William didn't suggest there was something wrong with the battery, he suggested cleaning the battery terminals. The connectors may look fine, and actually be tight, but could have a fine layer of oxidization between the post and the inside of the connection. This will definitely cause the symptom that you are describing. Remove both of the cables from the battery, clean the inside of the terminals and the battery posts. Wipe both posts and the inside of the terminals on the cables with the battery grease, to prevent corrosion, then reconnect and tighten the connectors. You may also use those green and red discs on the battery posts. I have them on all my cars and they really do work, preventing corrosion. These discs and the grease can be found at your local auto parts store, where they sell car batteries.
I hope this helps.
