Failed Battery Charge/Discharge Unit replacement complete during historic EVA

by Chris Gebhardt

NASA astronauts Christina Koch and Dr. Jessica Meir exited the International Space Station to begin an Extravehicular Activity – EVA, or spacewalk – on Friday that largely saw the duo replace a failed Battery Charge/Discharge Unit on the Station’s P6 truss structure. 

The unit failed to reactivate after the first two of a five spacewalk sequence to replace 12 old batteries with six new and improved Lithium-ion batteries. The replacement work was successful.

Unit failure:

Overall, the International Space Station has 24 Battery Charge/Discharge Units (BCDUs) along its Integrated Truss Structure – the part of the Station that houses the solar arrays and all of the batteries and electrical distribution systems for the outpost.

BCDUs control the amount of charge put into each battery on the Station’s truss during the 16 sunlight orbital passes each day as well as the amount of power taken off the batteries during the 16 orbital night passes each day.

In preparation for the first two EVAs earlier this month to begin the battery replacement sequence on the P6 truss, three BCDUs on the same power channel as the first batteries to be replaced were turned off.

The two post-battery swap configurations: without an old nickel hydrogen battery on an Adaptor Plate and with an old nickel hydrogen battery on an Adaptor Plate. (Credit: NASA)

Last Friday, all three were commanded to reactivate as their associated batteries had completed their replacement sequence.

Two of the BCDUs reactivated without issue, but a third failed to do so. 

According to NASA, “On Friday, October 11th, after the completion of US EVA 57 P6 Battery EVA #2, the BCDU 2B2 converter did not close when commanded.  The Li-ion battery connected to BCDU 2B2 was safed and still shows good health & status. 

“The other two BCDUs, 2B1 & 2B3, were successfully activated and are healthy and nominal. BCDU 2B2 troubleshooting was completed … to power cycle the BCDU and the converter; however, it was not recovered.”

This resulted in only two-thirds of the potential power from this particular BCDU/battery channel being available for the Station.

When the failure occurred, NASA decided to stop the current battery replacement spacewalks and change the next EVA in the sequence to a removal and replacement operation for the failed BCDU.

While a single failed BCDU might not seem like a critical issue, it follows on the heels of a similar failure earlier this year when a BCDU on a channel of newly replaced batteries failed as well.

A second failure in rapid succession opens the question as to the exact cause of the BCDU failures, if they are connected, and if they are in someway related to the spacewalks or the new Lithium-ion batteries.

The previous failed BCDU was resolved when the Station’s Canadarm2 performed a removal and replacement operation robotically.

That robotic activity is not an option for the currently failed BCDU as it is located on the extremity of the Station’s Integrated Truss Structure, out of reach of Canadarm2. 

The removal and replacement process is also not possible using the Orbiter Boom Sensor System (OBSS) arm left behind by Shuttle Endeavour after her final mission in May 2011.

The OBSS was left on the Station to allow an extended reach of Canadarm2 to extreme points on the Station in certain situations.

This is not one of those situations as the OBSS does not have controllable grapple fixtures to allow it to grab items on the Station – as required for the removal and replacement activities associated with the failed BCDU.

With the need to manually replace the BCDU, NASA re-evaluated US EVA-58 – which was originally the third spacewalk in the P6 battery replacement sequence.

Scott Parazynski attached to the end of the OBSS – held by Canadarm2 – during an STS-120 spacewalk. (Credit: NASA)

During this re-evaluation, NASA decided to change the astronauts assigned to the spacewalk by removing Dr. Andrew Morgan and replacing him with Christina Koch.

Koch is tied with Morgan as the most experienced US-segment spacewalker currently aboard the International Space Station – with three EVAs to each of their credit.

However, all three of Koch’s EVAs have dealt with the Station’s power and electrical distribution systems, whereas only two of Dr. Morgan’s have done so.

Replacing Dr. Morgan with Koch exemplified NASA’s commitment to putting the most qualified astronaut on a spacewalk.

Dr. Morgan’s replacement with Christina Koch subsequently paired her with Dr. Jessica Meir, who was already slated to perform U.S. EVA-58 under its original plan.

Another reason for removing Dr. Morgan from this EVA is that he is slated to perform five back-to-back spacewalks in November and December with European astronaut and current Station Commander Luca Parmitano to repair the Alpha Magnetic Spectrometer experiment.

The first and foremost task of the BCDU replacement EVA saw Koch and Dr. Meir translate out to the P6 truss and perform a manual replacement.

In preparation for this activity, the Station’s Canadarm2, with the spare BCDU, translated to the appropriate area as close to the P6 truss as it can get.

The replacement operation lasted just over three hours, after which Koch and Dr. Meir performed a rearrangement of Multi-Layer Insulation blankets on other elements of the Station’s power system.

After that, the duo translated back to the central core of the Station and then outward to the European Space Agency’s Columbus laboratory where they prepared the “porch” section of the module to accept future external payloads.

After the replacement activities of the BCDU, there will only be two or three remaining BCDU spares on the Station and none on the ground, according to NASA in a teleconference detailing this spacewalk.

According to NASA, the failed BCDU will be brought inside the Station and prepared for a now-planned return to Earth in early-January 2020 aboard the SpaceX CRS-19 cargo Dragon – the only craft currently capable of returning payloads from the International Space Station.

But a much larger and critical question now faces the International Space Station Program: what is causing the BCDU failures and are there any links to the new Lithium-ion batteries being installed on the outpost?

Until some kind of answer can be determined, the current battery replacement sequence is on hold.

As of publication, there are no plans to delay the pending 31 October Russian EVA nor is there any, at-present, interference with the planned 2 November launch of the Northrop Grumman Cygnus resupply cargo craft to the Station

More so, NASA says the BCDU failure and investigation will not affect the planned five spacewalk series in November and December to repair the Alpha Magnetic Spectrometer experiment that was mounted to the outside of the Station in May 2011 during the STS-134 flight of the Space Shuttle Endeavour.

Related Articles