Stitch wrote:3) To that end, the 787 batteries need to be able to be quickly recharged to a level necessary to support those critical systems if discharged to support other functions. Example would be using battery power to perform an engine start then having to perform an RTO soon after.
This keeps getting thrown up as a reason, but it is a red herring. Firstly, all designs of Li-ion cells that I have met feature fast-charging. How fast is open to discussion.
Currently, one of my cars has a battery issue, and after a difficult cold start, it might not have enough reserve to repeat those starting duties if I go to the shops and turn the engine off. The answer, and it's not exactly a new idea, is to run the engine for a while longer than normal, purely in order to recharge the battery. So I reach the shop 10 minutes late. Big deal!
The answer for the B787 is the same. IF ground power is unavailable, and IF the APU sucks up an excessive amount of battery charge getting started, then..... you simply delay take-off for 10 minutes to allow the APU (and/or the main engines) a chance to recharge the battery unit(s).
The 787 "has very high power generation capability, making a total of 1.5 megawatts in flight from 6 Variable Frequency Starter-Generators (VFSG), with two on each engine, and two in the aft Auxiliary Power Unit (APU)."
The limiting factor is almost certainly overheating of the Li-Ion cells if you try to charge them too quickly. But 10 minutes extra on the ground will most likely make a big difference, and yet it shouldn't be necessary in 99.9% of cases. Or is the 787 still having serious issues with GPUs?