system
Given the following post, respond as if you're commenting on Reddit:
human
How much voltage deviation is acceptable?

Like if you have an USB Stick, which needs 5V, but your System only provides 4.8V. Why does it still work?
Similarly, when is a battery 'empty' or 'loaded'? If you try to measure battery voltage, you get a result, but how do you find out, if the voltage is sufficient to operate the system?
gpt
Well, you can never get a voltage to be *exactly* 5V, because there will always be some noise and measurement error. So as a matter of good engineering practice, we try to design circuits so that their behavior is stable even if the supply voltage varies slightly. Of course, this has to be balanced against all of the other design requirements.

For example, digital electronics use transistors, which (to drastically oversimplify) switch on or off at a particular gate threshold voltage. But that's not an instantaneous on/off process; if you take a transistor that's just barely on, and decrease the voltage a bit, it will still conduct but not quite as effectively. Eventually you reach the point where the device doesn't function properly or at all, because some internal component is too far out of its expected state.

As another example, anything with capacitance (either capacitors that were explicitly added to the design, or the internal capacitance of other components) will charge/discharge more slowly when the applied voltage is lower. Some components (e.g. microprocessors) can operate at lower voltages if the clock speed is reduced.

The allowable voltage range of a particular component is one of many parameters listed in its datasheet; that's how the designer knows that they're using it correctly. As a general rule of thumb, most designs can tolerate at least a 5% variation in supply voltage, and some can handle a lot more.