EVA-39: Spacewalkers complete the upgrading of ISS batteries
Two astronauts aboard the International Space Station (ISS) have completed a long-planned process to upgrade the power storage batteries outside the station. The upgraded batteries will give the ISS better power storage capacity throughout the rest of its planned lifetime. The second spacewalk came one week after the first – highly successful – EVA.
Battery replacement Overview:
All electrical power on the ISS is generated via the station’s solar arrays, which transform sunlight into electrical energy. However, during times when the ISS is passing through “orbital night”, the solar arrays 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.
The batteries are charged via the solar arrays during the approximately 45-minute period when the arrays are in direct sunlight each orbit and are discharged as they power the station’s loads during the 45-minute period of darkness per orbit.
In total, the ISS has eight separate power channels, with each channel having three batteries – although one battery is considered as a “string” of two separate battery units connected together, making for six batteries per channel in actual fact, and thus 48 batteries on ISS in total.
Each of the current batteries is of Nickel-Hydrogen (Ni-H2) type, which have typically always been used in space applications due to their long lifetime, 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.
The batteries were launched in four batches, attached to the station’s four solar array Truss segments (P6, P4, S6, S4), with each array containing two power channels, and so 12 batteries in total.
The oldest Truss segment, the P6 Truss, was launched in 2000, however its 12 batteries were replaced with new Ni-H2 batteries during the STS-127 and STS-132 Shuttle missions in 2009 and 2010.
The P4 and S4 Truss segments were launched in late 2006 and early 2007, with the S6 Truss being launched in 2009. As such, the oldest batteries on station are now around 10 years old and are reaching the end of their design life.
This means that replacement batteries are required in order to sustain the ISS out to its currently planned retirement date of 2024. However, Ni-H2 batteries are now considered to be old technology, as most of the station’s systems were designed throughout the late 1980s and early 1990s.
The ISS Program have therefore decided to modernize the station’s batteries during the replacement process, by moving to modern Lithium-Ion (Li-Ion) batteries. These battery types work via lithium ions moving 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, since they do not require pressure vessel containers for the storage of hydrogen gas, meaning Li-Ion batteries have very high energy density compared to Ni-H2 batteries.
This has many benefits to the ISS program, as it means that just 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 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 susceptible to battery memory, negating the need for battery conditioning to be performed. Li-Ion batteries do have some drawbacks however, namely the fact that they are much more sensitive to overcharging, which must be prevented via battery management and protection systems.
In addition, Li-Ion batteries typically have shorter lifetimes than Ni-H2 batteries, as they cannot sustain as many charge/discharge cycles before suffering notable degradation. However, the ISS Li-Ion batteries have been designed for 60,000 cycles and ten years of lifetime.
In addition, they will incorporate cell balancing and adjustable end of charge voltage technology in order to maximize their lifetime.
Li-Ion batteries have experienced notable issues in the past, in the form of overheating and “thermal runaway” problems on the Boeing 787 Dreamliner aircraft.
However, the Li-Ion batteries that will be used on the ISS, although manufactured by the same company (GS Yuasa), have been designed incorporating lessons learned from the 787 issues, and have passed rigorous space certification tests.
Specifically, the ISS Li-Ion batteries include two redundant controls against thermal runaway, voltage and temperature monitoring of individual cells, circuit protection & fault isolation of individual cells, and thermal heat barriers between cell packs incorporating 787-derived safety additions.
In terms of construction, each ISS Li-Ion battery contains 30 individual cells, packed into a box which maintains the same dimensions and mounting interfaces as the previous Ni-H2 batteries, but at a significantly lower weight (430 pounds as opposed to 740 pounds).
A single Li-Ion battery will replace the functions of two Ni-H2 batteries, but since two Ni-H2 batteries were connected together in a “string” and considered as a single battery, this means that adapter plates are also needed in order to connect the single Li-Ion battery to the connections for the unneeded second battery in each string, thus completing the circuit.
The move to Li-Ion batteries is just the latest in a series of technology upgrades designed to address obsolescence issues and equip the ISS with 21st-century technology to support research. Recently, astronauts began replacing old General Luminaire Assembly (GLA) filament lights with modern Solid Sate Lighting Assembly (SSLA) LED lights.
In addition, the process of upgrading the station’s command and control computers to modern standards is still underway via the Enhanced Processor & Integrated Communications (EPIC) upgrades. Also, the process of upgrading the station’s external cameras to High Definition (HD) standard has begun.
Since only six Li-Ion batteries can be launched at a time via the annual Japanese HTV cargo craft, which is enough to replace the batteries on two of the station’s eight power channels, it will take four years in total to replace all the station’s Ni-H2 batteries with Li-Ion batteries.
That process began with the first of two upcoming spacewalks to install Li-Ion batteries on power channels 3A and 1A, both located on the S4 Truss Integrated Electronics Assembly (IEA).
