syseng68k
Charger and battery testing is one thing. But I'm thinking more of battery/load compatibility. That's not something I'd expect Thales or GSYuasa to be able to do easily. One needs a relatively accurate simulation of variable frequency drives, power supplies and other stuff on the battery busses. The best place to find that is on the airplane.
This brings up some interesting philosophical points in handling development and certification testing. The 777 was developed with an "iron bird". A mockup of all the aircraft systems where compatibility issues could be researched. But that was expensive. It may have cost Mullaly his shot at CEO, but he's doing fine at Ford. As far as I know, nothing to this extent was done for the 787. Flight testing is going to be more expensive than an iron bird test.