Gopalakrishna M. Rao

Effect of KOH concentration and anions on the performance of an NiH2 battery positive plate

Abstract The capacity and voltage behavior of electrochemically impregnated sintered nickel positive plates was examined by galvanostatic charging and discharging in a flooded electrolyte cell. Three different concentrations of potassium hydroxide (KOH) (40%,31% and 26%) and 31% KOH containing dissolved nitrate, sulfate, or silicate were investigated. The end-of-charge voltage at C 10 charge and at 10°C showed the following order: 40% KOH > 31 % KOH alone, and in the presence of the anions > 26% KOH. The mid-discharge voltage at C 2 discharge was higher in 26% KOH, almost the same for 31%Ao KOH with and without the added contaminants, and much lower for 40% KOH. The plate capacity was marginally affected by cycling in all cases except for 40% KOH, where the capacity declined after 1000 cycles at 80% depth-of-discharge (DOD). At the end of cycling all the plates tested experienced a weight loss, except in the case of 31% KOH, as a result of active material extrusion. Cyclic voltammetry of miniature electrodes in 31% KOH showed that the cathodic peak potentials are less polarized at −5 °C (compared to 25 °C) in the presence and absence of silicate. This indicates a slightly higher voltage during discharge in an NiH2 battery. Furthermore, the features of the current-potential profile were practically unchanged in the presence of silicate.

NiH/sub 2/ reliability impact upon Hubble Space Telescope battery replacement

The NASA Hubble Space Telescope (HST) was designed to be deployed and later serviced for maintenance and upgrades, as required, by the space shuttle fleet, with a 5-year mission life for the batteries. HST was deployed 380 miles above the Earth, from Space Shuttle Discovery, on April 25, 1990. Four servicing missions, (SM1, SM2, SM3A, AND SM3B) have been performed. Astronauts have replaced or modified optics, solar arrays, a power control unit, and various science packages. A fifth servicing mission, SM4 scheduled for early 2004, is planned to replace the batteries for the first time. The HST is powered by solar array wings and nickel hydrogen (NiH/sub 2/) 22-cell batteries, which are grouped into two parallel battery modules of three parallel batteries each. With a design life of 7 years at launch, these batteries have surpassed 12 years in orbit, which gives HST the highest number of charge/discharge cycles of any NiH/sub 2/ battery currently in low earth orbit (LEO) application. Being in a LEO orbit, HST has a 36-minute umbra period, during which spacecraft power requirements normally force the batteries into discharge, and a 60-minute sun period, which is available for battery recharge. The intent of this paper is to address the issue of NiH/sub 2/ battery reliability and how battery capacity degradation can impact scheduling of a servicing mission to bring replacement batteries to HST, and extend mission life until deployment of Next Generation Space Telescope (NGST), planned for 2010 at the earliest.

Electrical and thermal characteristics of lithium-ion cells

The 18650-type lithium-ion cells are characterized by a cell resistance of 130 m/spl Omega/ capacity of 1.27 Ah at 25/spl deg/C, and a mid-discharge voltage of 3.6 V. The capacity loss in the 72-hr stand test was 3.39 percent. The heat dissipation properties were determined using a radiative calorimeter. During charge, initial endothermic cooling and subsequent exothermic heating beyond 55-percent state of charge were observed. At C/2 rate of discharge (which is considered medium rate), the heat dissipated was 17 mW/cm/sup 3/. The heat dissipation profile during discharge is also unique in the presence of a minimum that is different from that observed for Ni-Cd, Ni-MH, and Ni-H/sub 2/ cells.

Voltage and capacity stability of the Hubble telescope nickel-hydrogen battery

Abstract The power system of the Hubble Space Telescope includes two orbital replacement units, each containing three nickel-hydrogen (NiH2) batteries of 88 Ah capacity. Since launch in April 1990, the batteries have completed 23 000 charge and discharge cycles and continue to meet the power demands of the satellite. The voltage, capacity, and pressure characteristics of all six batteries were analyzed to determine the state of health of the battery and to identify any signs of performance degradation. The battery pressures have changed to varying degrees. The end-of-charge pressure for battery 4 increased by 96 psi, while that for battery 3 decreased by 37 psi. The voltages of the individual cells show a decay rate of 0.69 mV per 1000 cycles, and the capacity of the batteries has apparently decreased, possibly due to the system being operated at a lower stage of charge. Autonomous battery operation involving charge termination at a preselected voltage continues to restore the energy dissipated during each orbit. The accumulated data on voltages and recharge ratios can be used to design new temperature-compensated voltage levels for similar missions that employ NiH2 batteries.

