I've had my 2006 Cadillac srx 4.6 with about 46k miles on it and for about 4 months now. I have not had any problems until now and I had a question for those who know Cadillacs. I left the radio on while the car was off. It drained the battery to where the starter wouldn't turn. I got a jump from the truck next to me and went home. A day or so later the battery was dead first thing in the morning. I charged it and the auto parts shop said the battery was good but alternator was bad. So this prompted a couple questions...is this about the time for an alternator to be out? Also, upon turning the key the volts will be at "X" and when i start the car it gets up to 14.4 volts which is normal. If the alternator is bad then why does it charge up to full power after starting the car? the battery only dies when the car is off from what I can tell. If this is an alternator problem then is this replacement doable or must it be done by certified mechanic?