Alternator, Battery, wiring?

Hi, I've got a 1987 Olds Cutlass Supreme with the 3.8L engine.
Recently having problems with what I thought was the Alternator. Car was draining the battery while driving.

Had the Alt tested and it was GOOD!
So I had the Battery load tested and it came back good.

Here is what I personally have found.

Tester on the batter with the car running. 11.5 volts.
Tester on the Battery Ground and the positive on the Alt. 11.5volts
Tester with the Ground on the ALT and Positive on the ALT 15volts
Tester with the Ground on the Alt and Positive on the battery 15volts

I did this with the headlights running and the car at idle speed, I believe about 650 or 750RPM's.

Anyone tell me what is going on. Why do I keep draining the battery.
Is my Ground from the battery to the Engine bad? Should I replace the ground battery cable?

Or the positive cable? Or both?

Anyone that can help...please do... I have no freedom as I can only get to work and to home, and have to charge the battery everyother day or I am not going anywhere.

Thanks
Hearts88