Batteries come in different types, with the most common being Lithium-Ion (Li-Ion) and Lithium-Polymer (Li-Po). These are used in almost all modern laptops because they are lightweight, provide high energy efficiency, and do not suffer from the old "memory effect" found in older battery technologies. Li-Ion batteries are widely used for their balance of cost and performance, while Li-Po batteries are slimmer, lighter, and allow flexible designs, making them popular in ultrabooks and thin laptops. Older types like Nickel-Cadmium (Ni-Cd) and Nickel-Metal Hydride (NiMH) are now outdated, as they were bulkier, less efficient, and prone to memory effect. When choosing a laptop battery, it's important to check compatibility in terms of voltage, size, and connector type, while also considering the battery's capacity, measured in mAh or Wh, for longer backup time.