AGAINST
The APU gen may be thrown off-line and all youve done is drain the batt during the start.
45 seconds after the loss of both engine generators, the APU is available for START. If the EMER GEN is available, the APU start will not adversely effect the batterys due to the power being supplied by the EMER GEN.
If the APU GEN is then thrown "Off Line" no adverse battery drain would have occurred. This is the reason for
not attempting a start when "ON BATT" only.
Clarification: No adverse battery drain, meaning that the battery power used for start, should be replentished with the APU GEN running. The reason for not attempting an APU start on BATT only, is in the event that the APU doesn't start, thus decreasing the already minimal (30 min) battery life.