Hi,
TURIN:
Redundancy (DC EXT PWR) was necessary.
The Hot Bat Bus is connected to the Main DC Busses via the Hot Bat Bus Contactor.
So, 787 has:
1) Relay/contactor inside battery (as we imagine from pictures) for batt. ON/OFF
2) BDM between STD batt. connector and HOT BATT BUS
3) HOT BATT BUS contactors to DC BUSsess (LH and RH)
Electrically speaking is similar what first thought.
The APU Bat when towing only powers the Position (Nav) lights.The Main Bat powers the essential towing items such as VHF1, brakes (& Anti collision beacons-I think).
Indeed, brakes, comm and Anti collision beacons, certainly.
I initially thought of APU batt. for this items but to save weight and cost you donīt have higher current cabling between batteries. Just control wiring.
There is a very simple indicator on the flight deck that shows battery state when the a/c is unpowered. It simply shows High, Medium, Low. When beginning a tow, this will give the engineer/mechanic a reference to how long they have to complete the tow. I have also heard that automatic load shedding will occur if the bat state gets too low, IE. VHF1 is lost. etc.
I consider it enough! The Li ion actually doesnīt require a voltage indicator. itīs voltage span is small. 3 levels is perfect. Simple and efficient.The load shedding is a good approach to allow a prioritized graceful degradation. Avoiding damage to any cell. (other batteries are more tolerant)
The design sounds perfect.
green granite:
Since the discharge curve for lead-acid and Li-Ion batteries are very different Peukert's law would be much nearer to being negligible for Li-Ion, and if the batteries are allowed to rise in temperature to 131°F you get nearly as much energy no matter how hard you drive them.
One of the appeals for designers adopt it.
no matter how hard you drive them.
For a 65A (75 A nominal) What would be a designers (787) peak limit?
(For MAIN batt. i mean) This specific aspect concerns me.
For APU i guess there is no big problem. Soft start, etc.
My concern in the MAIN also is: Li ion use was really very important?
WHY?