Effect of storage on performance of SUPER nickel-cadmium cells

A study was undertaken to examine the capacity maintenance features of SUPER nickel-cadmium cells when stored for extended periods to determine whether the features change when the same kind of positive plates as that used in nickel-hydrogen cells are used. The cells maintained their capacity when stored at 0/spl deg/C in the discharged state and at 0/spl deg/C in the charged state by continuously trickle charging. There was a capacity loss when stored in the open-circuit condition at 28/spl deg/C. A cycling test at 17 percent depth of discharge for 2400 cycles using cells stored at various conditions showed that cells maintained good end of discharge voltage regardless of their storage history. However, the EOD voltages of stored cells were lower by 10 mV compared to those of fresh cells. The capacity at the end of the cycling test decreased for the stored cells by 2-7 Ah. The storage related capacity loss is lower for SUPER Ni-Cd cells compared to that of Ni-H/sub 2/ cells containing a hydrogen precharge. The results suggest the pivotal role of hydrogen pressure in the capacity loss phenomenon.

Orbital performance of AMPTE/CCE nickel-cadmium batteries with a large number of charger-induced, very small, charge/discharge cycles

Abstract The AMPTE/CCE spacecraft used switching regulators to short-circuit the excess solar array current and control the charge to each of its two 28 V, 4 Ah nickel-cadmium batteries. This digital regulation of the battery overcharge current caused approximately 10 × 10 6 , very small battery charge/discharge cycles during the long sunlight periods of the highly elliptical, 24 h orbit; a unique system influence that did not significantly degrade battery performance during the mission life. The occurence of three very long eclipses at the beginning, middle and end of the mission, provided a rare opportunity to analyze the performance of the batteries for the mission life. This paper presents the power system design, and the battery performance for about five years of mission life.

Evaluation of Earth observing system AM-1 battery cell bypass protection

The Earth Observing System (EOS) AM-1 spacecraft, one in a series of Mission To Planet Earth (MTPE) low Earth orbit (LEO) spacecraft, is designed for a lifetime of 5 years. The spacecraft bus design incorporates cell bypass circuitry, similar to that tested in the two onboard batteries. After initial measurements of the resistance in the bypass circuit design for the batteries, a concern regarding the effects of an inadvertent bypass switch closing was identified. Investigation of the magnitude of damage that would result from a bypass switch closing across a fully charged Ni-H/sub 2/ cell was initiated after the short circuit current values were calculated based on measured resistance data. The calculated short circuit current was identified as being higher than specification values for the bypass switch. In order to evaluate the cell bypass circuitry design, a 50 amp-hr Ni-H/sub 2/ cell was outfitted with an EOS AM-I bypass harness, fully charged, and then subjected to a simulated inadvertent switch closing. Results of the first test indicate that the bypass circuit harness design permits short circuit discharge currents in excess of 600 amperes peak and 400 amps continuously for over 2 minutes. Other observations included the loss of the bypass harness as well as a temperature rise on the battery cell dome to almost 100/spl deg/C. The post performance evaluation of the test cell together with recommendations for possible modification of the bypass circuit design for EOS AM-1 spacecraft batteries to increase the probability of post failure survival are presented in this paper.

AMPTE/CCE battery and charger performance

Switching regulators were used to short-circuit the excess solar array current and control the charge to each of its two 28-volt, 4 ampere-hour (Ah) nickel-cadmium batteries. This caused approximately 10-million, very small battery charge-discharge cycles during the long sunlight periods of the highly elliptical, 24-hour orbit; a unique system influence that did not significantly degrade battery performance during the nearly 5-year mission life. Three very long eclipses at the beginning, middle and end of the mission, provided a unique opportunity to analyze the performance of the batteries.

Reactions during reversal of an Ni-H2 cell

Abstract The chemical reactions that occur during reversal of aerospace-design nickel-hydrogen (Ni-H 2 ) cells are examined by determining voltage changes and heat dissipation. Radiative calorimetry is used to measure the rate of heat dissipation during charge, discharge, and reversal. The heat dissipated during reversal at C/10 rate (8.1 A) for a positive precharge cell is four times greater than that for a hydrogen precharge cell. For a cell design with a positive precharge, the reversal reactions consist of completion of nickel electrode discharge and hydrogen evolution on the nickel (positive) electrode, platinum oxide/ hydroxide formation, and subsequent reduction by hydrogen at the hydrogen (negative) electrode.

Performance comparison between Ni-H/sub 2/ dry sinter and slurry electrode cells

The electrical and thermal performance of dry sinter and slurry process electrode cells manufactured for the Hubble Space Telescope (HST) batteries have been characterized for a matrix of operating conditions over the temperature range from 14 to 86/spl deg/F at various charge control levels. The dry sinter process electrode cells tested are similar to the onboard HST Ni-H/sub 2/, cells. The slurry process electrode cells were developed to be less susceptible to electrode expansion and impedance changes with life. Both cell types were impregnated by the aqueous electrochemical process. Test conditions included standard capacity tests and electrical cycling using 96-minute cycling regimens incorporating constant depth-of-discharge (D0D) cycles. The dry sinter process electrodes have higher operating capacities to 1.20 V/cell, but both electrode types have similar heat dissipation for the conditions tested. The results of the testing included cyclic heat generation during a typical 96-minute cycle, operating capacity data vs. cutoff voltage to generate a temperature-compensated voltage curve, and voltage characteristics suitable to develop a voltage prediction model. Analysis of data shows differences in the discharge voltage plateaus at all temperatures for the operating conditions evaluated. This is the basis for recommended changes in the battery charge control.