The following is written in the NG Maintenance Manual (It appears to be ambiguous)
Voltage (DC VOLTS) and load (DC AMPS) show on the
alphanumeric display when you put the selector in any of the
TR positions or any of the BAT positions.
Only voltage shows when you put the selector to the BAT BUS
or STBY PWR positions.
(EDIT) I see (below) I've included data for the BAT meters position (I had confused it with "BAT BUS", but I'll keep the data for reference.
You see the voltage and current output of a battery with
the DC meter selector in the BAT position or AUX BAT position.
If the battery’s charger has power, you see the output voltage of
the battery or its battery charger, whichever is more.
The main
battery charger is in the charge mode when you see a positive
DC AMPS indication while the DC meter selector in the BAT
position. Use the AUX BAT position to monitor the auxiliary
battery charger.
The main battery charger cannot go into the charge mode
during any of these conditions:
* Fueling station door open
* APU start
* Standby power switch (P5-5) in the BAT position
* Standby power switch (P5-5) in the AUTO position, battery
switch ON, and DC BUS 1 and AC TRANSFER BUS 1 do not
have power
* Main battery overheat.
The auxiliary battery charger cannot go into the charge mode
during any of these conditions:
* Standby power switch (P5-5) in the BAT position
* Standby power switch (P5-5) in the AUTO position, battery
switch ON and DC BUS 1 and AC TRANSFER BUS 1 do not
have power.
* Auxiliary battery overheat.