Method and apparatus for improving multi-cell battery safety and / or life expectancy using controlled compression

Abstract

A method and apparatus for improving multi-cell battery safety and / or life expectancy using controlled battery compression. A strap circumscribes a multi-cell battery to apply to the battery a compressive force generated by a compression generator. A controller controls the compressive force in accordance with measured or received battery characteristics.

Description

METHOD AND APPARATUS FOR IMPROVING MULTI-CELL BATTERY SAFETY AND / OR LIFE EXPECTANCY USING CONTROLLED COMPRESSIONBACKGROUNDField

[0001] Embodiments of the present invention generally relate to battery energy storage systems (BESS) and, in particular, to a method and apparatus for improving BESS safety and / or life expectancy using controlled compression.Description of the Related Art

[0002] Battery energy storage systems (BESS) generally comprise a battery formed of a plurality of battery cells, an energy conversion unit or units and a battery management unit (BMU). The BMU controls battery charging and discharging via the energy conversion unit(s). The energy conversion unit(s) are typically at least one DC / AC bidirectional microinverters that convert stored DC power to AC power to discharge the battery and convert AC power to DC power to charge the battery.

[0003] The battery is typically manufactured using lithium-ion cells. The cells are stacked adjacent to one another and the stack is compressed to maintain a uniform temperature across all the cells. The applied compression is static - applied during manufacturing at a fixed pressure level. However, battery cells expand and contract with ambient temperature, charge / discharge cycle, and state of health (SOH) of the cells (i.e., battery age). This expansion and contraction cause the pressure on the cells to vary. Such pressure variation shortens the life expectancy of the battery.

[0004] Additionally, lithium-ion cells are subject to thermal runaway that can lead to a fire within the battery. Thermal runaway occurs as the result of physical damage and / or operational parameters such as extreme temperature, inappropriate charge rate, incorrect charge state, etc. When a cell experiences thermal runaway, the cell (known as the trigger cell) may expand and damage neighboring cells which then also experience thermal runaway. This cascading effect of cells experiencing thermal runaway may result in a catastrophic fire or explosion.

[0005] Therefore, there is a need for a method and apparatus to control cell compression to improve battery safety and / or life expectancy.SUMMARY

[0006] A method and apparatus for improving multi-cell battery safety and / or life expectancy using controlled compression is provided substantially as shown in and / or described in connection with at least one of the figures, as set forth more completely in the claims.

[0007] Various features and advantages of the present disclosure may be appreciated from a review of the following detailed description of the present disclosure, along with the accompanying figures in which like reference numerals refer to like parts throughout.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] So that the manner in which the various features of the present invention can be understood in detail, a particular description of the invention, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

[0009] FIG. 1 depicts a front plan view of a battery comprising pouch cells and compression bands in accordance with at least one embodiment of the invention;

[0010] FIG. 2 depicts a front perspective view of a battery comprising prismatic cells and compression bands in accordance with at least one embodiment of the invention;

[0011] FIG. 3A depicts a side view of a battery comprising pouch cells in a compressed state in accordance with at least one embodiment of the invention;

[0012] FIG. 3B depicts the battery of FIG. 3A in an uncompressed state in accordance with at least one embodiment of the invention;

[0013] FIG. 4A depicts a top view of a compression generator in accordance with at least one embodiment of the invention;

[0014] FIG. 4B depicts the compression generator of FIG. 4A in an uncompressed state in accordance with at least one embodiment of the invention;

[0015] FIG. 5 depicts a flow diagram of a method of operation of a compression control system to improve battery safety in accordance with at least one embodiment of the invention;

[0016] FIG. 6 depicts an impedance graph created using electrochemical impedance spectroscopy (EIS) in accordance with at least one embodiment of the invention;

[0017] FIG. 7 depicts a flow diagram of a method of calibrating a compression control system in accordance with at least one embodiment of the invention; and

[0018] FIG. 8 depicts a flow diagram of a method of operation of a compression control system to improve the life expectancy of a battery in accordance with at least one embodiment of the invention.DETAILED DESCRIPTION

