Why is my Subaru Forester losing it's charge?

-We have a Subaru Forester that makes a loud alarm sound that would seem to tell us that the battery's lost it's charge because we can't start it. But when the battery's been on charge all day it will start, but when I went to go out in it this morning, again it didn't start - although the engine turned over once then went dead. This alarm sounded once. Any ideas?Once batteries reach a certain age and usage they become incontinent and unable to hold a charge - sounds like your battery is about to die.
Time for a new battery, it's normal for a battery to stop holding charge. Just get a new one and all will be well.
Weak battery, most likely.