A large amount of preparation work has been carried out prior to the spacewalk, with the first step being to transfer channel 3A’s loads onto another channel and fully discharge its batteries.
In addition, to reduce the number of spacewalks required, a large amount of robotic activity has taken place ahead of time via the Special Purpose Dextrous Manipulator (SPDM) “Dextre” robotic hand, in an increasing move toward the use of robotics for more complex station servicing tasks.
The new Li-Ion batteries were launched on the Japanese HTV-6 vehicle last month, mounted to an external cargo carrier known as the Exposed Pallet (EP). The EP can hold nine batteries in total, however it only launched with six batteries, as this is all that is required to replace the 12 batteries of the two power channels on each Truss segment.
Each of the six Li-Ion batteries was mounted to an adapter plate during launch, but these will be separated during the installation process, with the Li-Ion battery being installed on the ISS in place of one Ni-H2 battery, and the adapter plate being installed in the place of the second Ni-H2 battery per each string.
The Li-Ion battery will then be linked to the adapter plate via a cable, which will essentially allow the single Li-Ion battery to be connected to the slots previously occupied by the two Ni-H2 batteries.
However, since the EP only has space for nine batteries for disposal, and since 12 Ni-H2 batteries are being replaced in total, this means that three unneeded Ni-H2 batteries must remain on the ISS.
These three batteries will be installed on top of the adapter plates, although they will be unused and completely disconnected from the power channel, and are likely to remain on the ISS for the remainder of its life due to lack of available disposal options.
Once arrived at the ISS, the EP was extracted from HTV-6 robotically, and placed on the Payload ORU Accommodation (POA), a grasping point for temporary payloads attached to the station’s Mobile Base System (MBS).
Over the past week, the SPDM, controlled from the ground, has removed four of the six Ni-H2 batteries from power channel 3A on the S4 IEA, installing three of them into three empty spaces on the EP, and another onto the SPDM’s own Enhanced ORU Temporary Platform (EOTP) storage interface.
The SPDM then removed three new Li-Ion batteries from the EP and installed them into the vacated Ni-H2 slots on the S4 3A IEA, leaving three adapter plates exposed on the EP ready to be used by the spacewalkers. The only robotics issue encountered was an inability to drive a bolt on one of the new batteries, a task which will be undertaken by the spacewalkers instead.
The spacewalk – US EVA-38 – was performed by astronauts Shane Kimbrough as EV-1, wearing the suit with the red stripe and making his third EVA, and Peggy Whitson as EV-2, wearing the all-white suit and making her seventh EVA.
Upon egress from the Quest airlock, the first order of business was for Kimbrough to translate out to the Crew & Equipment Translation Aid (CETA) cart on the S1 Truss and retrieve an Articulating Portable Foot Restraint (APFR), which he then installed on the power channel 3A side of the S4 Truss IEA.
Whitson, meanwhile, translated to the EP installed on the MBS POA, and began to remove two adapter plates. Kimbrough then joined Whitson on the EP and help her remove the adapter plates, following which they each carried one adapter plate to the S4 IEA worksite.
Once at the worksite, Kimbrough installed one of the adapter plates into an empty slot that was previously occupied by an Ni-H2 battery, but which was removed by the SPDM. He drove two bolts to connect the adapter plate to the IEA. He then connected a single data cable to link a new Li-Ion battery – previously installed by the SPDM – to the adapter plate, creating a single string.
Kimbrough then released a still installed Ni-H2 battery – a large box-like object with two bolts (labeled H1 and H2) and blind-mate connectors – from a second string on the IEA, and installed it onto the adapter plate of the first string for long-term stowage.
Kimbrough then installed the other adapter plate into the newly vacated slot from the Ni-H2 battery of the second string, and connected its data cable to a pre-installed Li-Ion battery to complete the installation of the second Li-Ion battery string. By this time, the spacewalkers were one hour ahead of schedule.
A Ni-H2 battery from the third and final string on the IEA was then uninstalled from its slot and installed onto the adapter plate of the second string for stowage, which left an empty battery slot in the third string.
Both spacewalkers then headed back to the EP and retrieved the third adapter plate, translated back to the S4 IEA worksite, and installed the adapter plate into the empty battery slot on the third string.
Following the mating of the data connector between the adapter plate and Li-Ion battery on the third string, the 3A power channel Li-Ion battery replacement was complete, and the channel can be re-commissioned and begin re-powering loads.
Kimbrough then headed to the 1A power channel on the other side of the S4 IEA and drove a bolt that the SPDM was unable to accomplish.
Thanks to being ahead of their timeline, get-ahead tasks were provided, with a repair on an external light for Whitson conducted while Kimbrough inspected the AMS-02 experiment.
They also installed an ethernet cable related to commercial crew vehicles that will be arriving at the ISS in the next few years.
Both spacewalkers then cleaned-up the worksite and head back to the airlock to conclude the EVA.
