Car won't start after sitting for a week – battery or alternator?

[jimmyjanny]

I left my 2012 Nissan Sunny parked at the airport for 8 days while I was traveling, and when I came back, the engine barely cranked and then just clicked. Got a jump start from a friendly guy and it ran fine all the way home. But now I'm paranoid it's going to die again. The battery is about 2.5 years old and I'm in Deira so I need something local. I'm looking for a 50AH Battery Replacement in Deira Dubai that won't overcharge me just because I'm in a hurry. I saw Amaron's battery page and they seem to have options. My question is, how do I know if the battery is actually dead or if my alternator isn't charging properly? I don't want to buy a new battery if the problem is something else. Also is 50AH the right size for a Sunny? I've seen different numbers online. Any recommendations for a mobile battery service that comes to Deira and tests the charging system before selling me a battery? Would really appreciate some advice from people who've been through this. Thanks.