12V "23A" battery.
Also known as GP23A, 23AE, A23, V23GA and MN21.
Note - "23A" is the part number for the battery, it is NOT 23 Amp (a 12 Volt 23 Amp battery would be the size of a shoe box).
Measures 28.5mm long x 10.3mm diameter.
This battery must not be confused with the similar single-cell 1.5 Volt N battery.
The higher voltage is necessary for the radio frequency-transmitting and receiving devices which A23 batteries often power.