This is the issue with my Mechman alt. When I'm in drive, my engine idles right below the idle turn on speed, so if I'm at a stoplight or stop sign, I get no charge (but why would I need a charge at those places anyway?) Once I rev my engine, RPM"s hop up and boom, I have a charge.
In park however, I've got the RPM's turned up a bit so it idles at the turn on speed of the alt, so in park (when giving Demo's and whatnot) I get charging.
This is the issue with putting big alts on small engines. =/