The Battery full charge alarm circuit for a general battery charger. It is simplified circuit and cheap, use one transistor BC557 and display with 2 LED.
It can charge any size of the Gel cell batteries and extend the life of the Gel Cell battery. While the circuit is running, the LED indicates charging.
Modify cheap mobile phone charger inside is simple DC switching regulator circuit and can apply many ways, such as DC power supply
This is how to modify old Lead-acid battery charger or convert power supply to battery charger in automatic. To protect over charging by CA723 and power SCR