PPRuNe Forums - View Single Post - 787 Batteries and Chargers - Part 1
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Old 11th Mar 2013, 00:10
  #924 (permalink)  
syseng68k
 
Join Date: Jun 2009
Location: Oxford, England
Posts: 297
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Not a good idea to enclose all the protective & contol circuits & components
in an area of a battery case which has known potential fire & heat issues as
well.
Agreed - we trashed this part of the design mercylessly a few weeks ago,
just as soon as pics of the enclosure became available .

Could someone who has watched the current levels of the B787 APU battery
indicate that 45Amps without reducing for 15 mins after starting the APU
is a normal expected level as it seems a bit high.
That had me thinking about an overall scenario for the failure, as follows:

1) Apu startup, very high peak current demand, which significantly warms up
the cells

2) Immediate 45 amp charge current, which warms up the cells even more, with
one or more exceeding data sheet limits. The single thermal sensor doesn't see
this in time to shut down the charger, due to thermal resistance and lag across
the package. If it does see it, it's late, and the out of limit conditions
persist for long enough to cause some permanent, though not initially fatal cell
damage.

3) Cell eventually gets tired of being abused, thermal runaway in a single cell,
cascading to others. Game over.

I think temperature is the key here. These events may have been waiting
to happen for some time, but ambient temperature and low battery cycles have
mitigated it up to now. As the cells age though, ie: more and more flights
and charge discharge cycles, their ability to withstand abuse will be degraded.
The fact that it didn't happen in flight, yet, could simply be that that the ambient
temp may have been much lower than that on the ground.

Just a theory, but seems quite feasible ?.

To add to the mix, we have no clue what the controller software is up to,
algorithms used, how various limits are decided against time and other factors
directly or indirectly influenced by the charger and/or the bmu software...
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