Electricity: battery or capacitor charging or discharging – Battery or cell discharging – With charging
Reexamination Certificate
2002-01-24
2004-11-09
Vu, Bao Q. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell discharging
With charging
C320S136000
Reexamination Certificate
active
06815930
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to protecting battery cells from various fault conditions and unequal cell performance during normal operation. More particularly, the invention relates to a cell protection circuit that provides over-current protection to battery cells.
2. Background Information
Batteries are useful for a variety of purposes, but generally must be operated in accordance with various criteria to ensure the safety and reliability of the battery and the device for which it provides power. The protection circuit described herein has been developed for use in connection with lithium batteries that are used in downhole tools. Such tools may be used for open hole logging and/or drilling purposes. Although the following background information and description of the protection circuit may be presented in the context of protecting high voltage, lithium battery packs, the protection circuit is useful for batteries used in a variety of other applications and for non-lithium battery types, particularly those that have similar safety and reliability concerns. Accordingly, the disclosure and claims which follow are not limited to the context in which the protection circuit is discussed below.
By way of definition, a “cell” is an individual location where chemical energy is converted into electric energy. A “battery” or “battery pack” is a collection of one or more cells connected in series or in parallel to produce more current, voltage or power than is provided by an individual cell.
The selection of battery type and configuration for downhole tools is influenced by various considerations. Downhole tools are typically packaged so as to have a diameter less than four inches so as to fit within a standard 8¼ inch diameter drill pipe. For obvious reasons, space is therefore at a premium for a downhole tool and thus battery packs should be as small as possible. Further, downhole tools usually experience relatively high temperatures presenting a potential hazard for the tool and its battery. It is not uncommon, for example, for the tool to operate at temperatures exceeding 150° C. or even 175° C. Also, the relatively high cost (labor and materials) of a seismic or drilling operation makes it desirable to reduce cost whenever possible. In light of these considerations, lithium cell chemistry is used in a majority of downhole tool applications today when surface power is not provided by a wireline or other means. Lithium cells, and particularly, lithium thionyl chloride (Li/SOCl
2
) cells provide high energy densities (i.e., a relatively large amount of energy given the size and weight of the cell when compared to other types of cells) and excellent high temperature performance. Thus, relative to many other types of cells, lithium cells last longer and operate better at higher temperatures with lower total cost.
Despite the advantages of lithium cells, they are not problem free. For instance, the discharge profile of a lithium cell must be carefully controlled to obtain the available energy from the cell and prevent hazardous conditions. Two of the most prevalent conditions that interfere with optimal Li/SOCl
2
cell performance are excessive anode “passivation” and cathode “freeze-over.” Anode passivation refers to the formation of a layer of lithium chloride (LiCl), which is also known as solid electrolyte interphase (SEI), on the anode surface. Cathode freeze-over refers to the formation of LiCl discharge products in the outer portion of the cathode which blocks access to unused reaction sites.
A thin SEI layer is always present on the surface of the anode. This layer is formed as a result of the reaction between the lithium and the thionyl chloride electrolyte in the cell, and the layer begins to form as soon as a cell is filled with electrolyte. The LiCl generally is a desirable feature for long term storage of such cells because it helps to minimize or prevent self-discharge. It is only when the cell is placed into service that passivation becomes a problem. Anode passivation is responsible for the condition known as “voltage delay,” which is the initial drop in potential observed when a load is first placed on a cell. The voltage drop is caused by the SEI layer which acts as a series resistor. As current flows through the cell, the SEI layer begins to evaporate resulting in an associated increase in cell terminal voltage. This process is called “depassivation.” In a freshly manufactured cell, the drop in running potential may last for less than a second, but in a heavily passivated cell (i.e., a cell with thick SEI layer), the voltage may drop below its nominal voltage (e.g., 3 volts) for an extended period of time.
As noted above, the performance of Li/SOCl
2
cells also can be detrimentally affected by cathode freeze-over which is the formation of LiCl discharge products in the outer portion of the cathode to the extent that it blocks the electrolyte's access to unused reaction sites. The discharge of Li/SOCl
2
cells results in the formation of LiCl in the cathode. If the cell is discharged at low rates (e.g., a current density of less than 2 milliamps per square centimeter), the LiCl will be evenly distributed throughout the carbon cathode, which results in efficient use of the active sites available for the reduction of SOCl
2
. At discharge rates greater than 2 mA/cm
2
, the reduction of SOCl
2
occurs predominantly on the outer surfaces of the cathode. The outer surface of the cathode effectively “freezes over” with LiCl, and the inner active surfaces become inaccessible. Unlike passivated anodes, which can be recovered via a depassivation process, cathodes that have been frozen over are irreparably damaged and capacity loss will result.
As noted above, passivated cells can be depassivated. This can be accomplished by placing the cells under load in a predetermined manner. Initially, the cell voltage will drop (below 3 V) due to the passivation, but gradually increase as the cell becomes unpassivated. One suitable way to depassivate a pack of cells is to place the cells under a light current load and then, as the voltage increases due to depassivation, increase the current draw on the pack.
For many cells, the voltage will begin to rise in about 15 minutes. A severely passivated cell may have a cell voltage below 3 V for a prolonged period of time (e.g., more than one hour). Any load that results in a cell voltage below 3 V for a prolonged period of time may cause cathode damage and reduce cell capacity. Batteries used for high voltage downhole tools typically are constructed from dozens or even a hundred or more series-connected lithium cells. Depassivating a pack of 100 cells might successfully depassivate most of the cells in the pack, but some cells may remain depassivated due to variations between individual cells. It is difficult to determine whether a few cells out of a hundred are passivated. Thus, as the current load is increased during the depassivation process, some cells that are still passivated will experience an increasing current demand. Because of the passivation that remains on such cells, the voltage of such cells will drop as the current load is increased. As explained below, this voltage drop can be harmful to the cell.
Cell voltage generally decreases as the current demand on the cell increases. Also, cell voltage will generally decrease as a cell ages and nears the end of its useful life. Most cell manufacturers recommend that their cells not be discharged to a point where the cell voltage is below a minimum level (e.g., 2 V). Forcing a cell below 2 V may cause bulging of the cell due to the build up of gaseous discharge products in the cell. It is also widely known that lithium cells exhibit safety concerns when the cells are discharged into reversal (the cell voltage reverses polarity). Besides cell reversal, the cells can also vent var
Alliburton Energy Services, Inc.
Conley & Rose, P.C.
Vu Bao Q.
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