BATTERY CHARGE LIMITER (BCL) (continued)APU STARTINGThis topic explains the APU BCL functions and operation.The charge and discharge cycles are similar with BCL 1 and BCL 2.The main functions of APU BCL are to control the APU BATcontactor for the control, monitoring and protection of the APU BAT.When the APU starting sequence is initiated, the APU BCL closesthe APU BAT contactor 5PB. This provides power to start the APU.As soon as the APU MASTER P/BSW is set to on, the APU ElectronicControl Box (ECB) provides a signal to the APU BCL to closecontactor 5PB.If the APU Transformer Rectifier (TR) is operating, its output isconnected to 309PP. If the APU TR is not operating, contactor 7PUsends a ground signal to the APU BCL to increase the currentlimitation. This allows APU STARTING on battery alone.To prevent complete discharge of APU BAT during APU start, APUBCL opens contactor 5PB in the following cases:- if APU BAT voltage is lower than 9.5 V during 1.5 seconds,- or if APU BAT voltage is lower than 12 V during 15 seconds.