Cockney Steve,
Turin...Yes, I understand that in an emergency, you MIGHT want to pull the last dregs out of the MAIN battery....but isn't that why it carries 2 apparently identical units?
No, the APU and Main batteries perform completely different and seperate functions. You cannot use power from the Main Battery to start the APU anymore than you can use the APU battery to power the brakes unless you physically disconnect them and swap their positions. Do-able on the line to get the a/c back home but not a switchable feature from the flightdeck.
Even allowing for the extra cost of certifying these units, they really don't come into the realms of "disposable"....
In the event of an emergency they will be.
Normal ops they shouldn't be.
BUT they're tens of thousands of dollars a pop and at that price, if I were a "punter" I.E. a Boeing customer, I'd expect my Lithium batteries to be effectively managed to give me COST-EFFECTIVE Emergency power
It's not beyond the wit of smart engineers to design a proper, effective battery-management system,possibly with automatic sensing to access that final, destructive reserve of energy.
Absolutely.
....If they can do it,as hobbyists, why can't the might of Boeing, Thales and Securaplane , manage a "floating " emergency battery and an APU starter-battery. Even making the (probably wrong) assumption that the APU is used at every start to give start power for the main engines, there's still the pushback, engine -warm-up and taxiing times for the APU battery to recharge.
Assuming the APU battery was used to start the APU of course, which during normal turnround ops it won't be as ground power will be attached.