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My MacBook Air will NOT hold a charge. When it’s plugged in the light on the charger is really faint. Occasionally I can get it to turn on and it’ll stay on and work fine. Usually it won’t startup completely before it dies and it has to be plugged in to work.

Review this Apple T/N Apple Portables: Troubleshooting MagSafe adapters Often time a bad cord or connector can be the cause. Also make sure the charger is a real one! OEM MagSafe Chargers vs Cheap Imposters: Teardown for Truth Fakes can kill your system! Lacking safety features, cheap MacBook chargers create big sparks