Yes, you can charge with the same transformer, but not 8v. What you need is a primary voltage control, either variable or fixed. This is usually done using a triac or 2 SCRs and another tapping in the transformer (I used 140v). The control of the triac/SCRs is done using phase angle control with either opamps (a complicated circuit) or if you can use microcontroller, then much simpler with a microcontroller. On the other hand, you CAN use a fixed voltage tapping, ie input 220v into another winding (eg 180 or such) adjusted such that the output is 12 or more voltage. However this would change the charging current with changing input voltage, so thyristor based is better.
Hope this helps.
Tahmid.