Flat Battery 05 320

[packman10_4]

Hi people , ive just got a 05 320 petrol est , it was ok for a few weeks then the battery died just would not hold a charge. Ok not a problem got the correct size battery 80amh 640 v and fitted that this was ok for 4 weeks then again last night well 3 am this morning the alarm went off and the battery is dead as can be.

anybody heard of this before ? if so what the deal as im getting racked off.