If it sits for 5 days every week, the message will probably come back. Used to happen to me every day - second short journey of the day. Got a good battery (Bosch Silver S4027/069) and never saw the message again. Old battery was only 18 months old, but was cheap rubbish.
But before shelling out on a new battery, test the battery voltage with engine running - should be about 14 volts. If not, either alternator is on way out, or connections between alternator and battery are bad.