Abstract:
A zero gravity simulator rotates an electrolytic device, such as nickel hydrogen battery cells, to simulate the effect of zero gravity on the battery cells by providing a cumulative zero gravity vector upon the device under test. The simulator has electrical connections to the device under test for monitoring the electrical performance of the device for determining the expected performance of the device under test when deployed and used in space.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention was made with Government support under contract No. F04701-93-C-0094 by the Department of the Air Force. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of battery testing. More particularly the present invention relates to the testing of electrolytic battery cells used in zero gravity space environments. 
     BACKGROUND OF THE INVENTION 
     Nickel hydrogen and sealed nickel cadmium batteries have been used in spacecraft applications and have been designed to operate in an electrolyte starved configuration where there is no free liquid electrolyte within the cell. All of the electrolyte is contained by capillary forces within the pores of the electrodes, wall wick, and separators within the cell. This starved configuration simultaneously enables uniform transport of both gases and liquid electrolyte within the electrode stack and in the gas spaces surrounding the electrochemical components within the cell. An excessively starved cell will perform poorly because the separators having large pores will become dry, making the cell unable to support high rates of ionic current flow through the separators. 
     At the other design extreme, an excessively flooded configuration will not allow uniform transport of gases within the cell. For nickel hydrogen cells, the flooded configuration can result in two problems with gas transport. The first problem occurs when some areas of the hydrophobic side of the negative plates become flooded with electrolyte to limit the accessibility of hydrogen gas to the platinum catalyst in the negative electrode. The second problem occurs when free electrolyte is present in the regions through which oxygen must flow as the oxygen escapes the electrode stack during overcharge. Bubbles of high-pressure oxygen can accumulate in such regions of free electrolyte. These bubbles of oxygen, when contacting the platinum catalyst, can ignite to cause small explosive thermal popping events. Such popping events can occur either over the surface of the negative electrode, or at the edges of the negative electrodes where large amounts of oxygen can be channeled to the edges of the negatives. In the back-to-back stack design of large nickel hydrogen cells, popping at the edges of the plates is generally the more significant. Significant popping events can result in cell short circuits as a result of damage to the edges of the plates or separators. 
     Ground life-test cycling of nickel hydrogen cell designs can be very misleading in identifying popping problems during prospective spacecraft usage as a result of excessive electrolyte. Cells are typically tested in a vertical configuration that gravitationally drains all free electrolyte into a pool in the bottom of the cell case. Alternatively, testing cells on their sides has been found to also not represent the zero-gravity environment of space because the electrolyte tends to settle towards the downwards side of the electrode stack. Horizontal life testing has often led to early failures due to popping problems, although horizontal life testing also represents a worst-case stress condition for popping problems. Popping damage can lead to short circuits and failures of the battery cells. These and other disadvantages are solved or reduced using the invention. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to simulate the distribution of electrolyte in a nickel hydrogen cell operated in the zero-gravity environment of space. 
     Another object of the invention is to test nickel hydrogen battery cells containing different levels of electrolyte for susceptibility to popping damage during electrical cycling. 
     Another object of the invention is to provide a test system that simulates zero-gravity during cell operation with electrical charging cycling and monitoring to detect popping events during operation. 
     The present invention is directed to simulating zero-gravity during the operation of devices, such as battery cells, which contain both fluids and gases that can redistribute to affect performance in the presence of the gravitational field of the earth. The simulation of zero gravity leads to ground test results that are equated to performance expectation of the cells operating in a spacecraft in space. The test results are particularly useful in predicting the performance of nickel hydrogen battery cells that depend on controlled movement of both gas and liquid electrolyte for proper performance. The system provides simulated zero-gravity performance at a small fraction of the cost and time required carrying out a space experiment. The system simulates zero-gravity by time-averaging the gravitational field of the earth to zero on a time scale consistent with the rate of movement of fluids within the device. This test is done rotating the device under test in a rotating test fixture for a simulated zero-gravity life test for electrolytic battery cells, such as nickel hydrogen battery cells, containing differing levels of liquid electrolyte. The system provides simulated results consistent with the performance of these battery cells when in space. The system can rotate large battery cells that are horizontally positioned and then rotated at a predetermined rate, such as at one revolution every minute, which rotational rate is consistent with the rate at which electrolyte moves through the electrode stack within these nickel hydrogen battery cells. The system can be operated continuously for many months within an environmentally controlled chamber at a controlled temperature. The system can effectively provide for any constant or variable temperature profile within a range of temperatures. The system has the ability to simulate the zero gravity environment of space quickly and cheaply for a wide range of devices containing materials or fluids that respond to gravitational forces on time scales of seconds to minutes. The system can detect failure events and can be used to model the expected performance of the battery cells operated in space. These and other advantages will become more apparent from the following detailed description of the preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a zero gravity simulator. 
