Rechargeable battery packs have multiple cells connected in series or parallel to form a single unit. These packs are found in smartphones, electric vehicles, and many other devices. They can be used multiple times which makes it more convenient than replacing batteries all the time. Different types of lithium-ion and nickel-metal hydride chemistries are used in rechargeable battery packs and they differ in energy density, lifespan, and cost. Protection circuits to stop overcharging and over-discharging are also commonly included.