That sounds like undervoltage condition as shakalnokturn said.
After you sort that out, you'll also need to make some changes to battery constant if you have a bigger battery and want the full run time.
Serial protocol info here:
https://networkupstools.org/protocols/apcsmart.htmlit's pretty straight forward to change the battery constant to something reasonable - SC1500I default battery constant should be 8F by the table I have:
Enter Y (UPS should respond SM);
Enter 1 (one, not el; wait 4 seconds);
Enter 1 (one, not el; UPS should respond PROG);
Enter 0 (zero, not oh; UPS should respond with current constant);
Write down the existing value so that if something goes wrong, you can at least put it back to that value;
Enter + (plus) or - (minus) to increment/decrement the value;
Enter R to reprogram constant value (UPS should respond Bye);
Enter Y (UPS should respond SM);
Enter 0 (zero, not oh; UPS should respond with the new constant);
exit terminal.[/li][/list]
You can also tweak the battery voltage measurement to ensure it doesn't float them too high.