Which factor can affect the APU’s starting capability?

The starting capability of the Auxiliary Power Unit (APU) is significantly impacted by low battery voltage. The APU relies on electrical power from the aircraft's batteries to initiate the starting process. If the battery voltage is low, there may not be enough electrical energy to power the starter motor effectively. This situation can lead to a failure to start or an extended starting time.

High ambient temperatures can have varying effects on engine performance, but they do not directly impair the APU's ability to start as significantly as low battery voltage does. Regular maintenance is crucial for the overall health and performance of the APU, but it does not influence the immediate starting capability at a given moment. Excess hydraulic fluid may cause operational issues elsewhere in the aircraft systems, but it does not directly relate to the APU's ability to start; hence, it is not a primary factor in the starting process. Focusing on battery voltage provides insight into a critical aspect of APU operation that needs to be monitored to ensure reliable starting under various conditions.