Finally an over current scenario identified as potential causes for 787 battery problem

The burned out batteries on the 787 have grounded the 787 fleet.


I've been waiting to write a blog entry on the 787 battery issue, and was amazed the amount of coverage discussing over voltage.

Japan transport-ministry investigator Hideyo Kosugi said the state of the battery indicated “voltage exceeding the design limit was applied” to it.

Eventually it was discovered over voltage was not an issue.

30 years ago I spent a lot of time working as a reliability engineer, tearing apart the insides of semiconductors to figure out why they failed.  Semiconductors failed for basically reasons of manufacturing defect (lifted bond), over voltage, or over current.  It was pretty cool milling the plastic down to create a trough down to the semiconductor, then using sulfuric acid to remove the remaining plastic to expose the semiconductor die and its wire bonds.  For over voltage you would look for where there was a break in a connection that was very small and hard to see.  But, when it was an over current problem, many times the plastic was cooked from the heat and the sulfuric acid would not remove the plastic.  These hard crusted plastic burned areas looked like the above picture.

Finally there is coverage for improperly wired batteries causing over current in the battery.

According to the AP, on the ANA flight, the Japanese Transport Ministry noted:

Flickering of the plane's tail and wing lights after it landed and the fact the main battery was switched off led the investigators to conclude there was an abnormal current traveling from the APU due to miswiring.

I am sure many of you have seen over current failures and you recognized as well that the 787 batteries were subjected to over current.