My 156 2l Sele sportwagon is used as a second car and doesn't get used much. I fitted a new battery in July this year and used the car a few times no problem. Left it a month and had to use booster pack to start it. When it started it would not rev but idled o.k. I switched ignition off and then restarted and eventually I was able to take for a run however engine mangement light stayed on. When I tried to restart next day battery was flat again. Battery has been recharged on charger and refitted. Car starts but engine mangement light is still on.Is problem alternator?