This is probably one cheap electronic device that is common in most Indian homes. Mostly stops working completely in a year’s time. Consists of an AC input section with a capacitive dropper and indicator LED followed by a 4-diode bridge rectifier that feeds directly to the battery. There are different battery options mostly 4V. I seem to have a 2.4V(2 Cell Rechargeable 2.4V 600mAh) one. There is no protection for charge current manipulation here(to save a few cents there). Probably the reason why these batteries don’t last more than a few recharge cycles.
This is followed by the high voltage section with a feedback transformer driving a transistor followed by a slightly weird voltage multiplier section to generate a high voltage which gets stored at a large capacitor at the end. Some units come with a discharge resistor to discharge that capacitor(So that there are no accidental zaps). This high voltage section is connected to 2 metal outer grids and 1 inner metal grid in the bat. When a mosquito comes in contact with the inner and outer grids, this capacitor energy is discharged across the insect to burn it to create the classic pop sound.
The most common thing which goes bad in these devices is the output capacitor(you can test to see if there is a very small spark across the terminals) or like in my case here, the battery loses its retaining capacity. You can test by charging it fully on AC, then with a metal part(insulating the section you hold) discharging the grids, you will hear a classic pop initially then it will go down in sound after 3-4 tries as the battery discharges fast. Replace the battery in that case.