2004 MPV dies on low rpm

[maz-vic]

Batterie does not get charged in idle or low rpm while driving
I'm thinking loose belt but the tensioner sits on the mark in the middle
When i press on the belt in front of the tensioner I can easily push it down to the tensioner
On higher rpm voltage at the batterie measures not more than 12.7V

Anybody any suggestions?

Thanks in advance

[maz-vic]

Does anybody know how to change the alternator?

[maz-vic]

just replaced the belt -alternator is charging -get 14.7V at the batterie, but it is whining pretty loud and got hot in a few minutes
I could turn the pulley by hand when the belt was off, but it had a bit resistance

[maz-vic]

let the alternator cool down started the car again -whining is gone, but now it is not charging under 2000 rpm