battery warning light

Hi I had a new alternator put in a few weeks back and since then after a couple of weeks my battery light comes on not as soon as i start my car but after I've been driving for a while.I've replaced the battery but problem still not gone away.I think the alternator is working as when car is stationary the volt is 12.5 and when started goes to around 13.7 . It seems to me that the alternator for what ever reason isn't working or charging properly whilst driving but I'm no expert and new your help thanks