Since you have cleared the alternator of any guilt, next steps could be.

(1) Check voltage across batery terminals - engine running and not. touch the tops of the terminals, not the cables.

(2) Check wires between alternator and battery - is the charge getting to ther battery? The fat red wire from the alternator connects to the large stud on the starter. That wire seldom goes bad, but there's always a first time.

(3) Clean thoroughly the battery cables and lugs.

the voltmeter is showing a full charge when the car is running but showing nothing when I'm trying to start it, it can't be the alternator again can it?

That depends on how the voltmeter is wired in. I am assuming you have an in-dash Volvo voltmeter, right? If the hot side of the voltmeter is connected to fuse #1, #2 or #3, that would give the zero reading during starting. Those fuses are not powered when the key is in Postion III.

Good Luck,

Bob

:>)