Electricity: measuring and testing – Electrolyte properties – Using a battery testing device
Reexamination Certificate
2000-06-23
2001-04-17
Tso, Edward H. (Department: 2838)
Electricity: measuring and testing
Electrolyte properties
Using a battery testing device
Reexamination Certificate
active
06218843
ABSTRACT:
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.
REFERENCES:
patent: 5379657 (1995-01-01), Hasselman et al.
Matsumoto James H.
Prater Alonzo
Smith Dennis A.
Zimmerman Albert H.
Reid Derrick Michael
The Aerospace Corporation
Tso Edward H.
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