I want to buy an extra charger for my 2012 cMBP so that I always have one at school. I don't care about charging as fast as possible, but I don't want to use anything that is "not recommended" by Apple. Do I have to go with a 2012 charger? I don't mind buying an old used one, how far back in time can I go? And do I have to stay with MBP charger, or can I go for the MBA charger (should be cheaper since less watt)? Thanks!