Rechargeable batteries are batteries that can be restored to full charge by the application of electrical energy. They come in many different designs using different chemistry. They are also called storage battery or secondary cell. Attempting to recharge non-rechargeable batteries may lead to a battery explosion. Additionally, one must be careful to recharge a rechargeable battery before it is completely discharged, or reverse charging may occur. Contradicting the previous point, some kinds of rechargeable batteries need to be fully discharged every so often to maintain the ability to deeply discharge. There exist fully integrated battery chargers that optimize the charging current.