i have a rev. A macbook, and recently its been having some issues while on battery mode.. this has happened about twice, so i'm unsure ifi ts something to worry about. but on occation when its around 30-40% remaining, the power completly cuts out and it refuses to turn back on untill its plugged in. i even checked the battery on the underside, and it displayed 2 lights. could i have a problematic battery that needs replacing?