not to be too simplistic, but as the calendar pages go by, there will come a point in time when going down to the local apple store and buying a bucket full of iphones and plugging them into the system will be cheaper than the cost of grounding a multi billion dollar program over the concern of a battery mass. The NiCd battery has been running for a fair old time, the guys in the hangar will be pretty familiar with them, and there are enough ex douglas guys there to string some more cabling as they were fond of doing.
Right now, the simple solution is a least risk solution.