@FlightlessParrot
If I recall correctly 70 lbs LiOn equivalent NiCd will be around 350 lbs. Still NiCd cannot meet 787's hungry power requirements. 120 lb add on is better than 280 lb.
@Turin
120 lbs per battery. It is a heavy thick steel casing to contain thermal runway conditions. Rework and additional weight associated is common on frames, parts already built. But putting a battery in a steel case, forever!!! Is FAA going asking every future battery in a steel case.
@tdracer
Boeing is just an integrator, they have to take whatever partner gives. Welcome to the risk sharing partnership world of B787 program, outsourcing on steroids.