I tried some of the small mini alternators and I found about the same thing on all of them. I even have one for sale. None of the ones I tried will charge or provide even voltage at an idle. If you raise engine rpm, they start charging, but then it falls off again at idle.

My full size 100a unit works very well, but its heavy. I wish I could run a mini, as I have a really nice one for sale, but maybe I will try one of the Mancini 60a units.

'70 Cuda,...605 EFI Hemi Street Car (6.20 best pass, 1.33 60ft)
https://www.youtube.com/watch?v=WYw6RA-k5Bk (6.25 at 108.75mph from inside car)
https://www.youtube.com/watch?v=3zQEb9uxFng (6.25 at 108mph from outside car)
https://www.youtube.com/watch?v=JCvfzsC4NgM (9.9)

'66 Barracuda AWB Stretched nose Blown 440 Car in build stage

'71 Duster Drag Car 400 Low Deck 512 best 6.002 at 115.44mph
https://www.youtube.com/watch?v=Znuo3jMUXTk