2005 Vauxhall Astra infotainment system drains battery

i got a astravan had it for 6months ish i was on my on my way home driving along the engine managment light came on the lights went dim the needles were bouncing about power steering went but stayed running turned it of went to restart after 10-15 mins and battery was dead had it looked at and the garage done a voltage test on it and said the battery didnt have any ankerage amps or summat along them lines so i said replace the battery then it done the exact same thing so got a higher amperage battery and still the same problem continues but this time the engine died and refused to start whilst rolling tryed to bump but no joy and i just had a brand new genuin alternater fitted was ok for about 2 days now back to square 1 =( sorry about the essay

all the help i get will be much appreciated
kind regards
danny