A CE680 Captain discussed the DC GEN OFF left and/or right and/or APU CAS procedure. The Reporter contends the case of the APU not being available should be considered earlier in the procedure because of a potential for rapid battery depletion rate.

Narrative

CE680 AFM revision 9 dated 27 May 2010; Red CAS message DC GEN OFF left and/or right and/or APU is written incorrectly. Specifically: 1. Procedure divisions throughout this checklist are incorrect. 2. Sub-divisions (A) IF NEITHER GENERATOR COMES ON LINE and (B) IF ONLY RIGHT GENERATOR COMES ON LINE under division IF EITHER OR BOTH GENERATOR (S) VOLTAGE IS EQUAL TO OR LESS THAN 29 VOLTS are written incorrectly. They both assume that the APU can be started and that it will supply electrical power. If the APU cannot be started (APU MEL'ed; will not start; too high to start); then electrical power will be depleted at a rapid rate causing an unsafe condition. Following the checklist in case (B) (assuming APU is not available) will result in the left bus receiving power from the battery only and the right bus receiving power from the right generator. It should say that the bus tie should remain closed if the APU in not available. Case (A) should have you follow the steps for IF VOLTAGE IS GREATER THAN 29 VOLTS ON BOTH GENERATORS. Have the manufacturer correctly write this procedure.