Issue: Alternator not charging at full capacity.

Before you say its the alternator, i've Replaced it with a new one. This is the cars 4th alt. The original stopped working. Replaced with a used one. The used one didn't work. Then replaced that used one with a new one. The new one didn't worked. Replaced the new one with another new one. Same deal.

When i manually charge my battery, it drives for about a day or two, and then the battery dies again. If i drive at night, the battery will die that night. (Because of Head Lights)

The alternator is putting out a hair over 12 volts. And when the lights are on, it puts out a hair under 12 volts.

So, if its not the alternator, what could it be?

I've been dealing with this for weeks. Please help!

Thanks in advance!