A rechargeable battery, also known as a storage battery, is a group of two or more seconary cells. These batteries can be restored to full charge by the application of electrical energy. In other words, they are electrochemical cells in which the electrochemical reaction that releases energy is readily reversible. Rechargeable electrochemical cells are therefore a type of accumulator. They come in many different designs using different chemicals. Commonly used secondary cell chemistries are lead and sulfuric acid, nickel cadmium (NiCd), nickel metal hydride (NiMH), lithium ion (Li-ion), and lithium ion polymer (Li-ion polymer).