787 Batteries and Chargers - Part 1

[bill good]

B787 battery

Keep in mind there are two failures of batterys from two different locations each with very simular end results. The report on the first indicates 17 mins after the APU was started from the APU battery, the voltage dec from 32V to 30V which indicates the first time a short is present. Since the current information is being derived from the neg of the battery & is after the shorted cell it is the sum of both charge & the discharge of the faulty section of the battery & thus would not be the correct recorded value. Because the breakdown of the insulation barrier between the cells has effectively reduced the number of cells over voltage would now appear on the cells before the short resulting in damage but lesser damage than the cells with the higher circulating current involving the path via the case to the ground point of the battery case & A/C bonded points. Further just to make sure the whole thing is destroyed all the wires from every connection appears to be taken thru a stainless "protective " shield which is bonded to the case to short out all wires from each cell to the external world of the case once the heat melted the insulation. i.e. just about 100% chance of every cell being destroyed. (The local heat build up is seen in the pictures is clear & reports of flame by the fire crew confirms) I can't help thinking that if the case was not bonded this event would have not flagged so many design oversights & safety issues. 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. All in all how did this design get this far?? 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. There are more concerns but the battery case & installation of this case is the primary cause. The swiss cheese model follows after this level with secondary oversights as time will show.

ps Looking at the specs for this cell the CC part for a cell start point 3.75V which would be degraded for a battery to 29.6V (est) & at 10A is 1 hr. Again anyone know if the battery in the B787 gets a CC to start the recharge process? Applying 32V initally seems very strange & would like to see how long a battery would last being tested on the bench by taking it to a low SOC & applying the 32V that is being stated as normal. (i would be in the next building) Best est 300-500 cycles.

Regards