     FIG. 2 depicts a stack of nickel hydrogen battery cells disposed in a rotating structure. 
     FIG. 3 depicts the core within a battery cell stack. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the invention is described with reference to the figures using reference designations as shown in the figures. Referring to FIG. 1, a zero gravity simulator includes a temperature chamber  10  having a temperature controller  12 , cooler  14 , heater  16 , fan  18  and temperature sensor  20  for controlling and monitoring the temperature within the chamber  10 . A plurality of battery cells  22   a ,  22   b ,  22   c , and  22   d  are affixed to a rotator  24  that is rotated by an AC motor  26 . AC power  27  is distributed to a control unit  28  for controlling the motor  26  for rotating for rotator  24  at a predetermined number of revolutions per minute rate. The rotator  24  is rotated about a horizontal axis relative to earth gravity. The rotator  24  includes two end plates  30   a  and  30   b  respectively attached to left and right axle portions  32   a  and  32   b , that in turn rotate within and are supported by left and right stand portions  34   a  and  34   b , respectively. The left and right stand portions  34   a  and  34   b  are supported by a base  36  within the temperature chamber  10 . Electrical power and monitoring wires  38  are connected to each of the battery cells  22   abcd . As shown, four battery cells  22   abcd  are supported in the rotator  24 . The rotator  24  is rotated at a predetermined rate. A tab  40  and tab sensor  42  are used to monitor and control the rotational rate. The rotator  24  is rotated using a drive chain  44  coupling the rotator  24  to the AC motor  26 . A hanger  46  is supported by a hanger stand  48  and is used to support the wires  38  connected to a data acquisition system including a computer  50 , multiplexer voltmeter  52 , power supply  54 , audio interface  56  and thermal coupler interface  58 . The data acquisition system controls and monitors the battery cells  22   abcd  during testing. 
     Referring to all of the figures, and more particularly to FIGS. 2 and 3, the wires  38  include +/− power lines  60   a  and  60   b , respectively, +/− voltage sense lines  62   a  and  62   b , respectively, an audio sense line  64  and a temperature sense line  66 . The +/− power lines  60   ab  provide power to the battery cell  22  and the +/− voltage sense lines  62   ab  provide sensing voltages of the battery cell  22 . The audio sense line  64  is connected to an audio sensor  68  attached to a battery cell pressure vessel  70  for sensing audio popping sounds within the battery cell  22 . The temperature sense line  66  is connected to a thermocouple  72  that is also attached to the battery cell pressure vessel  70  for sensing the temperature of the battery cell  22 . The battery cell  22  is constructed as a stack of discs  74 . The discs  74  include alternating anode and cathode metal electrodes disc  74   a  and  74   b , and separator disc  74   c . The insulating circular disc shaped separator  74   c  is disposed between each pair of electrodes  74   a  and  74   b . A center weld disc  74   d  is disposed in the center of the stack of discs  74 . The electrodes  74   ab , separators  74   c  and weld plate  74   d  are circumferentially aligned as the stack  74  within the pressure vessel  70  around a stack core  76 . The discs  74  are abutted together around the stack core  76  using end caps  78   a  and  78   b . The stack core  76  has a center grooved rod  80 , that is mated to protruding contacts  82   a  and  82   b  of the alternating electrodes  74   a  and  74   b  extending inwardly into the stack core  76 . The stack core  76  serves to align the circular shaped battery electrodes  74   ab , separators  74   c  and weld plate  74   c  along the length of the pressure vessel  70 . The +/− power lines  60   ab  are routed through sealed openings  86   a  and  86   b  to the electrodes  74   ab  using spotwelded contact points  84   a  and  84   b  disposed within the contacts  82   a  and  82   b , respectively within grooves of the stack core rod  80 . The cathode and anode electrodes  74   ab  of the battery cell  22  are electrically connected together using the +/− power lines  60   ab . The center weld plated  74   d  is affixed to the temperature pressure vessel  70 . The center weld plate  70  has a diameter of the pressure vessel  70 . The electrode discs  74   ab  and separators  74   c  have a diameter less than the weld plate  74   d  so that the weld plate  74   d  serves to suspend and electrically isolate the electrodes  74   ab  within the pressure vessel  70 . The weld plate  74   d  is affixed to the stack core  76  between two sets of battery electrodes and serves to rigidly suspend the stack of battery electrodes  74   ab  within the pressure vessel  70 . The pressure vessel  70  is a hydrogen gas reservoir. The oval shaped, but substantially cylindrical pressure vessel  70  is rigidly affixed to and disposed within a cylindrical temperature coffin  88  having openings  90   a  and  90   b  for communicating wires  60   ab ,  62   ab ,  64  and  66 . The coffin  88  provides thermal heat coupling to the battery electrodes  74   ab  during testing. Lastly, the electrolyte  92  is disposed in a predetermined quantity within the pressure vessel  70 . The battery cell  22  is of a conventional pressure vessel design, but other types of electrolytic battery cells may be tested. 
