Abstract:
An application for a battery pack that includes a set of walls made of sturdy material, power interface terminals and battery cells/electronics held within the walls. The battery cells are separated from the walls by a highly insulating material, thereby reducing the temperature that the battery cells reach during heat sterilization cycles performed on the battery pack after, for example, exposure of the battery pack to pathogens.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a Continuation-in-part of U.S. patent application titled “BATTERY CUSHION AND INSULATOR,” Ser. No. 12/786,473, inventor Steven Tartaglia, filed May 25, 2010. 
    
    
     FIELD 
     This invention relates to the field of batteries and more particularly to a system for insulating battery cells situated within battery packs from external heat introduced during thermal sterilization. 
     BACKGROUND 
     Battery cells such as flooded lead-acid, absorbed-glass-matt (AGM), lead-acid, Nickel Cadmium (NiCd), Nickel Metal Hydride (NiMh) and the like perform best at certain temperature ranges and are easily damaged when exposed to very high temperatures. When such battery cells, either standalone or within a battery pack, are exposed to certain high temperatures, various physical changes occur internal to the battery cells such as boiling of the electrolyte, etc. In an extreme case, such as boiling of the electrolyte, high pressure results within the sealed cell, leading to possible deformation of the outer case, deformation of the anode/cathode arrangements and, possible out-gassing or leakage of electrolyte, the later of which resulting in a totally useless cell. 
     Many hospital or surgical related devices use battery packs, providing enhanced range of motion to surgeons and reducing the chance of a power cord finding its way into a bad location. There are many examples of such devices for drilling into bone, sawing bone, screwdrivers, making incisions, etc. These devices use battery packs that attach to the devices and provide power during, for example, an operation such as an orthopedic operation. In most systems, the battery pack is removable for charging in a charging station. 
     A recent search indicates that many battery packs for such devices contain the heavy metal cadmium, Cd. The labels of these batteries show the chemical symbol, Cd, along with a crossed-out trash can, meaning that these batteries are not to be disposed of in ordinary trash due to land, water table and air pollution from landfills or incinerators. Notwithstanding, the cost for such battery packs are around $150-$400 for a used pack and $300-$500 for a new pack. 
     Some battery packs for medical devices are single-use, in that after usage, the entire pack is discarded. This creates a dilemma because the battery packs are often exposed to body fluids, making them a bio-hazard. Bio-hazard material is often incinerated to neutralize the hazard, but most batteries cannot be incinerated due to pollution and/or explosion issued. Therefore, the battery packs should not be placed in bio-hazard disposal containers, yet, since they are now bio-hazardous, the exposed battery packs cannot be disposed in normal recycle locations. 
     Many battery packs are rechargeable and reusable, much like a battery pack for a home cordless drill. After use and before the next usage, the device and the battery packs must be sterilized to kill any pathogens present on surfaces and in cracks, etc. To sterilize a rechargeable battery pack, per one exemplary manufacturer&#39;s procedure, the device is separated from the battery pack and any debris or accumulation is cleaned. Next, sterilization is performed through an Autoclave Cycle. Autoclave cycles are, for example, 132° C.-137° C. for at least 15 minutes then 5 minutes drying time or 15 minutes of pre-heating, 132° C.-137° C. for at least 10 minutes, then 5 minutes drying time. Such cycles, although hard on mechanical devices such as motors, usually do not severely affect the life of the actual devices. Unfortunately, these heat cycles often severely affect the life of the battery packs. For example, nickel cadmium battery packs are known to severely degrade after such sterilization cycles. A battery pack that normally would function well after 200 charge/use cycles (at normal ambient temperatures) is only useful for around six charge/use cycles after being sterilized at such temperatures and periods of time. Furthermore, even though it is usable for six cycles, the amount of charge such a battery pack will hold after heat sterilization is severely decreased, often requiring swapping of battery packs during an operation. 
     Newer, ecology minded technologies such as lithium ion (Li-ion) and Lithium Ion (Li fe) normally provide more use/charge cycles than nickel cadmium, but are even more susceptible to issues related to high temperatures. In, for example, Lithium Ion battery cells, the thin Solid Electrolyte Interface (SEI) layer on the anode breaks down due to overheating caused by excessive currents, overcharging or high temperatures. The breakdown of the SEI layer starts to occur at the relatively low temperature of 80° C. Once the SEI layer is breached, the electrolyte reacts with the carbon anode at a higher, uncontrolled, temperature, creating an exothermal reaction which drives the temperature up still further. Therefore, it is important to assure that the core temperature of Lithium Ion cells remains well under 80° C., preferably under 75° C. 
