Batteries in clocks

Most clocks are designed to work with one or two standard Alkaline batteries .
Typical specifications of these batteries are:
LR6
Size AA
Voltage : 1.5V
Using rechargeable batteries may be cost effective in the long term.
However, the rechargeable NiMh batteries available do not usually fit into the slot in most clock models, so rechargeable NiCd batteries, which are slightly smaller in size have to be used.