Abstract:
A modular battery includes a housing, a first battery cell having a first electrode surface, a second battery cell having a second electrode surface, and a pressurizable bladder forcing the first battery cell against the second battery cell.

Description:
BACKGROUND 
     Modular batteries are batteries which comprise two or more battery cells or cell modules or cells. A common example of a device using a modular battery is a hand held flashlight which may use for example two C cells. 
     Recently, modular batteries have become important in many applications, including hybrid electric vehicles (“HEV”), plug-in hybrid electric vehicles (“PHEV”), and other applications. When used in HEV, PHEV, and other applications, in addition to being durable, safe and cost effective, modular batteries are required to deliver a great deal of power. 
     Applications of modular batteries, like the hand-held flashlight, require the use of multiple battery cells connected in series. However, the modular batteries for HEVs and PHEVs, for example, may differ from the modular C cells used in a common flashlight. 
     U.S. Patent Publication No. 2009-0239130 A1 discloses a modular battery with interconnectors, and is hereby incorporated by reference herein. 
     SUMMARY OF THE INVENTION 
     The present invention provides a modular battery comprising a housing, a first battery cell having a first electrode surface, a second battery cell having a second electrode surface, and a pressurizable bladder forcing the first battery cell against the second battery cell. 
     The present invention also provides a method for forming a modular battery comprising: placing a first battery cell having a first electrode surface in a housing, placing a second battery cell having a second electrode surface in the housing, placing a bladder over the second battery cell, and pressurizing the bladder to force the second battery cell against the first battery cell. 
     The present invention also provides a method for operating a modular battery comprising: inflating a bladder to a predetermined pressure, the bladder forcing a first battery cell against a second battery cell; and monitoring the bladder pressure during operation of the modular battery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described with respect to a preferred embodiment, in which: 
         FIG. 1  schematically illustrates a cross-section of an embodiment of the present invention with a modular battery having six cell modules within an enclosure with feedthroughs; 
         FIG. 2  shows a plan view of the  FIG. 1  embodiment; and 
         FIG. 3  shows an alternate embodiment of the present invention. 
     
    
    