[0019] Embodiments of the present invention comprise a method and apparatus for improving multi-cell battery safety and / or life expectancy using controlled compression. The apparatus comprises one or more compression bands circumscribing a plurality of battery cells to apply a compressive force to the cells. Such compression improves thermal uniformity throughout the battery to reduce the chance of a cell experiencing thermal runaway. Upon detection of thermal runaway of a particular cell (the trigger cell), the at least one compression band is released (i.e., placing the cell(s) in an uncompressed state) which allows the trigger cell to expand. Thereby minimizing the heat transfer surface area. This expansion facilitates cooling of the trigger cell and reduces the probability that the trigger cell will not trigger other cells to experience thermal runaway.

[0020] In addition, the apparatus may actively control the pressure applied by the at least one compression band to the cells. Battery function is monitored using electrochemical impedance spectroscopy (EIS) and cell compression is automatically controlled through adjustment of the at least one compression band in response to measured impedance changes. Through active compression control, the pressure applied to the cells is stabilized in response to at least one battery characteristic such as, but not limited to, ambient temperature changes, battery age, charge / discharge cycles, voltage, current, impedance, etc. Such pressure stability extends the life expectancy of the battery.

[0021] In one embodiment, the at least one compression band may be positioned to circumscribe a stack of pouch cells. In another embodiment, the at least one compression band may be positioned to circumscribe a battery comprised of an array of prismatic cells.

[0022] The compressive force is applied by tightening the at least one band around the battery. The tightening force may be applied via a compression generator that is manually operated or operated via an electric motor. In one embodiment, the compression generator comprises a ratchet mechanism coupled to a compression controller. An embodiment using an electric motor may apply a dynamic compressive force where the force varies with operational characteristics such as, but not limited to, one or more of temperature, voltage, current, impedance, battery state of charge, battery state of health, and the like. In all these forgoing embodiments, the controller may be coupled to the compression generator to release the compressive force when a cell is detected to be or about to be experiencing thermal runaway as well as to actively control the applied compressive force.

[0023] FIG. 1 depicts a front plan view of a controlled compression battery energy storage system (CC-BESS) 100 comprising a battery 102, at least one compression band 120A and 120B, and a compression controller 108 in accordance with at least one embodiment of the invention. In the depicted embodiment, the battery 102 comprises a plurality of pouch cells 106 arranged in a stack. The battery 102 is circumscribed by at least one compression band (two are shown as 120A and 120B),where each compression band 120A and 120B comprises a strap 118A and 118B and a compression generator 104A and 104B. The straps 1 18A and 118B are fabricated from a flexible material that is able to be wrapped around the battery 102 such as nylon or metal. In one exemplary embodiment, the compression generators 104A and 104B are ratchet mechanisms wherein ends of the straps 1 18A and 118B wrap around a bobbin. When the bobbin rotates, the bands become tight and exert compressive force upon the battery 102. In one embodiment, the applied force may range from 100 to 300kgf. The compressive force can be released when a cell is experiencing or is about to experience thermal runaway. The battery may swell between 0 and 4 percent. The operation of the compression generator 104A and 104B is described in detail with respect to FIGs. 4A and 4B below.

[0024] The compression generators 104A and 104B are controlled by the compression controller 108. In one embodiment, the compression controller 108 may control an electric motor in the compression generator 104A and 104B to tighten and / or loosen the bands 118A and 118B. In other embodiments, the bands may be manually tightened and the controller 108 operates a release device that releases the compressive force upon detection of a cell experiencing or about to experience thermal runaway.

[0025] The compression controller 108 comprises at least one processor 110, support circuits 1 12 and memory 114. The at least one processor 110 may be any form of processor or combination of processors including, but not limited to, central processing units, microprocessors, microcontrollers, field programmable gate arrays, graphics processing units, and the like. The support circuits 112 may comprise well- known circuits and devices facilitating functionality of the processor(s). The support circuits 112 may comprise one or more of, or a combination of, power supplies, clock circuits, communications circuits, cache, and / or the like. The support circuits 112 may also comprise an interface to a battery management unit (BMU) and / or comprise sensors for detecting when a cell is experiencing or about to experience thermal runaway. The sensors may also monitor at least one battery characteristic such as, but not limited to, temperature, voltage, current, and / or impedance. The BMU maysupply signals to the controller 108 that are used to detect when a cell is experiencing or about to experience thermal runaway.

