Battery replacement intervals

This is the tenth in a series of units that will educate the reader on the part played by a battery in an uninterruptible power supply (UPS) system. So you bought a battery with a nominal design life of “X” years. The realistic service life is actually significantly less than that, but you don’t know what it is. At around the “half-life” (i.e., half way through the design life) an individual cell-unit fails, then another. You replace the cell-units with new ones (maybe under depreciated warranty, or maybe at full price). Service technicians come in to replace the cells. Each visit costs money and disrupts the service while the work is being performed. You’re told that the new cells create an imbalance with the rest of the cells in the string. At some point you must decide that the entire battery string needs replacement. How do you know when that time has arrived? How do you know if your battery will be able to ride through the next power interruption? Several factors must be considered, including: How many cells (or multi-cell units) are in the string? What has been the maintenance history of the battery? Has the battery been exposed to abnormal conditions, such as extreme heat? Are there other strings in parallel with this one? Do you have a monitoring system capable of taking ohmic measurements on individual cell-units? Do you have reliable records capable of comparing the current measurements to the original base-line measurements? What is the impact of “down time” when a battery or battery string is removed from service for cell replacement? What is the labor cost of cell replacement? How did the battery perform during the last discharge (or planned discharge test)? A lead-acid cell or battery is considered to have reached “end of life” [...]