Historically most clocks were either wind up versions, which meant that the owner had to wind it up each day to ensure alarm went off at the correct time, as new designs developed and the use of electricity as the power source came about many alarms were then powered by mains electricity. As technology developed and electronics became small and used less power the first battery powered clocks came about.
There are two main types of battery powered clocks, the first is the more traditional looking analogue alarms, these have the classic clock face with hour and minute hands, the second type are digital alarms, these have a digital display and normally have a lcd format. Most battery powered clocks have been designed to use AA batteries, normally they use 1 or 2, but some models can use up to 4 batteries in total.
The AA battery size was standardized by the American National Standards Institute in 1947, but had been used in flashlights and electrical novelties for some time before formal standardization. Battery nomenclature gives different designations depending on cell size and chemistry. For example, a zinc–carbon (Leclanché) AA cell is designated "15" by ANSI, and R6 in the IEC system.
The range of battery powered alarms is vast, and most modern designs use this versatile and flexible power source. The range includes travel clocks to full features radio alarms which as well as having an alarm feature can tune into your favourite stations to place music or listen to the news.
The smallest types of battery powered alarm are travel clocks, some are so compact that they do not use AA batteries but use the smallest batteries such as button cell batteries.
Many of the modern designs also have solar panels built in which can extend the life of the battery in the alarm. The batteries in these clocks are easily changed as there will be an easy accessible place to insert any new batteries.