Another spacewalk was required to complete the Li-Ion battery installation on channel 1A, and prior to that the SPDM was hard at work removing five old Ni-H2 batteries from the 1A power channel, installing three onto the EP in the spaces vacated by the adapter plates removed by the spacewalkers, and another two onto its EOTP.
The SPDM also installed three new Li-Ion batteries into channel 1A in preparation for the EVA.
Following the installation of three new Li-Ion batteries and three adapter plates, plus the stowage of two old Ni-H2 batteries, onto channel 3A during EVA-38, ground teams commissioned the new Li-Ion batteries as detailed in status notes on L2.
“The new Lithium Ion batteries installed on 3A were fully activated following US EVA 38 on January 6th and the 3A power channel was reconfigured to power its nominal loads”, stated the notes. Two minor issues were noted during the process, but did not have any impact.
“A Battery 3A2 heater 2 failure message was annunciated. The heater is functional and the message is thought to be due to the battery temperature not rising above setpoint in a duration specified in software. Teams are investigating a potential software update to increase the time duration parameter.”
“Additionally, BCDU (Battery Charge/Discharge Unit) 3A measurement out of range messages have appeared periodically, due to voltages in the BCDU slightly above a software limit. Teams are investigating whether any actions are needed.”
With channel 3A up and running with its new Li-Ion batteries, attention turned to installing new batteries onto the other power channel – 1A – located on the S4 Truss.
With channel 1A’s loads transferred to another channel and its six old Ni-H2 batteries drained, robotics operations got underway to remove three new Li-Ion batteries from the EP, and install them onto the 1A IEA.
Specifically, the SPDM removed two Ni-H2 batteries from the 1A IEA, with these old batteries then installed onto the EP in two spaces vacated during EVA-38 via the removal of adapter plates from the EP.
Two Li-Ion batteries were then removed from the EP and installed onto the vacated slots on the 1A IEA. A third Ni-H2 battery was removed from the 1A IEA and transferred to the EP.
According to L2 notes, one of the Li-Ion batteries that was removed from the EP had a “dynamic release and rebounded toward the EP. The fins on the underside of the battery were inspected and no issues were found”.
The following day, an Ni-H2 battery was removed from the 1A IEA and stowed on the SPDM, with the final Li-Ion battery then being removed from the EP and installed onto the 1A IEA.
However, during operations to fasten the H1 bolts on two of the Li-Ion batteries on the 1A IEA, the SPDM experienced difficulties, as described in L2 notes.
“After multiple attempts, the ROST (Robotic Offset Tool) was unable to engage the H1 bolt on the battery in slot 5, so the SPDM moved on to slot 1. The H1 bolt was fastened on the slot 1 battery, but, afterward, the SPDM had difficulty disengaging the ROST from the H1 bolt. After many attempts to release the bolt, the tool was commanded to backdrive the bolt one half turn and it released.”
Following a review, it was decided to proceed with removing another Ni-H2 battery from the 1A IEA with the SPDM, and complete fastening of the two Li-Ion H1 bolts during EVA-39.
With the SPDM having installed three new Li-Ion batteries onto the 1A IEA, and having removed two of the three remaining Ni-H2 batteries, leaving two empty slots and one Ni-H2 battery for stowage on the 1A IEA, the stage was set for EVA-39.
EVA-39 was performed by NASA astronaut Shane Kimbrough as EV-1, wearing the suit with the red stripes, and ESA astronaut Thomas Pesquet as EV-2, making his first ever EVA and wearing the all-white suit. The spacewalk got underway at 11:22 PM GMT.
The first task for the two spacewalkers upon exiting the airlock was to remove two adapter plates from the EP, install one into an empty slot on the 1A IEA, and connect its data cable to its corresponding Li-Ion battery.
The single remaining Ni-H2 battery on the 1A IEA was then be removed from its slot and installed onto the adapter plate for long-term stowage. The second adapter plate was then installed on the 1A IEA, and its data cable connected.
The final adapter plate was then removed from the EP and installed onto the 1A IEA and its cable connected, which completed the installation of the three Li-Ion batteries and their adapter plates onto the 1A IEA, with one Ni-H2 battery reaming attached to an adapter plate for stowage.
The troublesome Li-Ion H1 bolts were also be fastened by the spacewalkers.
They completed this task two hours ahead of the timeline, allowing for multiple get-ahead tasks to be conducted before the two spacewalkers headed back to the airlock to conclude the EVA just short of six hours after it began.
Following the conclusion of the EVA, robotics work will continue to install three Ni-H2 batteries stowed on the SPDM into the three slots on the EP that were vacated by the removal of the adapter plates during the EVA. The EP loaded with nine Ni-H2 batteries will then be re-inserted into the HTV for disposal.
In addition, channel 1A and its new Li-Ion batteries will be activated and loads re-transferred, which will complete the process of installing Li-Ion batteries onto both power channels of the S4 Truss.
(Images via NASA, JAXA and CSA).