quoting Mike Sinnet, VP of Engineering and chief project engineer for the 787:
The driving factor in our design was really the ability of the battery to discharge a large amount of energy in a very short period of time. And this was required for two different functions at an airplane level.
One was for starting the auxiliary power unit and the other was for being able to apply braking to the airplane in the event that all other power sources in the airplane were lost. The 787 braking system is an electrical braking system.
Which function (backup braking or APU start) places the greater demand on the battery system? I'll venture a guess and say its the APU start. That has been possible with NiCad technology for quite some time. And given the variable frequency drive motor/generator, I'd guess that the old school DC starters placed more strain on a battery than these LiON batteries will see.
The brakes, while critical, don't appear to have the same demand as APU start. There are a few pics of the system here:
Messier-Bugatti-Dowty
While there are no electrical demand specs, one can size up the electrical components (motors, connectors and wire bundles) and make some guesses about demand.
So falling back to a NiCad system may not be so far fetched as Boeing would like people to think. The main problem will be building up a 32V NiCad battery assembly and certifying it with a 32V charger. This would be prefferable to redesigning and recertifying the 787 with a 24V DC system (to use off the shelf battery systems). But its still possible.