The nine-volt Akku, or 9-volt Akku, is a common size of Akku that was introduced for the early transistor radios

The nine-volt Akku, or 9-volt Akku, is a common size of Akku that was introduced for the early transistor radios.
It has a rectangular prism shape with rounded edges and a polarized snap connector at the top.
This type is commonly used in BabySonic fetal doppler