B737 NG electrical

[TyRod]

Hello,

small questions about electrical system for the NG.

When the battery charger operates and charges the battery, does it also continue to feed the hot batt and switched hot batt busses? Or does it just charge the battery and the battery powers those busses?


Now, you decide to use the BATT position , what happens?
Does that mean that the battery will feed the corresponding busses or that the battery charger will do it if it is available?
I read that, in this situation, charging of the battery is inhibited.

I also read that in case of thermal runaway, hot batt and switched hot bat busses are lost. On the schematics, it is not clear. Initially, I thought that the battery charger will be disconnected from the battery but would continue to feed the hot batt and switched hot bat busses...


Thanks

[Skyjob]

When the battery charger operates and charges the battery, does it also continue to feed the hot batt and switched hot batt busses? Or does it just charge the battery and the battery powers those busses?

Each battery charger also operates as a TRU when not in the charge mode.
[737-600/700/800/900 MAINTENANCE MANUAL - ELECTRICAL POWER - DISTRIBUTION - GENERAL DESCRIPTION]

The main battery charger is the primary power supply for the hot and switched hot battery bus. The auxiliary battery and its charger connect in parallel when the dual battery remote control circuit breaker (RCCB) is closed. This happens only during conditions when the main battery must supply standby AC and DC power.
[737-600/700/800/900 MAINTENANCE MANUAL - DC GENERATION - DISTRIBUTION AND CONTROL - GENERAL DESCRIPTION]

The main battery charger and auxiliary battery charger give a DC voltage output to charge their respective battery. Each charger operates like a TRU after the battery gets to full charge. The main battery charger sends a constant DC voltage to the battery and the hot and switched hot battery buses. The auxiliary battery and auxiliary battery charger power DC buses only during non-normal conditions.
[737-600/700/800/900 MAINTENANCE MANUAL - DC GENERATION SYSTEM - GENERAL DESCRIPTION]

Now, you decide to use the BATT position , what happens?

Which switch are you referring to? Do you mean:

1. STANDBY POWER to BAT posiiton;
2. BAT to OFF position;

[NSEU]

Originally Posted by Tyrod

When the battery charger operates and charges the battery, does it also continue to feed the hot batt and switched hot batt busses? Or does it just charge the battery and the battery powers those busses?

Looking at the wiring schematics, there is no way of diverting the power from the charger to one user only. The charger (if outputting volts) will always feed both bus and battery. i.e. there are no tie breakers between these items (other than one CB which feeds both Bat and bus).

However, the "nature" of the power from the charger changes. There is 'full on' charge mode (Constant current) and there is trickle charge mode (Constant voltage/ 27.5v/TRU mode).

When the battery is fully charged (and more), the charger switches to constant voltage mode. In this situation, the battery charger supplies the Hot Bat Bus with this voltage and trickle charges the battery (I assume, since there is no way of outputting different power (individually) to Battery and Bus, "trickle charge" simply occurs because of the constant voltage output from the charger).

The battery charger goes into charge mode (constant current) when the battery voltage is less than 23 volts. The charger will overcharge the battery for a certain period. Then the charger switches to TRU mode and provides 27.5volts.

Because the charger trickle charges the battery, "charge mode" is not often required. It happens only if the Battery Charger input power is off for more than 1 second or if the battery voltage is less than 23 volts. Charge mode is also electronically inhibited if the fuelling station door is open, the APU is being started, the Standby Power switch is in BAT (or in AUTO and conditions require BAT mode) or if there is a main battery overheat.

Hope this makes sense.
Rgds
NSEU

[NSEU]

P.S. Looking at the wiring schematics, if a thermal sensor in the battery sees an overheat, the charger output is completely inhibited. Therefore, the charger will not supply power to either battery or bus. This would lead me to believe that if the battery is still operating, it would still supply power to the Hot Bat Bus.

I stand to be corrected