     It is recommended to store nickel cadmium battery cells at less than 45° C. and temperatures above this “can” cause the alkaline electrolyte to leak out. Storage of nickel cadmium, lithium-Fe—S and Lithium-Mg—O at temperatures above 60° C. violates most manufacturers&#39; recommendations and many warranties. 
     Unfortunately, when heated in an Autoclave to a temperature between 132° C. and 137° C., most battery cells will fail. Because each cell has an initial temperature and mass and the case, usually ABS plastic, has minor insulating ability, exposure to a temperature between 132° C. and 137° C. does not mean that, after 10, 20 or 30 minutes of exposure, the internal temperature will reach between 132° C. and 137° C., but it will likely be well over 80° C., thereby permanently damaging, for example, lithium ion battery cells. 
     What is needed is a battery pack that is suitable for sterilization in an Autoclave at sterilization temperatures for long enough periods of time as to properly sterilize the battery pack while preventing damage to the internal battery cell(s). 
     SUMMARY 
     A battery pack is disclosed including a set of walls made of sturdy material, power interface terminals and battery cells/electronics held within the walls and separated from the walls by a highly insulating material. 
     In one embodiment, a battery pack is disclosed including an enclosure with one or more battery cells held within the enclosure. An insulative layer separates the battery cells from an inside wall of the enclosure. A connection terminal is on or in an outside wall of the enclosure and two or more conductors connects the connection terminal with two or more terminals of the battery cells, the conductors pass through the insulative layer. 
     In another embodiment, a method of reducing battery cell failure during heat sterilization is disclosed including providing one or more battery cells, the battery cells being interconnected (series, parallel or series-parallel) to provide power to a device. The battery cells are surrounded in an insulative layer and connected to connection terminals by a plurality of conductors, the conductors passing through the insulative layer. The battery cells and the insulative layer are enclosed into a battery pack in which the connection terminals are accessible from outside of the battery pack. In this battery pack, the battery cells are insulated from heat applied to the battery pack during heat sterilization during a heat sterilization cycle, maintaining a safe temperature at the battery cells. 
     In another embodiment, a battery pack is disclosed including an enclosure that holds two or more battery cells. Conductive paths interconnect battery terminals of the battery cells in series, parallel or series-parallel configurations. An insulative layer separates the battery cells from an inside wall of the enclosure to reduce heat transfer from outside of the enclosure into the batteries during heat sterilization. A connection terminal that has electrical contacts is on or molded into the enclosure so that the electrical contacts are accessible from outside of the enclosure. Two or more conductors connect each of the electrical contacts of the connection terminal to a first terminal of a thermal breaker and two or more conductors connect a second terminal of the thermal breaker to the battery cells. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be best understood by those having ordinary skill in the art by reference to the following detailed description when considered in conjunction with the accompanying drawings in which: 
         FIG. 1  illustrates a perspective view of a typical battery pack of the prior art. 
         FIG. 2  illustrates a perspective view of a battery pack with insulated battery cells. 
         FIG. 3  illustrates a perspective view of a battery pack with insulated battery cells with thermal conductive breakers. 
         FIG. 4  illustrates a perspective view of a battery pack with insulated battery cells with thermal resistance. 
         FIG. 5  illustrates a perspective view of a battery pack with insulated battery cells with thermal heat absorbing mass. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Throughout the following detailed description, the same reference numerals refer to the same elements in all figures. 
     Referring to  FIG. 1 , a perspective view of a typical battery pack  10  of the prior art will be described. Typical battery packs  10  have a plastic enclosure  20 , usually made of Acrylonitrile butadiene styrene otherwise known as ABS. Within the plastic enclosure  20  are one or more battery cells  22  connected in series, parallel or series/parallel by interconnecting conductive paths  18 , typically flat metal sheets that are tack-welded to battery terminals. One or more battery terminals  23  are connected to a power connection terminal  12  by wires  14 / 16  or other conductive paths for the delivery of power to a device and for the charging of the battery cells  22 . 
