative isometric rear view of the fan mounting mounted to the battery assembly;
a is an illustrative isometric bottom view of the battery assembly and certain parts of the battery thermal management system;
b is a bottom view of the battery thermal management system with arrows illustrating the airflow path therethrough;
A battery thermal management system is provided to improve air cooling of a multi-cell battery pack to achieve a high rate of heat rejection necessary for high power applications while maintaining a uniform cell temperature useful for maintaining and extending battery life. The battery thermal management system is used to cool and optionally heat the battery pack so as to maintain the battery pack within a desired operating temperature range and to maintain a substantially uniform cell temperature within this temperature range. This is achieved by managing an airflow throughout the battery pack based on observed temperatures of various cells of the battery pack. The battery thermal management system comprises an air supply apparatus and an electronic control unit with a memory having encoded thereon a battery temperature control logic. The air supply apparatus guides the airflow through the battery pack and the electronic control unit uses the battery temperature control logic to control airflow based on the measured temperature of the various cells of the battery pack. Additionally, the battery temperature control logic may initiate a heating source for ensuring that the battery pack temperature does not drop below a minimum temperature threshold when required.
The multi-cell battery pack can be constructed of individual cells assembled in a single unit or using a module comprising of several cells that can be used to assemble packs of various size and configuration. Depending on the cell type, system voltage and capacity requirements, packaging constraints and a number of other application factors, the configuration of modules in the battery pack can vary widely.
Each module 14 is monitored for temperature by temperature sensors (not shown in
In the described illustrative embodiment, a battery pack is constructed using modules 14 comprising six lithium ion cells 12 arranged in two rows of three cells 12. Each row of three cells 12 is connected in parallel and the two sets of the cells 12 are connected in series. The cells 12 used in this system have the nominal voltage of 4.2V (fully charged) and 2.9 Ah capacity at a 1 hour discharge rate. The six-cell module 14 has nominal voltage of 8.4V and 8.7 Ah capacity. The modules 14 may be arranged into five blocks of eight modules each for a total of forty modules. That configuration results in a pack with nominal voltage of 336V at 100% state-of-charge and capacity of 8.7 Ah. The battery size is typical for an engine-dominant hybrid car or light truck with an electric motor of up to approximately 70 kW peak power. A single 2.9 Ah cell is capable of delivering peak current of up to 100 A and the full pack can deliver up to 250 A peak current. However, in actual operation, the peak loads typically do not exceed 150 A and average load is approximately 10-15 A depending on the duty cycle. The heat generated in the cell 12 during discharge and charge is primarily due to resistive losses. A smaller amount of heat is also generated or absorbed due to the enthalpy change associated with the electrochemical reaction.
At an average load of 15 A, the battery pack generates only about 100 W of heat which can often be removed by venting. However, in heavy stop-and-go driving, the battery can generate as much as 9000 W for short periods of time, resulting in high thermal loads requiring intensive cooling. In most applications, such high thermal loads are a rare occurrence, so the cooling system can be designed for moderate loads and the thermal management system has the capability of reducing the battery output in extreme conditions. A graph of the resistive heat loss by the cell 12 as a function of discharge current is shown in
a shows an illustrative isometric rear view of a module 14 with its CMB 24 having temperature sensors 28 which are aligned to insert through the sensor holes 30 in the side plate 26 when the CMB 24 is installed. As outlined with reference to
The CMB 24 is a PCB that includes electronic components designed to measure cell voltage and temperature, convert the measured values into digital signals and transmit the data to a central BCM (not shown). The temperature sensors 28 used in the present embodiment may be of the solid state type and are attached directly to the CMB 24, for example by soldering the sensor 28 directly to the CMB. The sensors 28 are mounted in such a way that their surface contacts the cell 12. A small pad of thermally conductive material such as a Phase Change Material (PCM) may be inserted between the cell 12 and the transducer to improve the temperature reading. Other transducer types and design options can be used to measure cell temperature. For example, a thermocouple can be attached to a cell and connected to the board using wires. In the present embodiment, only the temperature of one cell in the module is measured at two locations as module tests indicated that the cell temperature is substantially uniform within the module.
b shows an illustrative top view of a module 14 with its top cap 20 removed to demonstrate how the temperature sensors 28 on the rear of the CMB 24 inserted through the side plate 26 are aligned to physically contact the nearest cell 12.
