How do I assess capacity on a single-bank zone substation where a discharge test is impractical?

This is the real problem, and AusNet states it plainly: full discharge testing and discharge-rate testing are “impractical on zone substations with a single battery bank”, which “prevent[s] accurate measurement of the DC system available capacity.” Their response is a new standard requiring individual battery cell monitoring of voltage and impedance plus SCADA alarm monitoring, with research directed at estimating remaining useful life. The honest position: impedance monitoring is a trend indicator, not a capacity measurement, it catches open cells, dry-out and connection degradation reliably, and correlates with capacity fade, but it does not replace a discharge test. A second bank, or a portable load bank programme, is the only way to get a direct measurement.