Lyman,
The point of a battery is to provide short-term electrical power when other sources of it are not available?
Turning to the APU again, I think we need to understand the raisons d’ętre (missions) of both the APU battery and the APU itself. There seems to be an assumption by some posters here that:
(a) the APU is only to be used during in-flight abnormal or emergency procedures; and
(b) it should always be possible to start it without recourse to the APU battery.
Lacking access to the FCOM, I can only speculate, but an APU – even one that does not have to supply copious amounts of bleed air - weighs a great deal more than a couple of lead-acid batteries, and occupies more space. It must justify its existence. The notion that it is not to be used in “Normal Procedures” is therefore unlikely. That would also assume that all the airports on your B787 network have rapid-deployment “ground-conditioning” units and high-rated ground power at every gate or stand at which you will ever have to park. (Preferably, the tugs would supply similar electrics, to enable engine start during push-back.) And that is to ignore your diversion airfields.
Think of an airline like Ethiopian. Does Addis Ababa have such facilities? And what about the other African cities on their network; such as Cairo, Dakar, Johannesburg, Lagos, etc., that they may be planning to send their B787s to later?
Even at the most sophisticated airports, there will be occasions when the crew arrives at the aircraft only to find no suitable ground power available. That’s what an APU is for, and the only way to get it started will be with its dedicated battery. 40 or 50 mins later, you may be getting airborne, with an ETOPS leg later in the flight. Ceasing recharging the battery before take-off could leave you with an insufficiently-charged battery, but we have no data on that.
Depending on the certificated ETOPS assumptions for the aircraft, as I wrote in an earlier post, banning in-flight charging might necessitate starting the APU in flight prior to the ETOPS leg (using generated electrics). If that was unsuccessful, however, you might have to divert. So an interim solution could be to start it before departure, and leave it running until the end of the ETOPS leg. Fortunately, an idle APU uses modest amounts of fuel at high altitude.
The banning of recharging the APU battery inflight would not be tolerable in the long term. The advantage of a dedicated APU battery, such as Boeing traditionally provides, is that it can be used to start the APU even in the event of total generation failure. Aircraft using the main battery for APU start normally cannot risk that.