The Alfa 147 1.6 shares the same battery as a great deal of other models from the same manufacturer. If you wish to change it yourself, you should look for a minimum of 45 Amp-hours (Ah) and the following techie bits:
B13 hold down
242mm by 175mm by 190mm
The Alfa 147s are very sensitive to the physical size of battery. They are held in by a non-adjustable strap which means that the size needs to be exact. Refer to this web site for instructions with photos:
Calcium lead acid batteries are more expensive but offer better performance than standard lead-acid. Halfords website offers good advice on changing batteries.