A number of banks 16 may be joined to form a battery pack 18 as shown in
Having regard to
Referring particularly to
When the banks 16 are joined together to form the battery pack 18, the bottom diffusers 36 of each bank 16 contact diffusers 36 of adjacent banks 16 to collectively form a bottom diffuser plate. Similarly, each bank 16 has a top diffuser 70 with multiple airflow openings 74 there-through. When the banks 16 are joined together to form the battery pack 18, the top diffusers 70 contact diffusers 70 of adjacent banks to collectively form a top diffuser plate.
The top and bottom diffusers 36, 70 sandwich the battery modules 14 there-between; the space between the top and bottom diffusers 36, 70 is hereby referred to at the battery compartment. In the embodiment shown in
The bottom diffuser plate is mounted above a bottom plenum 51 formed below the battery pack 18. Air exhausted from the fans 52 into the plenum 51 enters the battery compartment through the openings 74 in the bottom diffuser plate (which can be referred to as the supply air diffuser). Air is discharged from battery compartment through the openings 74 in the top diffuser plate (which can thus be referred to as a discharge air diffuser). Such a configuration is shown in
Alternatively, air may enter the battery compartment through the top diffuser plate and be discharged through the bottom diffuser plate in which case the top and bottom diffusers plates would be referred to as the supply and discharge air diffusers, respectively. Alternatively, air may be exhausted over both top and bottom diffuser plates and enter the battery compartment through openings in both diffuser plates in which case both diffuser plates are supply air diffusers.
In a conventional arrangement, cells close to a fan receive most of the air flow, while cells farther from the fan may receive little or no air flow. In order to achieve a uniform air flow through the battery compartment, the air pressure is to be maintained as uniform as possible across the surface of the battery pack 18, thereby resulting in a substantially equal air flow rate past each battery. In the present embodiment, this uniform pressure distribution is achieved by utilizing an air supply apparatus including certain features such as specifically located and shaped air flow baffles 42, 73 and diffusers 36 and 70 with openings of specifically varying size and density.
Referring now to
While both top and bottom diffusers 70, 36 are shown to have openings of varying size in this embodiment, only the air supply diffuser needs to have such openings of varying size; the air discharge diffuser can have openings of the same size. The air discharge diffuse can be just a perforated panel to provide mechanical support and allow the air the exit the battery compartment.
Alternatively, the position and size of the openings 74 can be determined by experiment by using a test box of the size of the plenum 51 with a perforated top surface. The box is pressurized using the fan intended for use with the given battery pack. The perforated surface can be masked with masking tape and the air speed can be measured with an air speed meter. The size and position of the slots 74 or holes is adjusted to obtain substantially uniform air speed across the pack. It is recommended that the total area of the openings in the diffusers should be approximately 10% larger then the fan outlet cross-section to account for pressure losses and maintain even flow.
As can be seen in
The baffle 42, 73 characteristics are dictated by the characteristics of the air flow discharged from the fan 52. Referring to
It is noted in this embodiment, that the total plenum width is greater than the total width of the two fans outlets 60; in this case, the width of half of the plenum 51 is 40 cm and the width of each fan outlet is 30 cm. The outside edge of each outlet 60 is about 5 cm from the respective edges of the plenum 51. The total air flow across the entire outlet 60 is about 38 L/s. Therefore, the air flow for each of the sections should be about 9.5 L/s. Reference to
Air discharged from each outlet 60 is directed into four streams 61, 63 by the respective baffles 42, 73. The two outer streams 61 and the two inner streams 63 have substantially the same flow pattern. As the four air streams 61 and 63 from the fan outlet 60 have different widths (in this case 9, 6, 6 and 9 cm) and velocities, but substantially equal air flux, the plenum area supplied by each fan outlet is divided by the baffles 42, 73 into four sections of the same area but having different shapes.
