Sorry Voldenuit, misread your post as suggesting the fix for the APU battery issues was to go back to the traditional design.
No probs, I was pretty wordy and perhaps not all that clear.
I'm still not entirely sure that the main battery needs to be that much more powerful than on a traditional type. Do you have a reference for this? The following is my understanding, please correct me where necessary given your experience.
On a traditional type wing device is bleed air and flight control actuators are hydraulic. If you loose the engines then you've lost bleed air, and once the airspeed drops below windmill speed then hydraulic and generator power as well. By this point you need the APU up and running already otherwise you are on to the ram air turbine. In a traditional type the RAT will provide both electric and hydraulic power. As far as I'm aware, the main battery is only to keep the avionics up and running for 30 minutes so that the crew has basic instruments and radios to get the plane down safely.
Actually, planes have been gradually going electric for some time. The Airbus A380 for instance uses electrically powered backup flight control systems. It's just that the 787 uses more electric controls than previous aircraft. It's also worth noting that while only 2 of the main controls of the 787 are purely electric - the spoilers and horizontal stabilizer, the hydraulic power to the rest of the controls is generated electrically (I believe) on the 787 instead of mechanically from bleed air, so the 787 still does rely on electrical power a lot more than normal.
In a hydraulic system, there are hydraulic accumulators that store pressure and can provide continued pressure (and thus power) to the flight control surfaces in the event of total engine failure. The batteries on an electric aircraft provide the same function, acting as capacitors, if you will, in the event of loss of electrical power. So they would be sized to the requirement to power the critical electronics and control surfaces for emergency landings without power.
The second pushing factor for the battery size would be Extended Operations (ETOPS) rating. The FAA certifies certain twin engine plane types to be able to fly certain routes (I'm being a bit imprecise here, but bear with me) based on how long the plane can be certified to fly with one malfunctioning engine.Typical ETOPS ratings for twin engine aircraft are 180 minutes. In other words, a plane with an ETOPS rating can fly routes that place them within 180 minutes of an airport (for emergency landing) at any time. The 787 was designed to achieve an ETOPS rating of 330 minutes (currently, only a select few 777 variants have this rating). This means that in the event of a single engine failure, the combined energy available for flight controls (from the remaining engine, Ram Air Turbine, APU and both the main and secondary battery) would have to supply at least enough power for 330 minutes. Moving to a smaller battery would reduce the total available energy, and reduce the ETOPS rating for the aircraft. This might lock some routes out that were previously available to the aircraft, which would hurt the customers (the airlines).
Efficient weight usage is paramount in modern aircraft design, and I would say that the 787 has been optimized with a battery that's pretty close to what it needs for normal and emergency operations. Moving to a different battery chemistry or radically redesigning the existing cells would reduce the operational capability of the aircraft, which might make it less desirable to some customers. Because the 787 is more reliant on electrical power than competing aircraft, any change to its batteries would have a larger affect on the capabilities of the aircraft than on other planes.
Again, take what I've said here with some grains of salt. I'm not an expert in the specific areas of this discussion (emergency flight controls, power distribution and storage systems) or with the specific systems of the 787 in question. But I think I'm not too far off the mark when answering the question of "why doesn't boeing just move to a smaller/safer battery?". Oh, and lastly, certifying a completely new battery would probably take months if not years with the FAA, so a stopgap solution (containment structure, perhaps tightening QC on the battery manufacturer, more electronic safeguards on charge/discharge cycles) is pretty much what is needed right now. It may not be ideal, but with ANA having to cancell 1,800 flights because of the 787 grounding, it's what's needed. And at the very least, it has to be shown to be safe (as considered by both the FAA and Boeing) before it will be let back into the air.
Wind, Sand and Stars.