What is an indication that a battery may be failing when running?

A voltage below 12.0 volts indicates that a battery may be failing when running because it suggests that the battery is not holding a sufficient charge. A healthy, fully charged battery should ideally measure around 12.6 volts or higher when at rest. Values consistently below 12.0 volts can reflect a deeply discharged state or deterioration in the battery's ability to hold a charge, leading to poor performance or a complete inability to provide adequate power for operation. This indication is critical for diagnosing battery health, as it can lead to malfunctions in the equipment powered by the battery, ultimately affecting the overall performance and reliability of the system in which it is installed.