See 4 LM317 Lead-acid battery charger circuits for 6V, 12V, and 24V battery. With automatic charging and full charged Indicator using TL431. Easy to build.
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
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.