Here is a Battery backup circuit with regulated charger. The circuit immediately takes the 12 Volt load in the event of a mains failure. The circuit also has a constant voltage and current regulation section and a battery full indicator. This kind of circuit is used in UPS and Inverter. The circuit is designed for 12 volt battery but it can be used for 6 volt battery also with slight modification
1. From the power supply 15 volt 1 Amps enters into the circuit.
2. Transistor T1 is used as a constant Voltage and Current regulator. The 14 volt Zener diode provides fixed 14 volt bias to the base of T1, so its output will be always 14 volt. Resistor R1 fix the charging current from T1.
3. Back-up section has only two diodes D1 and D2. When mains power is available, 15 volt DC passes from the power supply, so D1 forward biases and provides 14 V DC to the load as well to the battery for charging. When power fails, D1 reverse biases and D2 forward biases to take up the load using battery power.
4. Battery status indicator consists of Pot VR1 and T2. Adjust VR1 till LED lights at 13.8 volts in the battery. So it will show Full charge when battery attains 13.8 volts.
5. For a 6 volt battery, simply change the Zener value to 8.2 volts. All other components remain same.