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The battery backup is designed to maintain a fully loaded xPM for a minimum of 20 minutes. It will shutdown when the voltage reaches 38 volts to prevent the power supply from going out of regulation and an alarm will be generated. Rechargeable batteries will lose their full charging capabilities over time and will need to be tested and replaced when they fall below 60 percent of their original capacity. The battery backup has been designed to operate in standby (float) service for approximately five years. The five years is based on the battery being kept at 20C (68F) and the float charge voltage being maintained between 2.25 and 2.30 volts per cell. This includes the battery being fully discharged once every three months. No battery should be left in service over five years, and if no maintenance is done it should be replaced every three years. The service life is directly affected by the number of discharges, the depth of discharge, ambient temperature, and the charging voltage. The expected service life can be shorted by 20% for each 10C that the ambient is above 20C. The batteries should never be left in a discharged state. This allows sulfating to occur which will increase the internal resistance of the battery and lower its capacity. The self-discharge rate is about 3% per month at an ambient of 20C. The self discharge rate doubles for each 10C in ambient above 20C. The discharged voltage of the battery should never go below 1.30 volts to maintain the best battery life. With this in mind it is recommended to periodically load test the batteries to ensure they have sufficient capacity to maintain the system during a power outage. Tests should be done on an annual basis and more frequently as they become older and begin to lose capacity. The load test is recommended off-process if possible as there will be no battery backup available while performing the test and recharging of the battery pack can take up to 16 hours. Having a spare available to swap, especially if doing on process, is a wise option leading to minimal time without battery backup and allowing the tested battery to be recharged on a bench outside the system for future swap with the next test. If regular maintenance is not performed the recommendation is to change at least every three years rather than every five.