Battery capacity tests evaluation for stand-alone photovoltaic systems

Charge controllers are included in most photovoltaic systems to protect the batteries from overcharge and/or excessive discharge. However determining the state-of charge of a battery is not easy because most battery systems utilize multiple battery cells in series, e.g. 48 and 120 V battery systems will normally consists of 24 and 60 cells in series, respectively. One controller is normally used to detect the voltage of the entire cells. While the intent of the controller is to charge each individual cell equally, voltage variations can occur from cell to cell for a number of reasons. The problem gets more difficult if the effect of age is considered. The voltage variations on individual cells over the life of a battery will increase as more efficient cells will tend to have a higher voltage, and less efficient cells will tend to have a lower voltage. The cells with low states of charge will limit the entire battery cell's performance. The most important feature of battery maintenance is the detection of pending faults and failures before they become catastrophic. In this paper we will discuss a practical approach to evaluate battery capacity test that gives an exact result about the condition of battery cells. A computer-aided battery capacity test evaluation which gives well ordered data sheet, shows the selected values in diagrams, calculates the partial and final results in the same way, so the results of different measuring series will be comparable. The program is able to compare the results of annual capacity test to regroup the battery banks in series/parallel connections based on their performance to prolong their lifetime. This result will be converted to savings for the maintenance engineer to achieve in operating the photovoltaic systems.