PPRuNe Forums - View Single Post - 787 Batteries and Chargers - Part 1
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Old 18th Feb 2013, 16:02
  #717 (permalink)  
RR_NDB
 
Join Date: Feb 2011
Location: Nearby SBBR and SDAM
Posts: 873
What is a Thermal runaway?

Hi,

Reading some posts in the many threads created after 787 battery incidents it seems useful to remember, some important points on "Thermal runaway":

1) Itīs a "positive feedback" mechanism. Once started (in a Li ion battery) it will make the device go to limits. E.g. leakage of electrolytes, generation of smoke and even fire outside the device.

2) A "positive feedback" to be interrupted (once started) would require to reduce the "device energy". In ANA TAK case this happened because battery was severely discharged by an "external short circuit". (caused by an internal short circuit of cell # 3 plus terminal region to ground through battery case). *

3) Electricity means (cut off, etc.) may be useless. It seems this is one characteristic of the BAD battery selected for the 787. This mean may be useless to stop battery charging or even delivering electricity (under load).

4) A simpler redesign of the battery would require reduce the likelihood of thermal runaway. For now a (technical) "quick fix" for 787 could be summarized:

a) Separate each LVP-65 ideally in individual chambers. (battery volume would be aprox. 30 ~ 40 % more)
b) Monitor each LVP-65 temperature (if not yet made)
c) Tweak software algorithms
d) Improve mitigation means. (better battery case, better venting, safer batteries location, etc.)

This is a Technical comment. Bureaucratic aspects (Cert., etc.) is another certainly much bigger issue for Boeing.

(*) This model can be explained by the ground wire that was fused by currents estimated in excess of 1,000 Amps. Itīs a model, that could be confirmed or not.
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