[0026] The memory 114 comprises one or more forms of non-transitory computer readable media including one or more of, or any combination of, read-only memory or random-access memory. The memory 114 stores software and data including, for example, control software 116 and a calibration database 122. The control software 116 contains instructions that, when executed by the processor 110, cause the controller 108 to perform operations that control the functionality of the compression bands 120A and 120B, e.g., control the compressive force the bands apply to the battery. The function of the control software 116 and the calibration database 122 are described with respect to FIGs. 5, 6 and 7 below.

[0027] In some embodiments, the controller 108 may be a stand-alone control unit. In other embodiments, the controller 108 may be built into the BMU as a hardware component, software component or a combination of hardware and software.

[0028] FIG. 2 depicts a perspective view of a battery 200 comprising prismatic cells 208 and at least one compression band (two are depicted as bands 204A and 204B) in accordance with at least one embodiment of the invention. In the depicted exemplary embodiment, the compression bands 204A and 204B comprise at least one compression generator 206 coupled to at least one strap 210A and 210B. The compression generator is coupled to and controlled by a controller, e.g., controller 108 in FIG. 1 ). As with the embodiment of FIG. 1 , upon detection that at least one cell 208 of the battery 200 is experiencing or is about to experience thermal runaway, the compression 206 generator releases tension on the straps 210A and 210B to release the battery 200 from compression. Consequently, the cell temperature will decrease, and thermal runaway may be avoided and / or the cell will not trigger thermal runaway in other cells. In addition, the compression bands 204A and 204B may be used to actively control compression of the cells to extend the life expectancy of the battery.

[0029] FIG. 3A depicts a side view of a battery 300 comprising a stack of pouch cells 302 in a compressed state in accordance with at least one embodiment of theinvention. Compression is supplied by compression bands 304A and 304B which apply force as shown by the arrows. Such compression maintains thermal uniformity across the cells.

[0030] FIG. 3B depicts the battery 300 of FIG. 3A in an uncompressed state in accordance with at least one embodiment of the invention. Upon detection that at least one cell 308 of the battery 300 is experiencing or is about to experience thermal runaway, the compression bands 304A and 304B of FIG. 3A are released to release the battery 300 from compression and allow the cell 308 to expand. Consequently, the cell temperature will decrease, and thermal runaway may be avoided and / or the cell will not trigger thermal runaway in other cells.

[0031] FIG. 4A depicts a top view of a compression generator 400 in accordance with at least one embodiment of the invention. The compression generator 400 comprises a bobbin 410 around which the ends 406 and 408 of a strap (e.g., 118A, 118B, 210A or 210B in FIGs. 1 and 2) are wrapped. The compression generator 400 operates as a ratchet strap and comprises a gear (integrated with the bobbin 410) and a pawl 404. The pawl is generally biased against the gear by a spring (not shown). The bobbin may be turned clockwise to wind the strap ends 406 and 408 around the bobbin 410. The biased pawl interacting with the gear ensures the bobbin turns in only the clockwise direction to apply tension on the strap and apply compressive force to the battery. The bobbin 410 may be turned using an electric motor or a mechanical winding mechanism. The pawl may be controlled by a solenoid to disengage the pawl from the gear to release the compressive tension on the strap.

[0032] In some embodiments, a gear and pawl ratchet may not be used, and an electric motor may control the amount of compression and its release. In a further embodiment, an electric motor may be used to dynamically control the compression force in a modulated manner, i.e., varying the compression force in view of battery parameters such as, but not limited to, temperature, battery state of charge, battery state of health, battery age, battery impedance, and the like. Such parameters may be provided by the BMU or directly monitored by the controller (108 in FIG. 1 ). In otherembodiments, the BMU may compute the amount of force that is necessary and send a control signal to the controller.