     Although not shown, for completeness, often such battery packs  10  include other devices such as electronic circuits that prevent over current, over voltage, under voltage, control charging, prevent over-temperature situations during charging, etc. All such devices are known and present in some battery packs, but have been left out for clarity reasons. 
     Although the size of the plastic enclosure  20  is shown exaggeratedly larger than needed, it is known that inside surfaces of such cases  20  often directly touch the battery cells  22  to support the battery cells  22 . It is also known that an air gap  24  separates the battery cells  22  from the inside surface of the plastic enclosure  20  in places where no contact is made. 
     When such a battery pack  10  is placed in a sterilizing chamber such as an Autoclave during a sterilization cycle, external temperatures rise to a temperature between 132° C. and 137° C. The internal battery cells  22  will approach the external temperature gradually depending upon the initial temperature of the battery cells, the mass of the battery cells and the thermal resistance between the exterior of the battery pack  10  and the battery cells. Since plastic, in particular ABS plastic, is not a good thermal insulator, the ambient temperature of between 132° C. and 137° C. quickly conducts through the plastic enclosure  20  and into the battery cells  22  that are in direct contact with the plastic enclosure  20 . Soon after, battery cells  22  that are not touching the plastic enclosure  20  (e.g. battery cells  22  that are surrounded by other battery cells  22 ) are heated by heat that is conducted through the battery cells  22  that are in contact the plastic enclosure  20  and internal air  24  that is also heated by the plastic enclosure  20 . Furthermore, the 132° C. to 137° C. ambient temperature heats the contacts and heat is conducted over the electrical conductors  14 / 16  and into several of the battery cells  22 . Since the thermal resistance of the battery pack  10  is low, the internal battery cells  22  approach the external temperature of 132° C. and 137° C. during the sterilization cycle. During standard sterilization cycles, the temperature of the battery cells  22  exceed maximum allowable battery cell temperatures, damaging or destroying some or all of the battery cells  22 . For example, most lithium ion battery cells  22  begin to deteriorate or are destroyed at 80° C. 
     Referring to  FIG. 2 , a perspective view of a battery pack  100  with insulated battery cells  22  will be described. The new battery packs  100  has a plastic enclosure  120 , made of any known plastic such as ABS, but preferably a heat resistant plastic such as Ultem from GE plastics. Within the plastic enclosure  120  are one or more battery cells  22  connected in series, parallel or series/parallel by interconnecting conductive paths  118 , typically flat metal sheets that are tack-welded to battery terminals. Any known or future battery chemistry is anticipated including, but not limited to, alkaline, lead acid, nickel cadmium, nickel metal hydride, lithium, lithium ion, mercury, lithium iron, etc. 
     One or more battery terminals  123  are connected to a power connection terminal  112  by wires  114 / 116  or other conductive paths for the delivery of power to a device and for the charging of the battery cells  22 . 
     The inside surfaces of the enclosure  120  are separated from the battery cells  22  by a heat insulating material  130 . The heat insulating material  130  replaces much of the air gap  24  of the prior art. When such a battery pack  100  is placed in a sterilizing chamber such as an Autoclave, external temperatures quickly rise to a temperature between 132° C. and 137° C. The ambient temperature of between 132° C. and 137° C. quickly conducts through the plastic enclosure  120  but is retarded from reaching the battery cells  22  by the insulating material  130 . The heat insulating material  130  greatly reduces conduction of heat into the battery cells  22 , thereby enabling long sterilization cycles at a temperature between 132° C. and 137° C. while maintaining battery cell  22  temperatures of well under 80° C. during the entire heat sterilization cycle. In testing, using the appropriate heat insulating material  130 , internal battery cell  22  temperature has been measured at a maximum of 62° C. The contacts still absorb heat and conduct the heat over the electrical conductors  114 / 116  and into several of the battery cells  22 , but this amount of heat is minimal compared to the heat conducted through the plastic enclosure  20  of the prior art. For example, by maintaining an internal battery cell  22  temperature of less than 62° C., most lithium ion battery cells  22  fully survive the standard sterilization process. 
     There are many known insulating materials  130  such as fiberglass, Styrofoam, wool, etc. Any such material is anticipated in the insulating layer  130 , but several materials provide excellent insulating properties in such tight spaces. In preferred embodiments, the insulating layer  130  is made of silica (silicon dioxide) or silica ceramic carbon nitride, either solid, woven, fibers or in other forms. This is the material used in the tiles that insulate the space shuttle during the high heat exposure during re-entry into the earth&#39;s atmosphere. 
