What is the fine-grained topic of the following text: "Some personal computers sometimes are being left on simply to serve as power supplies for the charging of the aforementioned portable devices via connections, such as Universal Serial Bus ("USB") connections, that provide power in addition to data (rather than charging those devices from the household electric service using their dedicated chargers), even though the power supply of a personal computer is much larger than is needed for such a function, and as such draws much more power than such a function would otherwise demand. Only return the topic.
Using personal computers as power supplies for charging portable devices