NiMH battery management circuit
It's useful to have a device that can operate on the power from a battery, and preferable that the battery doesn't need to be replaced. So, we use rechargeable batteries in many of our devices, from smartphones to medical thermometers.
There are many types of rechargeable cells, the best and most popular being nickel metal hydride (NiMH) and lithium polymer (LiPo).
Some devices can charge the battery in place, while other devices require a separate charging device.
In either case, a circuit exists that monitors the status of the battery, adding charge according to a program. The program usually depends on multiple factors, including the available power, the voltage of the cell(s), the temperature of the cells, and the increase over time of the voltage and/or the temperature of the cells during charging.
There are integrated circuits that can handle managing all that monitoring and charging, requiring a few extra discrete components. For example, Texas Instruments produces the BQ2002, available in an 8-pin DIP package (useful for teaching with breadboards). http://www.ti.com/product/BQ2002/description