     In operation, the zero gravity simulator is used to evaluate the upper threshold for electrolyte fill quantity in the cell design, that may be a 350 Ah cell design. The fill quantity is consistent with minimal risk of cell short circuits due to popping damage. During testing, electrolyte fill quantities are preferably correlated to temperature cycling and popping damage. 
     An initial electrolyte fill amount is the amount added at cell activation. The initial fill level tends to leave a pool of free electrolyte of about 100 cc in the pressure vessel  70 . A final fill amount is the amount remaining in the stack after all free electrolyte is drained. The system for simulating zero-gravity is to time average the gravitational field vector to zero within each cell. This test effectively simulates the zero-gravity environment in terms of producing no gravitational orientationally preferred region in which free electrolyte may pool. During testing, the cells  22  are continuously rotated while held in a horizontal position with the discs  74  being held in a vertical position within the cells  22 . The rotation period of the cells can vary, but will typically remain constant during each test. A one minute rotation rate is appropriate for most large nickel hydrogen battery cells. The cells are held in the rotator  24  in the temperature chamber  10  at a suitable temperature, for example, ten degrees Celsius. 
     Each cell  22   abcd  is be instrumented using the voltage sensing lines  62   ab , the temperature lines line  66  connected to the thermistor  72 , and the audio sense line  64  connected to the acoustic pickup  68 . The acoustic pickup  68  is monitored by the transient data acquisition system that is periodically triggered to record a transient over a predetermined duration and is used to detect audio frequency popping events. Each transient recording has a predetermined duration, for example, two seconds, with a predetermined number of data points, for example, one thousand data points, that are saved during the transient recording period for each of the signals being monitored. The transient recording occurs at spaced time intervals, for example, at two millisecond intervals. A minimum number of transients, for example, twenty-four transients, are recorded during each charge and discharge cycle, with additional transients being recorded when particularly significant audio frequency events were detected. An average number of about 48 total transients are typically recorded during each charging cycle. 
     The thermistor  68  is imbedded in an aluminum or carbon composite sleeve, not shown, and disposed in direct contact with the inconel pressure vessel  70 . The thermistor  68  may be mounted near the center of the electrode stack  74   abcd . A pair of thermistors  72  may be disposed on the vessel  70 , one on each side of each cell  22 , for improved temperature sensing. The thermistor  72  mounting area may be covered with a one square inch piece of insulating foam, not shown, to isolate the thermistor  72  from the chilled air in the temperature chamber  10 . The thermistor  72  detects any thermal transients that could be due to popping events. Such thermal transients are expected to have a duration on the order of seconds. 
     The four cells are operated for many charge and discharge cycles, for example 200, in the simulated zero-gravity environment over a long, for example twelve hour, electrical cycle to obtain an 80% depth-of-discharge and a 120% charge return. After testing, the cells are subjected to a charge retention test in the zero-gravity environment at 10 degrees C. Following the electrical testing, the cells are disassembled and inspected for signs of degradation or unusual changes. The components are inspected for indications of popping damage that is typically manifested as melted regions of gas screen, holes in the platinum black catalyst on the negative plates, and in severe cases loss of separator and fragmentation of the positive plate structure. During testing, the popping events can be detected by audio frequency measurements and with sufficient data averaging over long-term cycling, a semi-quantitative measure of popping susceptibility for nickel hydrogen cells can be obtained for a given level of electrolyte. From the audio noise analysis, the popping is much more severe in cells that have higher amounts of electrolyte. 
     The zero-gravity testing causes distribution of electrolyte to be uniform through the length of the stack. The zero-gravity simulation is designed to maintain a uniform distribution in and around the cell stack, while not allowing any excess electrolyte to pool in the domes of the cells. The acoustic sensors detect significantly greater noise levels during overcharge in the two cells that have higher amounts of electrolyte, a result of popping events in these cells. The zero gravity simulator is used to test the operation of the cells and to prevent pooling of the electrolyte. During testing, the cells are charged and discharged and then inspected for damage. Those with damage due to popping are likely to fail when in space at the predetermined level of electrolyte. As such, the zero gravity simulator is used to test battery cells at a predetermined amount of electrolyte, or any device containing a liquid that is intended to be operated in space. Those skilled in the art can make enhancements, improvements, and modifications to the invention, and these enhancements, improvements, and modifications may nonetheless fall within the spirit and scope of the following claims.