[0033] FIG. 4B depicts the compression generator 400 of FIG. 4A in an uncompressed state in accordance with at least one embodiment of the invention. Here, the pawl has been released from the gear and the bobbin 410 rotates counterclockwise to release tension from the ends 406 and 408 of the strap.

[0034] FIG. 5 depicts a flow diagram of a method 500 of operation of a compression controller 108 of FIG. 1 to improve battery safety in accordance with at least one embodiment of the invention. It is assumed, the compressive force has been applied to the battery prior to the start of method 500. The method 500 begins at 502 and proceeds to 504 where the controller receives or measures at least one battery characteristic (e.g., temperature, current, voltage, impedance, state of charge, state of health, and the like). At 506, the controller analyzes the at least one characteristic to determine if at least one battery cell is experiencing or is about to experience thermal runaway. In some embodiments, the functions of 502 and 504 may be incorporated in the BMU such that the BMU sends control signals to the controller. In some embodiments, the BMU or controller may utilize predictive algorithms or machine learning algorithms to predict that a cell or cells are about to experience thermal runaway.

[0035] At 508, the method 500 queries whether a cell or cells are experiencing or are about to experience thermal runaway. If the query is negatively answered, the method 500 returns to 504. If the query is affirmatively answered, the method proceeds to 512.

[0036] At 512, the method 500 controls the amount of compression being applied to the battery. This control response may be a simple release of all force (e.g., activate a solenoid to remove the pawl from the gear of a ratchet) or a dynamic release (e.g., control a motor to modulate the amount of force being applied to the battery until the battery characteristics improve).

[0037] At 514, the method 500 queries whether the process should continue. If the query is affirmatively answered, the method 500 proceeds along paths 516 and 510 to 504 to receive or measure additional battery characteristics, i.e., continue monitoring the battery characteristics. If the query is negatively answered, the method 500 proceeds to 518 and ends.

[0038] Through use of controlled compression, the CC-BESS is able to control the compression on a battery to improve battery safety through mitigation of thermal runaway and the trigger effect.

[0039] FIG. 6 depicts an impedance graph 600 created using electrochemical impedance spectroscopy (EIS) in accordance with at least one embodiment of the invention. In one embodiment, the control software (116 in FIG. 1 ) includes an EIS process. This process may be used to calibrate the controller as described with respect to FIG. 7 or to operate the controller to adjust the compressive force applied to the battery cells as described with respect to FIG. 8.

[0040] Graph 600 comprises a real impedance axis 602 and imaginary impedance axis 604. The EIS process applies a varying frequency voltage to the battery where, for example, the frequency of a sensing signal is varied from 1000 Hz to 0.01 Hz to produce traces 606 and 608 (i.e., a plurality of spectra). In other embodiments, different frequency ranges may be used. Impedance trace 606 was generated at the battery’s beginning of life (BOL) and while fully charged 100%, while impedance trace 608 represents the impedance of the battery after 500 charge / discharge cycles. This change in impedance (i.e., the difference between the traces) may be exploited by the controller to generate a control signal to adjust the cell compression to compensate for the age of the battery as well as other battery parameters.

[0041] FIG. 7 depicts a flow diagram of a method 700 of calibrating a compression control system (100 in FIG. 1 ) in accordance with at least one embodiment of the invention. The calibration process is performed on a battery to characterize the impedance of the battery over various parameters such as, but not limited to, charge / discharge cycles, age, temperature, compression pressure, and the like. Aseach parameter is varied, an EIS trace is generated and stored in memory. The set of EIS traces is later to control the compression as described with respect to FIG. 8 below.

[0042] The method 700 starts at 702 and proceeds to 704 where the temperature of the battery is established (e.g., apply fixed ambient temperature until thermal equilibrium is attained). At 706, the battery is fully charged (100% SOC). At 708, a compression pressure level is applied to the battery and, at 710, an impedance trace is measured using EIS. 708 and 710 are repeated as indicated by path 712 to develop a set of impedance traces over various compression levels.

