What happened
During the base leg of the approach, the crew attempted to start the auxiliary power unit (APually). Following the call for gear extension and flaps 15, the aircraft experienced a total loss of electrical power as the landing gear and flaps were extended. During this period, the APU failed to start, and the battery voltage indicated between 17 to 18 volts.
The flight crew performed checklist procedures in an attempt to restore power, but they were unsuccessful. Consequently, the crew initiated a go-around and completed a subsequent landing without further incident.
The investigation
An examination of the aircraft's electrical system revealed that the cockpit DC voltmeter was providing an inaccurate reading, appearing 2 volts high. Investigators determined that the battery had been discharged due to a loss of electrolyte.
Additionally, the investigation identified latent shorted failures within the blocking diodes of both the No. 1 and No. 2 generator control units (GCU). While the specific failure mode of these diodes could not be determined, their condition led to an excessive current draw from both GCUs during the crew's attempt to start the APU. This surge caused an immediate overload of the internal AC power supply input fuses within each GCU, resulting in the loss of both generators and the subsequent complete discharge of the battery.