Replacing batteries in space

Function of the batteries
All electrical power on the ISS is generated through the station’s solar panels, which convert sunlight into electrical energy. However, during times when the ISS goes through “orbital night,” the solar panels can no longer produce energy. As such, it is necessary for the ISS to store energy in batteries, which it can then use to power its systems during periods of darkness. Every 1.5h the ISS makes an orbit around the earth, 45 minutes of which is in sunlight. During this period, the batteries are charged by the solar panels and the batteries are discharged while feeding the station’s loads during the 45-minute period of darkness per lane.

The nickel-hydrogen (Ni-H2) batteries
The ISS has eight separate power channels in total, with each channel having three batteries – although one battery is considered a “series” of two separate battery units connected together, which actually amounts to six batteries per channel, and thus 48 batteries on ISS in total. Each of the old batteries is of the nickel-hydrogen (Ni-H2) type, which have generally always been used in space applications because of their long life, as they can withstand a large number of discharge cycles without major deterioration. In addition, Ni-H2 batteries are not susceptible to overcharging and countercurrent, giving them good safety properties.

However, one disadvantage of Ni-H2 batteries is that they are prone to “battery memory,” where the battery can lose some of its capacity if it is not fully charged and discharged during each cycle. For this reason, regular “battery conditioning” is performed on the ISS to prevent battery memory. Each of the station’s Ni-H2 batteries consists 38 individual cells (76 cells per string of two batteries), with each cell consisting of a pressure vessel containing gaseous hydrogen stored to 1,200 psi, which is generated during the charging process itself. The oldest batteries at the station are now about 10 years old and are reaching the end of their design life.

Lithium-ion (Li-ion) batteries
This means that replacement batteries are needed to maintain the ISS until its current planned retirement date of 2024. However, Ni-H2 batteries are now considered old technology, as most of the station’s systems were designed in the late 1980s and early 1990s. The ISS program has therefore decided to modernize the station’s batteries during the replacement process by switching to modern lithium-ion (Li-ion) batteries. These battery types operate through lithium ions that move between electrodes during the charging process, rather than pressurized hydrogen gas as used in Ni-H2 batteries.

As a result, Li-ion batteries are much lighter and smaller than Ni-H2 batteries because they do not require pressure vessel containers to store hydrogen gas, which means that Li-ion batteries have a very high energy density compared to Ni-H2 batteries. This has many advantages for the ISS program, because it means that only a single Li-ion battery can replace the function of two of the previous Ni-H2 batteries. This means that only half the number of Li-ion batteries (24) are needed to replace all of the station’s Ni-H2 batteries (48), which also halves the number of launches required. Li-ion batteries are also not sensitive to battery memory, so there is no need to condition the battery. However, Li-ion batteries have some drawbacks, namely the fact that they are much more sensitive to overcharging, which must be prevented through battery management and protection systems. In addition, Li-ion batteries typically have a shorter life span than Ni-H2 batteries because they cannot endure as many charge/discharge cycles before experiencing noticeable degradation. However, the ISS Li-ion batteries are designed for 60,000 cycles and a lifetime of ten years. In addition, they will include cell balancing and adjustable charge voltage technology to maximize their lifetime.

Li-ion batteries have experienced notable problems in the past, in the form of overheating and “thermal runaway.” The Li-ion batteries that will be used on the ISS, while manufactured by the same company (GS Yuasa), were designed with lessons learned from the problems, and have passed space certification tests. In particular, ISS Li-ion batteries include two schemes against thermal runaway, voltage and temperature monitoring of individual cells, circuit protection and fault isolation of individual cells, and thermal heat barriers between cell packs.

In terms of construction, each ISS Li-ion battery contains 30 individual cells, packed in a box that retains the same dimensions and mounting interfaces as previous Ni-H2 batteries, but with a significantly reduced weight (430 pounds instead of 740 pounds). A single Li-ion battery replaces the functions of two Ni-H2 batteries, but since two Ni-H2 batteries are connected in a “string” and are considered one battery, this means that adapter plates are also needed. This is to connect the single Li-ion battery to the existing connections for the unnecessary second battery in each string, thus completing the circuit.