Battery bank capacity testing methods

The only real way to know your battery bank's actual remaining capacity (versus its rated capacity when new) is a proper load test or a controlled discharge test with a shunt tracking amp-hours removed until cutoff voltage. Voltage alone under no load tells you very little about true capacity, especially on lithium which holds a flat voltage curve until it's nearly empty. Want to walk through a proper discharge test method, or are you trying to diagnose a specific suspected capacity loss?