An automated external defibrillator (AED) is a self-contained defibrillator device designed to be movable, and easy and simple to use. They are often shaped like briefcases so that they can be carried by a handle. An AED contains a battery, a control computer, and electrodes. When the electrodes are stuck onto the patient, the control computer will assess the patient, checking the rhythm of their heart. It will then charge itself to an appropriate power level and tell users that the person needs to be shocked. If the patient does not need to be defibrillated, the automated external defibrillator will not allow a shock to be administered. A button must still be pushed manually to trigger the shock, as the operator beforehand must be certain that nobody is touching the patient. Often, automated external defibrillators will have speakers which give instructions when they are opened.