BOSCH alternators are known for their low output voltage due to how their internal regulators are set at the factory.
One fix is to install an adjustable internal regulator.

I have one and I can adjust the charging voltage to an acceptible level up to about 15V.
See http://davebarton.com/AdjustableVoltage.html

Yes, it says Volvo, but they have one that can be used on the Vanagon alternator.
I have the internally adjustable one.

Todd
'88 Westy

 -------------- Original message ----------------------
From: Greg Marshall <Mr.Marshall@SYMPATICO.CA>
> Hi folks,
>
> I have a couple of quick electrical questions.  First, what kind of voltage
> output should an alternator be putting out?  I read 13.5 on mine, which seems
> low to me. Second, What are the top culprits for current draw on the battery
> while the van is sitting?  My van has been sitting for three days, and the
> battery is down to 11.5 volts. It makes it a bit cranky to start at that
> voltage.  It seems to crank fine after I've driven it for a bit, but let it sit,
> and down the voltage goes.
>
> Thanks,
> Greg.
> 82 Westy