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Changing batteries in Space

1 July 2020 

On 26th of june, two NASA astronauts took a spacewalk to replace old nickel-hydrogen batteries with new lithium-ion batteries. The new lithium-ion batteries are lighter and smaller than the old batteries and they also have more current capacity.

 

The use 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 is going through the "orbital night", the solar panels can no longer produce energy. As such, it is imperative that the ISS store energy in batteries, which it can then use to power its systems during periods of darkness. Every 1.5 hours, the ISS orbits the Earth, of which 45 minutes is in sunlight. During this period, the batteries are charged via the solar panels and the batteries are discharged while feeding the station's loads during the 45-minute darkness period per runway.

 

The nickel hydrogen (Ni-H2) batteries
In total, the ISS has eight separate power channels, with each channel having three batteries - although one battery is considered a 'string' of two separate battery units connected together, which basically 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 typically always been used in space applications due to their long life, as they can withstand many discharge cycles without significant deterioration. Moreover, Ni-H2 batteries are not sensitive to overcharging and reverse current, which means they have good safety properties.

 

However, a disadvantage of Ni-H2 batteries is that they are sensitive 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, “battery conditioning” is regularly performed on the ISS to prevent battery memory from building up. Each of the station's Ni-H2 batteries consists of 38 individual cells (76 cells per string of two batteries), each cell consisting of a pressure vessel of gaseous hydrogen stored up to 1,200 psi generated during the charging process itself. The oldest batteries on 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 required to keep the ISS until the current planned retirement date of 2024. Ni-H2 batteries are now considered old technology, however, 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 work via lithium ions that move between electrodes during the charging process, instead of hydrogen gas under pressure as used in Ni-H2 batteries.

 

As a result, Li-ion batteries are much lighter and smaller than Ni-H2 batteries, as they do not require pressure vessel containers for hydrogen gas storage, 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, as it means that only a single Li-ion battery can replace the function of two of the previous Ni-H2 batteries. This in turn means that only half the number of Li-ion batteries (24) are required to replace all of the station's Ni-H2 batteries (48), which also cuts the number of launches required in half. Li-ion batteries are also not sensitive to battery memory, eliminating the 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 avoided through battery management and protection systems. In addition, Li-ion batteries generally have a shorter life than Ni-H2 batteries, because they cannot withstand as many charge / discharge cycles before undergoing noticeable deterioration. However, the ISS Li-ion batteries are designed for 60,000 cycles and a service life of ten years. In addition, they will include cell balancing and adjustable charging voltage technology to maximize their life.

Li-ion batteries have experienced notable problems in the past in the form of overheating and “thermal runaway”. The Li-ion batteries to be used on the ISS, while manufactured by the same company (GS Yuasa), have been designed with lessons learned from the issues and passed rigorous space certification tests. In particular, the ISS Li-ion batteries include two controls 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 together in a "string" and are considered one battery, this means that adapter plates are also required. This is to connect the single Li-ion battery to the existing terminals for the unnecessary second battery in each string to complete the circuit.

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