Patent Application
20110091749
The present disclosure relates generally to battery packs comprising a plurality of electrochemical battery cells.
Electrochemical battery cells constructed in large arrays of individual cells (battery packs) are gaining popularity as a rechargeable power source for tools, vehicles and other powered devices. Some of these types of cells, such as Lithium ion (Li-ion) cells can experience failures leading to thermal runaway of other cells which can, in turn, cause damage to the battery pack and potentially to surrounding objects. For example, overcharging such a battery pack can lead to rising temperatures within individual cells and the battery pack overall, lithium plating within the cells, and formation of dendritic lithium within the cells which can cause internal shorts in individual cells. Such events can cause severe heating within the cells and an increase in internal pressure. For example, Li-ion cells containing Li—Co oxide (Lithium Cobalt oxide) cathodes may even catch fire when cell temperatures rise above about 150° C. Such damaging heat may be internally generated or externally applied. A cell having an internal short that leads to a temperature rise above 150° C., even locally inside the cell, may result in fire or explosion and cascading effects on nearby cells. Li-ion chemistries other than Li—Co oxide have different threshold temperatures for thermal runaway but are still subject to this potential failure.
To avoid such events, careful attention is given to the manufacture of the cells used in such battery packs as well as to the recharging and other interface electronics used with such battery packs. Nonetheless, given the large number of cells used in some battery packs, even with extremely low defect rates, cell failures will occasionally occur.
To relieve pressure in compromised cells, one or more pressure relief vent mechanisms are typically provided which vent hot materials from the cell at certain temperatures and/or pressures in order to relieve pressure within the cell and thus avoid an explosion. A problem with this approach is that when such vents open they release extremely hot materials such as gasses which can potentially direct excessive heat to adjacent cells within the battery pack causing a rapidly cascading failure. Such pressure relief vents generally comprise one or more apertures in the outer structure of the cell which may be coupled to a pressure relief valve of a type suitable for use in such cells. Such valves may include those of the type described in U.S. Pat. No. 7,195,839 where the vent aperture is formed by bursting a weakened portion of the cell container, those having a predefined aperture coupled to a pressure relief mechanism such as a valve or a burstable diaphragm or membrane, or the like.
To better understand the nature of the problem, the energy released during the combustion of a conventional Type 18650 3.6 V Li-ion battery cell having a 2.2 Ah capacity and 40 gram (g) weight built with 4 g of organic solvent in electrolyte and 7 g of graphite (carbon) in the anode is calculated as follows:
If the energy released during fire/explosion is used to heat a single cell, the temperature rise of the cell can reach (40g cell weight, specific heat of cell is assumed to be 0.5 joule/g*° C.): 71,600/40×0.5=3580° C. by solvent combustion alone. Even higher temperatures can be reached if part of the electric energy is released.
In a battery pack constructed with many cells connected together, a single cell fire/explosion can, without proper precautions, easily cause one or more neighboring cells to reach their respective thermal runaway temperature, thus potentially spreading a fire or explosion until the whole battery pack is consumed.
Some cells are sealed with a plastic grommet, typically made of polypropylene, which can soften if even briefly exposed to temperatures significantly above 160° C., which will compromise the ability to seal and result in electrolyte leakage. This leakage in the neighborhood of a fire can quickly add fuel to the fire.
Many chemistries have been applied to Li-ion cells, e.g., Li—Co oxide, Li-Manganese oxide, Li-Iron phosphate, Li-Nickel oxide. Each chemistry has its own thermal runaway temperature ranging from 150° C. to 400° C. or higher. While some are higher, and thus potentially safer, the safety level accomplished by higher thermal runaway temperatures is limited because all Li-ion cells currently manufactured employ organic solvents as described above in the electrolyte and once that starts to burn even the higher thermal runaway temperatures can be readily achieved.
Accordingly it would be desirable to provide an apparatus and method to safely contain and manage a cell-failure in such a battery pack.
In a battery pack comprising a plurality of electrochemical battery cells arranged in one or more battery pack modules having inter-cell spaces defined between the cells, a method and apparatus diverts hot material vented from a failed battery cell into adjacent inter-cell spaces without permitting the hot material to directly impinge upon an adjacent battery cell end. Optionally the heat evolved from the cell failure is also conducted within the structure of the battery pack by thermally conductive elements to further dissipate it.
The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more examples of embodiments and, together with the description of example embodiments, serve to explain the principles and implementations of the embodiments.
