Convenient and portable source of electricity are usually know as batteries, although most of the should really be called " power cell " . Strictly speaking, a battery is a set of power cells used together. Of the examples shown here, only the car battery is the true battery as it contains several cell in one case. All the other are single power cell. most batteries, including the commonly used alkaline manganese cell and most bottom battery, must be replaced when the chemicals they contain no longer provide electricity. other batteries, such as nickel-cadmium cell and lead acid car batteries are rechargeable and can be used over and over again.