A battery bank rarely announces that it has lost capacity. It is found at the moment it is needed, or it is found by measurement.

A maintenance visit records float and ripple voltage, per-cell or per-block voltage, connection resistance and torque, electrolyte level and specific gravity where applicable, ambient and cell temperature, and physical condition.

Out-of-tolerance cells are flagged with the measured values, and connections are re-torqued to the manufacturer's specification.