Carrier reefer 2500 X2 Alternator or belt problem

Hi everyone. My unit 2500 X2 from 2010 is giving me alarm A51 alternator not charging. First i noticed that the belt was worn out. I replaced it, but after 2-3 days I got the same alarm, a the new belt was worn out. I replaced it again, but same alarm came 2 days after. Took it to Carrier service, explained the problem and the technician told sme probably the alternator was not tight enough. Also checked the pullies and sad they looked fine. Now 3 days later, again with the same A51 alarm. I replaced yet another belt, but now the alarm wont clear. Just to mention the unit was 2 months not user prior to this issue. Anyone had similar experience or have a solution. Thanks