Hubble Space Telescope Nickel-Hydrogen Battery and Cell Testing - An Update 929089
Nickel-hydrogen (Ni-H2) technology has only recently been utilized in low earth orbit (LEO) applications. The Hubble Space Telescope (HST) program, over the past five years, played a key role in developing this application. The HST not only became the first reported, nonexperimental program to fly Ni-H2 batteries in a LEO application, but funded numerous, ongoing tests that served to validate this usage.
The Marshall Space Flight Center (MSFC) has been testing HST Ni-H2 batteries and cells for over three years. The major tests include a 6-battery system (SBS) test and a single 22-cell battery (FSB) test. The SBS test has been operating for 34 months and completed approximately 15,200 cycles. The performance of the cells and batteries in this test is nominal. Currently, the batteries are operating at an average end-of-charge (EOC) pressure that indicates an average capacity of approximately 79 ampere-hours (Ah). The watt-hour (Wh) efficiency is averaging about 82 percent, and the end-of-discharge (EOD) voltage is remaining stable at 1.31 volts (V)/cell. The batteries have been operating for nearly 15 months since the last capacity check was run and plans are to continue cycling without a capacity check for at least another 3 months. The FSB test has been operating for nearly 33 months and completed approximately 14,700 cycles. The performance of the cells in this battery test is also nominal. Currently, the battery is operating at an average EOC pressure that indicates a capacity of approximately 79 Ah. The Wh efficiency is averaging about 82 percent, and the EOD voltage is remaining stable at 1.32 V/cell. This battery has also been operating for nearly 15 months since the last capacity check was run and plans are to continue cycling just as the SBS test for at least another 3 months before a capacity check is considered.
Two special tests were conducted on the SBS and FSB over the past year. The first test required both the SBS and the FSB to be placed in a step-taper charge mode for approximately 4 months to evaluate the effects of operating the batteries in this less-efficient charge mode for an extended period of time. This test was developed in response to a possible need to free additional memory in the HST on-board computer. The electrical power system (EPS) could contribute to this end by eliminating its software-control (step-to-trickle) charge mode capability which requires significant computer memory. The results of the test showed little effect on the overall performance of the system when changed to a step-taper charge mode as long as the V/T level (temperature-compensated charge voltage cutoff curves) was lowered in conjunction with the charge mode change. The second test was an attempt to duplicate an anomaly observed on the HST EPS in which telemetry indicated 10 minutes into trickle charge a load current increase of approximately 20 amperes (A) resulting from an apparent temporary short to structure somewhere between the +CC solar panel assemblies (SPA) and the battery 1 positive lead. The current was high enough to lower the battery voltages to a point that the charge control relays closed, thus placing the batteries in a momentary high-rate charge mode. The test showed a current of approximately 70 A for less than 250 milliseconds would create such a voltage drop on the batteries. Because the telemetry rate is only two samples per second, a short pulse of this magnitude is possible and is the likely scenario for what occurred on the HST.