The placement and shape of the baffles 42, 73 should be such that the plenum sections supplied by each air stream 61 and 63 are approximately equal as each air stream 61 and 63 is substantially equal in flow volume. This can be achieved by entering the baffle shape data into a spreadsheet and integrating the data to obtain the area under the curve. This is shown in
In this embodiment, as the air flow exhausted from the fan outlet 60 is divided into four air streams 61 and 63, the calculated area should be equal to ¼ of the total area of the plenum 51. In the case presented in
The aforementioned fan outlet 60 and partitions 42, 73 ensure that the air flow is evenly distributed throughout the plenum 51 without “dead spots”. However, the pressure distribution within the plenum 51 is not uniform. It is highest in close proximity to the fan outlet 60 and decreases with the distance from the fan outlet 60. If the pressure distribution is not compensated for, the air flows through the shortest path directly at the fan outlet 60, with the majority of the cell assembly not receiving any cooling. In order to ensure equal flow through the pack 18, the bottom diffusers 36 are mounted above the plenum 51 having the opening pattern as previously discussed. The top diffusers 70 can also be provided with the same opening pattern for discharging air from the battery pack 18 (or alternatively serving as a supply air diffuser and receiving air from the fan outlet 60).
In one embodiment, the cooling air flows out of the plenum 51 through the diffuser 36 into the battery compartment and through the diffuser 70 and exits the battery enclosure through vents or air ducts if the battery is installed inside the vehicle. To maximize cooling efficiency and ensure uniform cell temperature, the flow path through the cell assembly may be optimized. The optimized path is determined by the cell arrangement and the spacing between the cells 12 of the battery pack 18. The air flow through the pack 18 can be either in the direction parallel to the cell axis (axial flow) or perpendicular to the cell axis (cross-flow). Axial airflow yields the most uniform heat transfer conditions if the cells are arranged in a single layer. If there is more than one layer of cells, the axial flow results in uneven cooling of the cells as the downstream cells receive less airflow than the upstream cells. Cross-flow cooling requires airflow perpendicular to the cell axis, and also results in uneven cooling of each cell 12, because each exposed side has a higher heat transfer rate than its back side. Another drawback of cross-flow cooling is that airflow is significantly obstructed by multiple layers of cells 12, resulting in increased pressure drop causing higher fan power demand and operating noise.
The arrangement of the cells 12 in the module is such that the air can flow freely through the pack 18 around the cells 12. The cells 12 are arranged in a single layer as shown in
In the present embodiment, an electronic control unit is used to control the air supply and ensure that the cells 12 remain within a predetermined allowable temperature range. The control unit has encoded thereon a battery temperature control logic for the battery pack 18. This control logic in conjunction with the air supply apparatus form the battery thermal management system. The battery temperature control logic of the battery thermal management system has two objectives. First, to detect potential overheating of the cell and second, to manage the air flow to remove the heat from the cells 12 in order to maintain the cell temperature within a selected temperature range or alternatively to manage the heating of the battery pack 18. Additionally, the battery temperature control logic may further detect if the cell 12 is too cold and initiate a heating pad to heat the battery pack 18. This will be discussed in more detail below.
As shown in
The temperature data may also be used to detect possible cell damage. The BCM software is calibrated to include a map of the typical cell temperature distribution across the pack. The BCM 80 compares the measured battery temperatures to the temperatures indicated by the pre-programmed map. The data is normalized to compensate for pack temperature level by referring each module temperature to the temperature of a reference module. In the present embodiment, the reference module is a module in the centre of the pack 18. If a module or a set of modules consistently shows temperature higher or lower than predicted, it indicates a potential cell 12 or module failure and the BCM 80 generates a warning message that is communicated to the vehicle diagnostics system.
