You are drawing conclusions that are not yet justified by your reported
tests. The voltage you should measure under load is the voltage at the
alternator. If the alternator has sufficient output voltage but that
voltage is not reaching the batteries then your problem is in the
wiring. You don't say what year this is but VW increased the capacity of
the charging wiring several times over the years. This area is a real
weak spot in all but the latest years. Easily improved if that proves to
be some or all of your problem.

Mark

Geza Polony wrote:

> Here's another wrinkle in the whole equation (to mix a metaphor): the
> voltage regulator/alternator system doesn't seem to be adequate to keep the
> batteries charged.
>
> Under load (fridge, AC, headlights, etc. on) my system barely generates 12.3
> volts at the vehicle battery, and about 11.8 at the aux battery. This is
> with a rebuilt alternator and new vehicle battery (a week old on both.) With
> no load, voltage is normal, about 13.8- 14 volts.
>
> In other words, there's not enough voltage to charge the batteries, no
> matter how you configure them, while driving.
>
> Those of you who have experimented with variable voltage regulators, what
> have your results been? There seems to be consensus that Bosch regulators
> are "lazy" when it comes to dealing with load, and that's what I'm seeing.
>
> If you're using a battery charger on the bench (or bedroom floor) to test a
> system, it doesn't seem like you'd get the same results as in the real
> world, where your alternator is trying to power umpteen things and charge
> two batteries at the same time.
>
> Maybe we could eat that cake at a free lunch, to mix another metaphor.
>