Wireless bell with self-contained power
Wireless doorbell for an apartment : Wireless Bell With Self Contained Power
Quote from Article entitled Wireless doorbell for an apartment:
Their disadvantage they have is that you need to keep your eye on the battery level or accumulators charge and change them from time to time. Element of power in the button, as a rule, is durable; it does not need changing more often than once every few years. The response device consumes more power, so the battery changing will be needed more often: once every few months, depending on usage. Therefore, choosing a bell for a door, it is better to prefer devices with a socket plug.
The full version of the article can be found here.