Alternator testing/replacing with supercharger

[Campbell0240 0]

Hi guys, i've recently replaced my 2.9 vr6 engine out f my corrado with a 2.8 stage 2 supercharged lump.

 

I replaced the battery at the same time and its lasted some time but recently as i've ben driving back and forth the garages its started to lose charge it seemed to completely run out and when i started it after charging it for 15 minutes it didnt seem to last anymore than 5 minutes it would cut out as the car was going?

 

The garage put a voltmeter across the battery to show it losing charge i just wondered how i test the alternator or any common problems that occur?

 

I do have a spare alternator off my 2.9 vr lump but i believe its a 70A not a 120A thats on the 2.8 i've put in? would that be sufficient?

 

Which leads to the next question how do i properly tension and release tension from the SC on the belt?