If these are STANDBY BATTERIES (back UP), why are they being charged inflight in the first place? Charging a Battery by definition means the power that the batteries supply can be provided by the charging system alone.
They have to have the juice to start the APU, and then the charging system carries the load. Once the APU starts, the Batteries are done. If the Batteries are charged by the APU, then obviously the APU is powering the system, not the Batteries.
Why even equip the aircraft with a BATTERY CHARGING SYSTEM? If the APU does not start, and the batteries need to power emergency lighting, they will, the APU, having not started, will be unable to charge (power) the system by definition. If the MAIN ship battery powers instruments and cockpit area lighting, same thing, if it needs to be charged, then the charging system (APU) will charge it, if it cannot, the batteries need to have the capacity to satisfy ETOPS, and if they do, they don't need charging....in the first place.
This applies if the BATTERIES are STAND ALONE. If provision for BATT/Charge together is to satisfy the safety reg, then the charger and BATTERIES constitute a dual (interdependent) system.
Last edited by Lyman; 22nd Jan 2013 at 20:42.