[0043] At 714, the battery is discharged to no charge (0% SOC) and, through path 718, the various levels of compression are applied at 708. With each compression level and the battery discharged, an impedance trace is created at 710.

[0044] At 720, the battery is incrementally aged (e.g., 100%, 95%, 90%, etc.) and, at 722, an EIS trace is created for each age level. At 726, the method 700 queries whether the next temperature is to be selected. If the query is affirmatively answered, the method 700 returns to 704 where the battery is adjusted to a new temperature level and the foregoing process is repeated to develop additional traces at the new temperature. If the query is negatively answered, the method 700 proceeds to 728 where the method 700 creates a look-up table or other form of database (calibration database 122 in FIG. 1 ) containing the impedance traces for the various parameters. The database contains the EIS trace track and trajectory information that occurs in view of changes in the battery parameters. The method 700 ends at 730.

[0045] The method 700 may be performed by a computer having the form of controller 108. The calibration method 700 may be performed on a sample battery or on the actual battery to be used in the CC-BESS. The calibration method 700 may be performed post-manufacturing, but prior to fielding the battery. In other embodiments, the calibration method 700 may be performed by the controller 108 after the CC-BESS is installed in the field.

[0046] FIG. 8 depicts a flow diagram of a method 800 of operation of a compression control system (100 in FIG. 1 ) to improve the life expectancy of a battery in accordance with at least one embodiment of the invention. The method 800 begins at 802 and proceeds to 804 where an EIS is performed on the battery (102 in FIG. 1 ). At 806, the EIS trace from 804 is compared to the calibration database to determine the track and trajectory of the measured EIS trace. From the database information, at 808, a compression level is determined.

[0047] At 810, the method 800 adjusts the compression level being applied to the battery. At 812, the method 800 queries if the compression adjustment process should continue. If the query is affirmatively answered, the method 800 continues along path 814 to perform another EIS measurement. In this manner, the method 800 continuously and actively controls the compression being applied to the battery. Such active control extends the life expectancy of the battery.

[0048] If the query of 812 is negatively answered, the method 800 proceeds to 816 and ends.

[0049] The method of improving safety (FIG. 5) and the method of extending battery life expectancy (FIGs. 7 and 8) may be performed together to improve both safety and life expectancy or they may be performed separately to either improve safety or improve life expectancy.

[0050] Here multiple examples have been given to illustrate various features and are not intended to be so limiting. Any one or more of the features may not be limited to the particular examples presented herein, regardless of any order, combination, or connections described. In fact, it should be understood that any combination of the features and / or elements described by way of example above are contemplated, including any variation or modification which is not enumerated, but capable of achieving the same. Unless otherwise stated, any one or more of the features may be combined in any order.

[0051] As above, figures are presented herein for illustrative purposes and are not meant to impose any structural limitations, unless otherwise specified. Variousmodifications to any of the structures shown in the figures are contemplated to be within the scope of the invention presented herein. The invention is not intended to be limited to any scope of claim language.

[0052] Where conditional language is used, including, but not limited to, “can,” “could,” “may” or “might,” it should be understood that the associated features or elements are not required. As such, where conditional language is used, the elements and / or features should be understood as being optionally present in at least some examples, and not necessarily conditioned upon anything, unless otherwise specified.

[0053] Where lists are enumerated in the alternative or conjunctive (e.g., one or more of A, B, and / or C), unless stated otherwise, it is understood to include one or more of each element, including any one or more combinations of any number of the enumerated elements (e.g. A, AB, AC, ABC, ABB, etc.). When “and / or” is used, it should be understood that the elements may be joined in the alternative or conjunctive.

[0054] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

Claims:1 . A method of improving multi-cell battery safety and / or life expectancy comprising: applying a compressive force to a battery at a compression level; measuring or receiving at least one battery characteristic; analyzing the at least one battery characteristic to determine whether the compression level should be adjusted; and controlling the compressive force being applied to the battery in view of the analysis of the at least one battery characteristic.