     The drawings are schematic in nature and not to scale. For clarity and ease of understanding, some elements have been exaggerated in size. 
     DETAILED DESCRIPTION 
     In order to be powerful enough for HEVs, PHEVs, and other applications, it is desirable to use modular batteries containing cells with a high surface to volume ratio, for example using a planar design for each cell of the battery. These cells may be, for example, about the size of a large book wherein the “front” of the book contains, for example, a positive terminal (also known as an electrode) and the “back” of the book contains, for example, a negative terminal. Unlike their cylindrical counterparts (e.g., C cell batteries) which use a raised dimple at one end of a cell to make electrical contact with the next cylindrical cell, substantially planar cells need not have such raised dimple(s). 
     For many applications requiring high electrical power including HEVs and PHEVs, it is desirable that the battery delivers electrical power at a high voltage in order to reduce the required current needed to supply the electrical power which in turn will beneficially reduce the need for high-current carrying materials to the devices using the electrical power. Electrical power is the multiple of voltage and current and high voltage delivery of electrical power to a device, for example an electric motor, will require thinner or less conductive current carriers (for example copper wire) to the device which will reduce their cost. Electric vehicles for example may require a battery to provide electrical power at 300 to 600 volts. This high voltage is typically achieved by externally connecting multiple lower voltage battery modules electrically in series. This is in part due to safety considerations in assembling and operating a series connected “stack” of typical “pouch” cells within a battery module, since at higher voltages and especially above approximately 60 Volts, there is a significant risk of electrical arcing and a severe shock hazard since the edge peripheries of “flat” cells such as typical “pouch” cells have their cell terminals exposed. For safety these cell terminals are connected electrically in series within a low voltage battery module, for example, having less than 60 volts. 
     An object of the present invention is to provide sufficient pressure to ensure that battery modules retain good electrical contact. Another alternate or additional object is to provide and/or use information on the pressure within the modular battery. 
     The present invention may be used with the modular battery disclosed in incorporated-by-reference U.S. Patent Publication No. 2009-0239130 A1. 
       FIG. 1  shows six cell modules  23  stacked one on another electrically in series and separated by the compressible interconnectors  24  which serve to electrically connect in series one cell module to the next cell module. Details of the cell modules are found in U.S. Patent Publication No. 2009-0239130 A1, although it is noted that other modules may be used in accordance with the present invention. Several interconnectors  24  can be present between two cell modules, for example 8 layers, each 10 mils in thickness. Thus the space between cell modules for example can be 80 mils, and compressible to 60 mils when in use. 
     For the lowest electrical resistance between cell modules  23  in the battery stack, pressure should be applied to the interconnectors  24  between the cell modules  23 . For other modular batteries pressure between the modules also may be important, even if no interconnectors are present. The modules  23  and interconnectors  24  are placed one after another in an enclosure  25 , which may have an interior electrically insulating material  25   a . Each module  23  may have a port  20  for an electrolyte, electrical feedthroughs  21  and burst disc  22  for pressure relief in the module  23 . 
     Multipin connectors  30 ,  31  can be provided for each of the cell modules  23 , as shown for example in  FIG. 2 . These can be connected to the individual cell modules  23  via sense lines  30   c ,  31   c  and as described in incorporated-by-reference U.S. Patent Publication No. 2009-0239130 A1. 
     A positive electrical bus  28  can be placed over the last cell module  23  in the enclosure, and connected to a positive power terminal  26 . A negative power terminal  27  can be connected via the electrically-conductive enclosure  25  to the bottom interconnector  24 , the enclosure  25  thus functioning as a negative bus  29 . 
     A burst disc  32  can be included in the enclosure  25 , to provide relief if pressure within enclosure  25  becomes too great. 
     More than 50 cell modules may be placed in the enclosure  25 , and preferably at least 20. 
     As shown in  FIG. 1 , the present invention provides an expandable bladder  33 , with a gas port  34  to allow gas entry. The bladder is sealed into the top of the enclosure  25  such that the bladder  33  is positioned between the positive electrical bus  28  and the top of the enclosure  25  and is pressurized by applying gas pressure through the gas port  34 . The expansion of the bladder  33  upon pressurizing the bladder with a suitable gas transmits pressure to the interconnectors  24  within the stack of cell modules  23 . 
     The gas may be air, or an inert gas, for example. 
     By pressurizing to a predetermined level via for example a detachable pressure source  110  such as a compressor, and then sealing the gas port  34 , consistent pressures can be applied from one battery to another during manufacturing. The actual pressure will depend on characteristics of the battery itself, and the desired pressure between the modules  23 , but pressures of up to 2 atmospheres or more may be used. 
     The bladder  33  may be made for example of rubber or other expandable material, and may include a strengthened area at its outer periphery, for example made of metal, so that the bladder expands mainly in the direction of the stack of modules. 
     A planar surface of the bottom of the bladder  33  preferably is at least 50% as large as a planar surface of the battery module  23 , and the planar surfaces of both the bladder and the modules  23  preferably are axially aligned so that the pressure from the bladder  33  is even about a planar center of the battery modules, or the axial center of the enclosure  25 . Port  34  preferably is also coaxial with the axial center of the enclosure  25 . 
     The present invention also provides a pressure sensor  112  for the bladder  33 , the sensor  112  continuously monitoring the pressure in the bladder  33  even after port  34  is sealed. Signals from the pressure sensor  112  can be sent to a controller  100 , for example a microprocessor or ASIC. Controller  100  and sensor  112  may be powered by their own power source, such as a separate battery, or by the battery module. 
     By continuously monitoring the pressure in the bladder  33  during service, additional battery safety information can be logged on a continuing basis as part of the overall battery communications and control system, of which controller  100  is part. Although a burst disc  32  is included in the enclosure  25 , upward trends in battery internal pressure could be an early indicator of catastrophic failure, possibly independent of voltage and temperature trends. For instance, undesirable gas generation within the cell modules  23  gradually increases back pressure on the bladder as the gas accumulates and provides an early warning which temperature and voltage monitoring may not provide since neither are a cumulative measure but are instantaneous measurements. 
     The controller  100  thus can be given one or more setpoint pressure values, and take action as the setpoint values are reached. For example, there can be a filling setpoint value which, when reached during pressurization by pressure source  110  can automatically activate a valve to seal port  34 , or indicate to operating personnel that port  34  could be sealed. A further setpoint value could be an error pressure, so that is a pressure in the bladder  33  falls below the error pressure, a warning is provided. Since pressure may vary during operation the battery, for example, in a vehicle due to road conditions, the controller  100  could also issue the warning solely if the pressure falls below the error pressure for a certain period of time. Other data, such as the rate of fall of the pressure with respect to time, dP/dt, or the acceleration of the pressure drop could be measured as well, and depending on the battery use, used to indicate an error or other warnings. The sensors or controller also could be operated in conjunction with other battery management systems or integrated with a central vehicle control system, 
       FIG. 3  shows an alternate embodiment of the battery of the present invention in which the interconnectors  24  do not touch the sides of the enclosure  25 , but the electrically-insulating sides of the modules  23  do. 
     It will be appreciated by those ordinarily skilled in the art that obvious variations and changes can be made to the examples and embodiments described in the foregoing description without departing from the broad inventive concept thereof. It is understood, therefore, that this disclosure is not limited to the particular examples and embodiments disclosed, but is intended to cover all obvious modifications thereof which are within the scope and the spirit of the disclosure as defined by the appended claims.