Before I get flamed, I would like to point out that, yes, I have searched
the archives (and got a lot of good info from there!).

By now we all know that a standard battery isolator drops the voltage
available to charge the batteries. In a Vanagon this leaves too little
voltage for the battery(ies) to be fully charged.

We also know that the simplest method of installing two batteries (apart
from wiring them directly together!) is the Hella 15A relay kit (with the
thin wires limiting the current to 15A). In my mind this 15A limit causes
two problems (most drivers will not find these to be insurmountable):

1. it takes a long drive to recharge a depleted auxiliary battery
2. it is impractical to run all accessories from the auxiliary battery
(which is what I've done on previous vehicles, headlights and all!).

Another solution is to modify the voltage rectifier on the alternator to
compensate for the voltage lost through the isolator - although some have
concerns about the alternator running hotter for two reasons:

1. it is putting out a higher voltage
2. energy is lost in the battery isolator - this energy has to come from the
alternator.

One other solution suggested in the archives is a dual output alternator -
one with two sets of coils on the stator. The only negatives I've seen in
the archives is the expense of these beasts and the fact that if one battery
is fully charged half of the alternator's capability is going to waste.

Now this has got me thinking - surely a far simpler way of building a dual
output alternator would be to have only the one set of coils, but two diode
packs? And couldn't a standard alternator be easily modified to achieve this
end?

Unless I get advice which sinks this idea, what I'm going to do is to tap
into the three-phase wiring prior to the diode pack. I'll have three wires
coming out of the alternator and going into an external diode pack made up
of three high current, low voltage drop, diodes. The cathodes of these
diodes will be connected to a terminal serving as the alternator's second
output - to be connected to the auxiliary battery.

I see this method as having several advantages:

1. The main battery's charging system will remain unaltered.
2. The two batteries will be properly isolated - reverse current will not be
able to flow through either diode pack, so a flat auxiliary battery will not
be able to discharge the main battery.
3. The full output of the alternator will be available to either battery.
4. Less current will be going through the existing diode pack, meaning the
alternator will run cooler.
5. Current will only pass through one diode on its path between the
alternator and a battery or accessory (avoiding the double voltage drop
encountered when using a standard isolator).
6. At about $60 for the parts this is still reasonably inexpensive (although
more expensive and much more work than the relay kit).

So, fellow listees, why should I not proceed?