787 Batteries and Chargers - Part 1

[RR_NDB]

Switching (clever design)

Hi,

TURIN:

The diode module is not "the switch that puts the bat on the bus" as you say. The diode module is between the bat and the hot bat bus. The hot bat bus is only connected to a main DC Bus when there is no other supply and the bat switch is on. The bat is not discharging unless there is a power failure of the main DC busses.

Exactly what i expected! And showing a VERY GOOD APPROACH used by the colleagues.

First a briefing on the DIODE function. Actually it is a SWITCH. A fast switch. What flips the switch? The voltage between the 2 (only) terminals. They are used as switches since the beginning. (there are other uses).

As i can imagine in 787 main battery circuit the diode has the (Safety) function:

Make impossible the DC bus charge the main battery. This as i commented earlier would not be the safer way to charge the critical cells. Reasons: The bus voltage varies (due varying loads on it) and the current management (to charge the battery) would require a more complex and costly scheme. They designed very well, imho.

As i can imagine the designers put a redundancy (wisely) with 2 "means" to turn off the battery, as you made implicit. I probably would design exactly the same.

The switching function i mentioned in earlier post actually is the inherent diode function:

When the bus falls below (i estimate ~ 30 V) the battery power (kept at itīs top-nominal capacity by a charger certainly connected between the diode module and the battery) flows AUTOMATICALLY to the bus. As we may imagine, several important modules would die (reset) if the DC bus fails. The bus voltage would be not less than battery voltage level (decaying) minus the diode module voltage drop (less than ~2 Volts).

To be continued

In the meantime please comment on RAT and itīs relation to the important DC bus.