Modifications in the charger circuit for 4,6 & 8 volts

Hello sir :o

Here i ve got a charger circuit diagram for 12 volts battery . .

Attachment 7255
Now i want to design the same thing for

1) 4 volts
2) 6 volts
&
3) 8 volts

So what modifications does component's value need in above 3 cases ?

please tell ! thanks :)

Attached Images