My Macbook Pro is exhibiting the exactly same symptom as I am typing this now; around 57% down to 7% instantly like in a second. I only noticed this as the "Low Battery" sign popped up at a corner. I don't think this is due to a faulty battery but the computer was programed to display a faulty reading the level. If 50% of energy had gone instantaneously like this, the computer should have caught in a big fire with an explosion. Apple does these stupid things, like making the device rejects third party parts or even real oem replacement parts if they are not installed by Apple technicians. This effectively prevents informed customers from self upgrading their Apple devices.