Date: Apr 09, 2019
Astronauts aboard the International Space Station (ISS) have now completed the third of a trio of spacewalks to continue the ongoing program to replace the station’s aging batteries and upgrade power systems. The spacewalks were originally deferred from last year due to the impacts associated with the aborted Soyuz MS-10 launch. EVA-52 was completed last week, while EVA-53 carried out a near-mirror of tasks to complete the battery swap. EVA-54 helped troubleshoot some of the earlier work and upgraded power systems
The station’s batteries are required to provide power to the ISS at times when it is in orbital darkness, during which time the solar arrays no longer produce power due to lack of sunlight. During “orbital day”, the station’s solar arrays charge the batteries so that they can provide power to the ISS during periods of darkness.
Each of the station’s eight power channels each have six batteries – although two batteries are considered as one as they are connected together in a “string”.
The “legacy” batteries on the ISS are of Nickel-Hydrogen (Ni-H2) type, which have typically been used in space applications due to their long lifetime and being able to withstand a large number of charge-discharge cycles without major degradation.
In addition, Ni-H2 batteries are not susceptible to over-charging and reverse current, giving them good safety properties.
However, a drawback of Ni-H2 batteries is that they are susceptible to “battery memory”, where the battery can lose a portion of its capacity if it is not fully charged and discharged each cycle. It is for this reason that regular “battery conditioning” is performed on the ISS, in order to prevent battery memory from occurring.
Each of the station’s Ni-H2 batteries consist of 38 individual cells (76 cells per two-battery string), with each cell consisting of a pressure vessel containing gaseous hydrogen stored at up to 1,200 psi, which is generated during the charging process itself.
