I ask myself why have Boeing gone with this technology?
My suggestion is because the a/c is all electric then there are ultimate failure situations where high energy is needed. On every a/c I have flown before the brakes for example have a brake accumulator. The brakes on this a/c are electric, so the backup has to be the same. A high energy battery?
You are not the only one who wonders why Boeing took that risk for the benefit of a few kilogramms....