My battery light keeps flashing in my 2001 Landrover Discovery. I put a new battery in it this past summer. I'm thinking its the alternator, what do You think?

I replaced the battery last summer in my 2001 Land rover Discovery, just this week while driving it to the store the battery light started flashing and my Land rover slowly died. I went to the junk yard and got a used alternator and put it on and , its still doing the same thing. If the battery is charged to full charge it will start and go but a couple of stops and starts again and Im back to square one. Tell me what I'm doing wrong please.