Voltage is a nominal measurement. The voltage is largely determined by the circuitry within the Electronic Components. The chemistry behind the circuit completion and provision for power supplies is the basis of measurement. Alkaline based batteries in comparison with lithium based batteries are specific to the size of volts that can pass through them. The specifications are therefore dependent on the main chemical component as well as the size and ultimate usage. The batteries are usually made up multiple cells in order to satisfy the need of a certain voltage load in the components. Usually the cells are connected together inside to make the particular load e.g. 6v or 24v.
The Power Supplies are pretty set in the requirements and usage. This is why a nominal voltage measure starts out as say 6v and drops down to 5.9v. It quickly drops to 5.8. When it slowly drips to 4v then its life expectancy is diminished and it has run its course. At this point many people toss out their gadgets or change the battery depending on which is cheaper. Rechargeable Batteries drop even faster than single use batteries. This is why many manufacturers currently produce their high consumption gadgets with reusable batteries in order for the customers not to feel short changed. The Doppler Effect is usually experienced in wavelength of these gadgets through the batteries in the circuit. The strength is heightened when they are in series and sequenced. The broken circuit ensures that battery life is thus extended.