Why are batteries necessary?
The battery is an indispensable component for operation of space vehicles including satellites. Satellites can generate electric power using solar cells while they are in the sun and receiving solar light. In the shade with no solar light, however, satellite operation is maintained by electric power stored in a battery. The battery is composed of cells. Since a single cell cannot provide enough voltage and capacity, multiple cells are connected in series (voltage rises) or in parallel (capacitance increases) to form a battery.
For example, the power required for a 4ton-class earth-orbiting satellite is around 3kW. In this case, electric power is generated by solar cells with a capability of about 6 to 7kW, and surplus power is stored in a battery. In order to reduce spacecraft weight, it is essential to reduce the cell mass. To this end, the Japan Aerospace Exploration Agency (JAXA) is actively engaged in research and development of cells. Technologies developed for communication systems, computers and automobiles have been introduced. Satellites require great power, 10 to 100 times than that of batteries used in computers, etc. Further, since it is difficult to replace batteries in space orbit, high reliability is required.
In this article, I will introduce space-application cells and their histories.
History of space-application cells
From the beginning of mankind’s space quest, securing electric power has always been a big issue. In the movie “Apollo 13,” there is a scene where an electrical engineer says, “Power is everything!” An accident had affected the power supply and endangered the astronauts’ return to earth. The electrical engineer delivers this phrase to convince all the staff who were involved with the accident. Without electric power, the spacecraft would not operate nor could people survive.
Batteries have a long history. At the end of the 1700s, as modern chemistry made progress, Galvani described the principle of the battery. In the 1800s, a variety of cells were devised, including the voltaic cell. It is believed that the cell concept, “electromotive force is generated by placing two different materials (initially, metals) opposite each other and separating the two with an electrolyte,” was established at this time. In fact, however, the origin of the cell is much older. A type of cell was discovered from ancient ruins (around B.C. 200 to A.D 200) in Iraq. It is called the “Baghdad battery.” A copper tube is placed inside a clay vase and an iron core is put into the center of the tube. It is assumed that vinegar or wine was used as an electrolyte to make a battery cell.
In the cell’s long history, people became aware that some types of cells could be charged many times. A rechargeable cell is called a secondary cell while a non-rechargeable cell is called a primary cell. Among the rechargeable cells, the nickel-cadmium (so-called ni-cad) cell invented by Jungner in Sweden in 1899 was a relatively light secondary cell that found wide application in hand-held appliances. For example, the cells were used in the headlamps on miners’ helmets or in portable wireless equipment. Nickel oxyhydroxide is used for the positive electrode and cadmium hydroxide is used for the negative electrode. A high-density potassium hydroxide solution is employed as an electrolyte. The ni-cad cell requires some skill to be used properly. For example, it has a memory effect, in that its apparent capacitance degrades when it is repeatedly charged and discharged incompletely. The cell, however, has been fully examined over a long time. Because of its long history, we have mastered the skill to use the cell well. In addition, since the ni-cad cell can be used in a sealing condition, it is the major cell for satellites and rockets even today.
The nickel-metal-hydride cell was developed as part of the pursuit of lightweight cells. It replaces the negative electrode material of the ni-cad cell with a hydrogen-storing alloy, Metal Hydride. Since the cell can be smaller and lighter than the ni-cad cell, its use spread to early notebook computers and cellular phones. In Japan, the nickel-metal-hydride cell was adopted on the “Tsubasa,” Mission Demonstration Satellite (MDS), and will be used on the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) and scientific satellites “LUNAR-A” and “ASTRO-F.” For comparison, space-application ni-cad and nickel-metal-hydride cells with the same capacity are shown in Fig. 1.
In the nickel-metal-hydride cell, charge and discharge occur via hydrogen moving in and out of the negative electrode’s hydrogen-storing alloy. A high-pressure nickel-hydrogen cell with the hydrogen gas sealed directly in a pressure container was also developed. This high-pressure cell is used primarily for satellites and submarines, while other types of cells are sold in the commercial market. Fig. 2 shows an external view of the high-pressure type. This type hardly deteriorates with repeated charges/discharges and shows robustness against over-charging and over-discharging. In Japan, the high-pressure cell was first used on the Engineering Test Satellite “Kiku-VI” and thereafter installed on the satellites “Kakehashi” and “Kodama.”