The battery voltage should be 12.6 volts. If it's less than that, it's "flat".
You can try to charge it up for a few hours and then re-test the voltage in the morning (it will read high, if you test it just after you removed the charger - it takes half an hour to give a "resting" reading).
If it's 12.6v then great.. if it's 11.6v then the battery is not holding the charge and should be replaced.
If the battery is kept in a low state of charge, the acid attacks the lead plates and damages them (it dissolves the lead plates). In High charged batteries the acid doesn't attack the lead... So if you're storing the car, disconnect the battery (to remove any drain from radios, immobilisors etc.) and then charge it a few hours every two weeks.
Ralf S.