According to L3 Service Letter SL-80 (Rev. W), the condition of the batteries should be checked every 3 to 6 months according to the Periodic Check procedure outlined in the bulletin. If the "Voltage Level 24VDC LED" does not illuminate while the 20VDC is illumined, then the unit is incapable of providing emergency power. The "Cell Isolation Test" must then be performed to identify if the unit requires repair or charging.