My car (a 1991 Citroen BX 1.4) has been giving me alternator problems since I got it, the alternator failure came on every now and then and dimmed or disappeared when I revved the engine, my first move was to change the 10 year old battery.
Now one day my car didn't start and battery reading was 11v, so I decided to change the alternator and, as my repair guide specified, I fitted a brand new 50A alternator.
Now my car responds better but every now and then my lights will dim to a 10% brightness, windows roll up really slow, until I rev the engine ever so slightly and everything is completely back to normal as if my battery was completely charged, in laymans terms, one second of the engine revving every now and then is all it takes to keep my electrical system happy.
Still when parked my car's battery reads 11.8-12.5v and 0 milliamp draw.
It's no mystery, that's not how a car should behave, but I'm completely out of ideas here.