Why won't my Lithium-Ion Smart-UPS start when the battery is first connected when there is no utility power to the unit?
When you first connect the battery there is a delay before the unit can start on battery with no utility power present.
SMTL750RM2UC, SMTL1000RM2UC, SMTL1500RM3UC - All Serial Numbers
If the UPS is not connected to AC Power, then there will be up to a 1 minute delay between the initial connection of the battery and the UPS being ready to start.
Wait 1 minute before pressing the ON key, or connect the UPS to AC power to eliminate the delay.