Float voltage measurements on a cell or monobloc level are measured to ensure correct charging. Any cell that has a significant deviation of the float voltage from the mean of the string may indicate defects but will not provide any indication of the battery capacity or the remaining life.
Measuring the float current, the ambient temperature and the battery temperature can help prevent a possible thermal leak in order to avoid a consequent risk of plant safety. The float current may increase if one of the elements goes into short circuit, or will decrease due to the sulphation of the cells, as the batteries age as they approach the end of their life cycle but it is not a sensitive indicator for the state of charge of the battery but will indicate information for the state of health of the same.
The test to define the exact capacity that is still able to maintain the battery is through the complete discharge of the same and that determines its battery performance compared to the data provided by the manufacturer, generally in terms of capacity measured in Ah. To do this requires external load banks, the work is difficult and the battery under test must be disconnected from the load. In these situations, the system is unable to backup any mains disconnections with consequent discomfort for the effectiveness of the system.
The type of discharge test that follows this method with a discharge depth > 90% (DoD) provides a reliable measurement of battery performance. Usually this type of test is suitable for laboratories and not to be carried out in the field.
An alternative to the full discharge test is to perform a partial discharge using the system load. This technique usually involves disconnecting the primary power supply from the loads, moving all the system load to the battery. Battery performance is generally assessed "during discharge" since the depth of discharge is often too different to allow comparison with the performance data published by the manufacturer. Partial discharge can estimate residual capacity by extrapolation if the disconnection voltage is supplied by the manufacturer while not providing absolute precision.
The best solution is to always keep the battery disconnected from the load, and to be able to control it in charging and discharging through a microcontroller that stores its charging and discharging current and voltage and which is able to establish the accumulated capacity through algorithms in continuous self-learning.