Auxiliary power supplies include a capacitor (e.g., super capacitor) and a capacitor charging circuit, which is configured to provide a charging current to a first terminal of the capacitor. Enhanced failure detection is provided by a capacitor monitoring circuit, which may be electrically coupled to at least one terminal of the capacitor. The capacitor monitoring circuit is configured to detect when the capacitor is malfunctioning in an open condition as well as when the capacitor is malfunctioning in a short condition.