     Another preferred material for the insulating layer  130  is Aerogel, preferably silica aerogel. Aerogels are good thermal insulators because they almost nullify the three methods of heat transfer (convection, conduction, and radiation). As for conductive insulators, Aerogels are composed almost entirely from a gas, and gases are very poor heat conductors. Silica aerogel is especially good because silica is also a poor conductor of heat. Because air cannot circulate through the lattice of aerogels, aerogels are very good convective insulators. Aerogels are good radiative insulator, especially when carbon is added because carbon absorbs the infrared radiation that transfers heat at standard temperatures. The most insulative aerogel is silica aerogel with carbon added to it. 
     Although not shown, for completeness, often such battery packs  100  include other devices such as electronic circuits that prevent over current, over voltage, under voltage, control charging, prevent over-temperature situations during charging, etc. All such devices are known and present in some battery packs, but have been left out for clarity reasons. 
     Referring to  FIG. 3 , a perspective view of a battery pack  101  with insulated battery cells  22  with thermal conductive breakers  150  will be described. Although the battery pack  100  with the insulating layer  130  performs well in most heat sterilization procedures, some heat conducts from the contacts  12  and through the conductors  114 / 116  and into the battery cells  22 . To reduce the conducted heat, the battery pack  101  includes thermal breakers  150  that thermally disconnect the internal battery cells  22  from the terminals at a predetermined temperature. The thermal breakers  150 , for example, include bi-metallic strips  151  formed of a sandwich of two dissimilar metals that have different coefficients of expansion. At room temperature (e.g. less than 25° C.), both metals are of the same length. As the temperature of the bi-metallic strip  151  increases, for example, to 40° C., one of the metals expands more than the other, causing the bi-metallic strip  151  to bend outwardly away from the thermal breaker contacts  152 . Once the bi-metallic strip  151  bends away from the thermal breaker contacts, heat from the ambient is no longer conducted from the ambient, through the contacts  112  and through the conductors  114 / 116 , thereby reducing thermal buildup in the internal battery cells  22 . Such bi-metallic strips  151  are known for use in thermal cutoffs and circuit breakers. In this use, the bi-metallic strip  151  must carry sufficient current without heating to a point at which it opens the electrical contact between the bi-metallic strip  151  and the thermal breaker contacts  152 . 
     Referring to  FIG. 4 , a perspective view of a battery pack  102  with insulated battery cells  22  and with thermal resistance  160  will be described. In order to reduce and/or delay the conduction of heat from the terminals  112  through the conductors  114 / 116  and into the internal battery cells  22 , the path length of the conductors  114 / 116  is increased, thereby since thermal transfer is inversely proportional to length, thermal transfer from the terminals  112  through the conductors  114 / 116  and into the internal battery cells  22  is reduced. 
     Referring to  FIG. 5 , a perspective view of a battery pack  103  with insulated battery cells  22  with thermal heat absorbing mass  170  will be described. In order to reduce and/or delay the conduction of heat from the terminals  112  through the conductors  114 / 116  and into the internal battery cells  22 , a heat absorbing mass  170  is inserted in the path of the conductors  114 / 116 . Since any mass absorbs calories of heat to increase its internal heat before passing that heat on, the heat absorbing masses  170  perform as heat sinks or thermal capacitors, taking time to heat before passing heat through the conductors  114 / 116  and into the internal battery cells  22 . Once subject to high ambient heat, the terminals  112  heat rapidly and the heat begins to conduct into the thermal masses  170 . Since it takes time for the temperature of the thermal masses  170  to increase, proportional to their mass, thermal transfer from the terminals  112  through the conductors  114 / 116  and into the internal battery cells  22  is reduced. 
     It is anticipated that an improved battery pack  100  include the insulative layer  130  along with any or none of the improved terminal thermal management devices (thermal breaker  150 , thermal resistance  160  or thermal mass  170 ). 
     Equivalent elements can be substituted for the ones set forth above such that they perform in substantially the same manner in substantially the same way for achieving substantially the same result. 
     It is believed that the system and method as described and many of its attendant advantages will be understood by the foregoing description. It is also believed that it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely exemplary and explanatory embodiment thereof. It is the intention of the following claims to encompass and include such changes.