2. The method of claim 1 wherein analyzing further comprises determining from the analysis of the at least one battery characteristic that at least one cell in the battery is or is about to be experiencing thermal runaway and controlling further comprises, upon determining that at least one cell in the battery is or is about to be experiencing thermal runaway, releasing the compressive force to enable the at least one cell that is or is about to be experiencing thermal runaway to expand.

3. The method of claim 1 wherein controlling further comprises adjusting at least one compression band that circumscribes the battery to alter the compressive force.

4. The method of claim 3 wherein the compression band comprises a compression generator and at least one strap, and further comprising controlling the compression generator coupled to the at least one strap to adjust the compression force applied to the battery by the at least one strap.

5. The method of claim 4 wherein the compression generator comprises an electric motor that is activated to create a dynamic compressive force.

6. The method of claim 1 wherein the at least one battery characteristic comprises one or more of temperature, voltage, current, impedance, battery state of charge, and battery state of health.

7. The method of claim 1 wherein at least one battery characteristic is impedance measured using electrochemical impedance spectroscopy (EIS).

8. The method of claim 7 wherein analyzing further comprises measuring an EIS track and trajectory and controlling further comprises determining the compressive force to be applied to the battery using the EIS track and trajectory.

9. Apparatus for improving multi-cell battery safety and / or life expectancy comprising: at least one strap adapted to circumscribe a battery comprising a plurality of cells; at least one compression generator, coupled to the at least one strap, for causing the at least one strap to apply compressive force to the plurality of battery cells; and a controller, coupled to the at least one compression generator, for controlling the compressive force applied to the plurality of battery cells within the battery.

10. The apparatus of claim 9 wherein the at least one compression generator comprises an electric motor coupled to a ratchet.

11. The apparatus of claim 10 wherein the controller measures or receives at least one battery characteristic and controls a level of compressive force based upon the at least one battery parameter.

12. The apparatus of claim 11 wherein the at least one battery characteristic comprise one or more of temperature, voltage, current, impedance, battery state of charge, and battery state of health.

13. The apparatus of claim 11 wherein the controller analyzes the at least one battery characteristic to determine when that at least one cell in the battery is or is about to be experiencing thermal runaway and, upon determining that at least onecell in the battery is or is about to be experiencing thermal runaway, the controller releases the compressive force to enable the at least one cell that is or is about to be experiencing thermal runaway to expand.

14. The apparatus of claim 10 wherein the at least one battery characteristic is impedance measured using electrochemical impedance spectroscopy (EIS).

15. The apparatus of claim 14 wherein the controller measures an EIS track and trajectory and determines the compressive force to be applied to the battery using the EIS track and trajectory.

16. Apparatus for improving multi-cell battery safety and / or life expectancy comprising: at least one strap adapted to circumscribe a battery comprising a plurality of cells; at least one compression generator, coupled to the at least one strap, for causing the at least one strap to apply compressive force at a compression level to the plurality of battery cells; and a controller, coupled to the at least one compression generator, comprising a processor and a non-transitory computer readable medium for storing instructions that, when executed by the processor, cause the controller to perform operations comprising: measuring or receiving at least one battery characteristic; analyzing the at least one battery characteristic to determine whether the compression level should be adjusted; and controlling the compressive force being applied to the battery in view of the analysis of the at least one battery characteristic.

17. The apparatus of claim 16 wherein analyzing further comprises determining from the analysis of the at least one battery characteristic that at least one cell in the battery is or is about to be experiencing thermal runaway and controlling further comprises, upon determining that at least one cell in the battery is or is about to beexperiencing thermal runaway, releasing the compressive force to enable the at least one cell that is or is about to be experiencing thermal runaway to expand.

18. The apparatus of claim 16 wherein the at least one battery characteristic comprises one or more of temperature, voltage, current, impedance, battery state of charge, and battery state of health.

19. The apparatus of claim 16 wherein at least one battery characteristic is impedance measured using electrochemical impedance spectroscopy (EIS).

20. The apparatus of claim 19 wherein analyzing further comprises measuring an EIS track and trajectory and controlling further comprises determining the compressive force to be applied to the battery using the EIS track and trajectory.

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