The BCM logic to implement the temperature limits and assess cell temperature uniformity is shown in
If the average temperature of the battery is found to be steady or decreasing, the BCM 80 performs a check of temperature uniformity across the battery pack 18. In order to evaluate the temperature differences between the modules 14, the BCM 80 normalizes the temperature with respect to the subject module 14 (step 122), then reads a typical temperature value from a look up table (step 124). If the normalized temperature value is within 10% of the typical temperature value, then the BCM 80 proceeds to the next module 14 (step 142) and restarts the process. When the normalized temperature is outside the 10% range, the BCM 80 flags this module 14 as “out of range” (step 132) and increases a counter by 1 (step 134). The BCM 80 continues to test all the modules 14 and continues to increase the counter by 1 when the normalized temperature for a tested module 14 is out of range. When the counter value for a given module 14 exceeds a prescribed limit, in this case 100 (step 136), then a warning is displayed 138. The warning indicates that the module 14 is consistently warmer or colder than expected and the pack should be inspected for potential cooling or charge balancing problems.
The second function of the battery temperature control logic is to control the fans 52. In order to remove the heat generated by the cells 12, the air flow through the pack 18 must be sufficient to allow the entire heat from the batteries to be transferred to the air, given cell-to-air heat transfer characteristics and ambient temperature conditions. To achieve maximum battery life, it is desired to maintain the battery temperature within a narrow temperature range.
A conventional approach to control the battery temperature is to turn the fans 52 on when the battery temperature reaches a predetermined threshold and turn them off when the battery returns to the desired operating temperature. This approach requires high air flow as the amount of heat to be removed is proportional to the battery temperature.
However, temperature control may be improved by active fan management using the battery load energy as a reference variable rather than direct temperature. This is achieved by measuring the load current and calculating the energy flowing from and to the battery. The calculations use the cell internal resistance and current to calculate the resistive heat generated by the battery. The total heat generated by the cell includes the resistive heat and the heat due to enthalpy change. The formula to calculate the heat generated by the cell is:
Q=I
2
*R+Q
e, [eq. 1]
where: Q—total heat generated by the battery,
I—battery current
R—battery internal resistance
Qe—heat generated or absorbed due to enthalpy change.
Referring again to
The BCM logic required to control the air flow using the above described energy criterion is shown in
The following additional embodiments, or similar methods leading to the same result, are not ruled out. A thermal management system should adapt to all environmental conditions. While the battery pack 18 is operating, the object is to efficiently maintain optimal cell 12 operating temperatures. However, when the battery pack 18 is inactive, a means to prevent the cells 12 from becoming too cold to function must be considered. A heating pad (not shown) in the bottom of, or underneath, the enclosure 66 to keep the central core of the battery pack 18 warm enough during periods of low temperature inactivity can be used. This is accomplished by implementing a wake-up mode of the BCM 80, where the BCM 80 is automatically activated, for example, every one hour, when the vehicle is inactive to check the status of the system. If the battery temperature drops below, for example, 0° C., the heating pad is activated until next status check. In the event of extended inactivity, this is maintained until the battery state-of-charge drops below a minimum threshold, for example 20%, when the system shuts down. Also, a means to automatically block the air intake 64 to prevent cold air from entering the enclosure 66 during battery pack 18 inactivity may be employed.
Illustrative materials for constructing major elements of the thermal management system are mentioned herein. The enclosure 66 and its lid 68 may be molded of carbon fiber. The module's 14 mounting plate 26, top cap 20 and bottom cap 22 may be polyethylene. Cells 12 may be lithium based chemistry, with this embodiment utilizing a manganese dioxide positive electrode with a graphitic carbon negative electrode. The diffusers may be made of polyvinyl chloride (PVC). The rails 38 and 40, baffles 42, the structure of the fan mounting 56 and the electronics bay 46 may all be made of plastic. The fan 52 may include an aluminum motor 54 and impeller with a steel casing. All electrical connectors may be made of a non-corrosive conductor such as nickel-plated copper, and fasteners are made of stainless steel.
The present invention has been described with regard to a plurality of illustrative embodiments. However, it will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined in the claims.
| Number | Date | Country | |
|---|---|---|---|
| 60900334 | Feb 2007 | US |
