Lithium batteries usually come in two shapes: cylindrical and square. The inside of the battery is a spiral winding structure, and a very fine and highly permeable polyethylene film separator is used to separate the positive and negative electrodes. The positive electrode includes a current collector composed of lithium cobaltate (or nickel-cobalt lithium manganate, lithium manganate, lithium ferrous phosphate, etc.) and aluminum foil. The negative electrode consists of a current collecting electrode composed of graphitized carbon material and copper foil. The battery is filled with an organic electrolyte solution. It is also equipped with a safety valve and a PTC element (part of the cylindrical type) to protect the battery from damage during abnormal conditions and output short circuits. The voltage of a single lithium battery is 3.7V (3.2 for lithium iron phosphate positive electrode V), the battery capacity cannot be infinite, so single-cell lithium batteries are often processed in series and parallel to meet the requirements of different occasions.