A battery often energizes a powered surgical tool used in an operating room to perform a surgical procedure. The use of a battery eliminates the need to provide a power cord connected to an external power source. The elimination of the power cord offers several benefits over corded surgical tools. Surgical personnel using this type of tool do not have to concern themselves with either sterilizing a cord so that it can be brought into the sterile surgical field surrounding the patient or ensuring that, during surgery, an unsterilized cord is not inadvertently introduced into the surgical field. Moreover, the elimination of the cord results in the like elimination of the physical clutter and field-of-view blockage the cord otherwise brings to a surgical procedure.
In an operating room, batteries are used to power more than the tools used to perform the surgical procedure. Batteries are also used to energize the power consuming components integral with a personal protection system surgical personnel sometimes wear when performing a procedure. This system typically includes some type of hooded garment. Internal to the garment is a ventilation unit for circulating air within the garment. Some of these systems also have lights for illuminating the surgical site or radios that facilitate conventional spoken level conversation with other persons involved in performing the procedure. Each of these units, the ventilation unit, the light unit and the radio, requires a source of power. By providing this power from the battery, the need to attach cords to each individual wearing such a unit is eliminated. This, in turn, reduces number of cords in the operating room persons would otherwise have to avoid. Further, eliminating these cords likewise eliminates the restrictions of movement they place on the individual using the system.
An integral part of any battery-powered device is, naturally, the battery. Most battery-powered surgical devices used in an operating room are designed to be used with rechargeable batteries. These rechargeable batteries typically include one or more NiCd cells. Once a battery is discharged, it is coupled to a complementary charger. The charger applies a current to the battery's cells to store energy in the cells.
Unlike other rechargeable batteries, a rechargeable battery intended for use with a surgical tool must be sterilizable so that it can be placed in close proximity to the open surgical site on a patient. Often, these batteries are sterilized by placing them in an autoclave wherein the atmosphere is saturated with water vapor (steam), the temperature is approximately 270° F. (132° C.) and the atmospheric pressure is approximately 30 psi (Gage) (1552 mmHg). The repetitive exposure to this environment causes a battery cells' ability to store electric charge to degrade. Often this is referred to as degradation in the “state of health” of the battery.
The Applicant's U.S. Pat. No. 6,018,227, BATTERY CHARGER ESPECIALLY USEFUL WITH STERILIZABLE RECHARGEABLE BATTERY PACKS, issued Jan. 25, 2000 and incorporated herein by reference, discloses a means to determine the voltage at load of a battery. Inferentially, this is a measure of the internal resistance of the battery. Unfortunately, this information alone does not provide a complete measure of the battery state of health. For example, this information alone does not provide information if the stored energy is sufficient to power the device to which the battery is attached for the time required to perform the surgical procedure. This means that, during the performance of a procedure, if the battery's stored energy appreciably depletes, the procedure is interrupted to replace the battery. This increases the overall time takes to perform the procedure. This interruption runs contrary to one of the goals of modern surgery which is to perform the procedure as quickly as possible so as to lessen the time the patient's internal organs are exposed, and therefore open to infection, and the amount of time a patient is held under anesthesia.
Moreover, there is an interest in having surgical equipment provide data regarding their own operating states to other equipment in the hospital. For example, some motorized surgical tools are provided with internal temperature sensors. In the event a bearing assembly internal to a tool of this type malfunctions, tool temperature will start to rise. This rise in temperature is detected by the complementary sensor. The output signal from the sensor can then be read by a remote device in the hospital. This gives hospital personnel notice that the tool may be approaching a critical malfunction and should be repaired or replaced.
Corded surgical devices provide these types of operating state data. These communications systems are relatively simple technically and economical to provide because the signals are forwarded to the complementary control consoles through the cords through which power is supplied to these devices. One can also provide the data from these devices through wireless communications systems. One system is disclosed in the Applicant's U.S. Patent Application No. 60/694,592, POWERED SURGICAL TOOL WITH SEALED CONTROL MODULE, filed 28 Jun. 2005, the contents of which are published in U.S. Pat. No. 7,638,958 B2, incorporated herein by reference. A disadvantage of the above-mentioned system is that it requires the addition of a wireless communications system into the operating room. The expense of providing such a system limits the locations where they are installed.
The Applicant's Assignee's U.S. Pat. No. 5,977,746, RECHARGEABLE BATTERY PACK AND METHOD FOR MANUFACTURING SAME, issued 2 Nov. 1999 and incorporated herein by reference, discloses a rechargeable battery especially designed to withstand the rigors of autoclave sterilization. The battery of this invention includes a cluster of cells that are bound together by top and bottom plastic binders. Conductive straps extending between openings formed in the binders connect the cells. One of the straps is a fuse that opens upon a more than a specific current flowing through it. More specifically, the current through the fuse heats the material forming the fuse so a section of the fuse vaporizes. This vaporization of the fuse section separates the rest of the fuse into two sections.
The above battery pack has proven useful for storing the charge needed to energize a cordless surgical tool. However, the cells internal to the battery pack can generate significant amounts of heat. This causes the temperature of the cells to rise. Sometimes, the temperature rise between the cells is uneven. This uneven thermal loading of cells can result in an electrical imbalance of the cells. If the cells become so imbalanced, both the immediate utility of the battery to supply energy at a particular time and its useful lifetime may diminish.