In the drawings:
Example embodiments are described herein in the context of a battery for an electrically powered or partially electrically powered vehicle such as an automobile. Those of ordinary skill in the art will realize that the following description is illustrative only and is not intended to be in any way limiting. Other embodiments will readily suggest themselves to such skilled persons having the benefit of this disclosure. Reference will now be made in detail to implementations of the example embodiments as illustrated in the accompanying drawings. The same reference indicators will be used to the extent possible throughout the drawings and the following description to refer to the same or like items.
In the interest of clarity, not all of the routine features of the implementations described herein are shown and described. It will, of course, be appreciated that in the development of any such actual implementation, numerous implementation-specific decisions must be made in order to achieve the developer's specific goals, such as compliance with application- and business-related constraints, and that these specific goals will vary from one implementation to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the art having the benefit of this disclosure.
As pointed out above, a major potential safety issue with battery packs, and particularly battery packs used in vehicles, is the potential for a single cell failure to cause a cascading failure in a number of other battery cells. In order to avoid that consequence, it is important to avoid having the heat from the single cell failure impinge directly upon a single or a few adjacent cells.
To thus ensure that the heat evolved from a single cell fire/explosion will not spread solely to the immediately neighboring cells, a battery pack in accordance with the present invention is designed in such a way that the energy released due to a cell failure is dissipated somewhere other than only in the immediately neighboring cells. If the place that the heat is dissipated has a large heat capacity, and can readily absorb the heat released, the temperature rise of adjacent cells will be reduced. Making use of the cells in the battery pack as a sink for the heat released is an economically practical approach. For example, if the energy of 71,600 joules is absorbed by 100 cells, then the temperature rise of these cells will become 3580° C./100=35.8° C. This temperature rise will raise the temperature from the typical battery operating temperature of about 40° C. or under to about 40° C.+35.8° C.=75.8° C., which is far from the thermal runaway temperature of Li-ion cells. The key to the success of this approach is to effectively spread the heat generated in a single-cell fire/explosion to many cells, and/or other battery pack structure and/or the environment.
Turning now to the figures,
In one embodiment 53 commercial cylindrical Type 18650 batteries of 2.0 Ah capacity are connected in parallel to make a sub-module of 106 Ah/3.6V. These cells are designed with a crimp seal using plastic grommet as an insulator. The top is the positive end and is also where pressurized material will be ejected from in the case of a fire or explosion.
In operation, a cell failure results in the pressure relief vent(s) of a cell 22 opening and causing heated material to be released through pressure relief vents 56. The heated material travels through hole 58, into first diverter hole 60, through one or more second diverter holes 62, through third diverter holes 64 and into a corresponding inter-cell space 24 where it can dissipate into some or all of the rest of the battery pack without causing spot heating directly on an adjacent cell. Forced air ventilation is provided in one embodiment of the present invention which flows orthogonally to the longitudinal axes of the individual cells to remove normal heat dissipated in normal operation as well as a portion of the extraordinary heat caused by a cell failure venting heated material into the inter-cell spaces 24. Thus the inter-cell spaces 24 communicate with one another to allow this cross-flow forced air cooling in accordance with one embodiment of the present invention.
Those of ordinary skill in the art will now recognize, having the benefit of this disclosure, that other arrangements can be made to divert hot material vented from one end of the cells 22 into the inter-cell spaces 24, for example, the parts used could be combined into one or more assemblies fewer or more numerous than those illustrated herein.
In accordance with another embodiment of the present invention, the case 30/32 may be fabricated of a thermally highly conductive material such as aluminum which will also act as a heat spreader to quickly spread heat due to a cell failure throughout a module and its adjacent module where it can be sinked by more of the battery pack. The thermal mass of the case 30/32 in accordance with this embodiment may be designed to better sink more heat (e.g., by adding mass, and/or finned heat sink areas) if required by the application. In the embodiment shown in
In accordance with yet another embodiment of the present invention, the case portion 32 may be fabricated of aluminum and directly connected to the negative terminals of battery cells 22 thus eliminating the need for layers 36 and 42 and their corresponding diverter holes 62, 64. The thermal mass of the case 32 in accordance with this embodiment may be designed to better sink more heat (e.g., by adding mass, and/or finned heat sink areas) if required by the application.
While embodiments and applications have been shown and described, it would be apparent to those skilled in the art having the benefit of this disclosure that many more modifications than mentioned above are possible without departing from the inventive concepts disclosed herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.
