If it is a maintenance free battery(most newer ones are) I think you may be going to buy a new battery soon. If I were you try to locate an auto parts store that will check your battery for free. I know around here AutoZone, Pep Boys, Kragens(or Checker) will check your battery and alternator for free. If the battery and the alternator test good, it may be as simple as a bad connection, wiggle the cables on your battery, if they are loose tighten them, check all connectors going to the alternator, grounds, etc. Of course this could also be from racing underdrive pulleys.