Have a little Giant Sump Pump Backup System (SPBS-12). Been working fine for years, all of a sudden the Alarm keeps going off. If I hit the reset button, it stays off for a minute or so and then starts screeching again. The battery charge indicator LED on the charger is green (charged). The battery itself has a charge indicator and it is also green (70-100% of full charge). I have checked that there is power to the outlet. If the battery is OK and there isn't a power issue, what else could be causing the Alarm to go off?

Garden - Little Giant - Submersible Sump Pump, 1/4 Hp