Date:         Fri, 26 May 2000 17:58:15 EDT
Reply-To:     Ssittservl@AOL.COM
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         S Sittservl <Ssittservl@AOL.COM>
Subject:      Re: wiring auxiliary battery
Comments: To: Durbys@plaid.com
Content-Type: text/plain; charset="US-ASCII"

>  I have an 87 Wolfsburg Vanagon (not a camper) with only the one starting
>  battery.  If I were to add an aux/house battery, could I wire that battery
>  in series with the starting battery, BUT in between use a battery isolator?
>  I don't see how I could wire them in parallel if I use a battery isolator
>  between the two batteries.
>
>  Durby
>  87 Wolfsburg Vanagon

You don't want them in series.  Think of it this way:
each battery puts out 12 volts more at its "positive"
terminal than it takes in at its "negative" terminal.
So, if you hook the "positive" of one to the "negative"
of the other - that is, you put them in series - then
the second one's going to boost the output of the first
one by an additional 12 volts, for a total of 24.  This
will make all your stuff that's expecting 12 volts (fridge,
lights, radio, etc.) melt, catch fire, or do something
similarly exciting.  (This isn't world's most precise
description from a physics standpoint, but hopefully
it gets the idea across.)

So, you want them in parallel.  The "negative" side is easy:
all the negative sides of pretty much everything in your van
are all simply connected together (using the van's metal body
as a big fat wire), and that will be true of your batteries, too.

For the most part, you only need to think much about the
positive side.  That makes things easier, because now
you've only got one wire/terminal/etc. to worry about.
You can think of this kind of like the plumbing in your house:
faucets, valves, meters, etc. all generally go on the "incomming
water" ("positive") lines.  The "drain" ("negative") lines require a
lot less thought - just make sure everything dumps into some
adequately-sized sewer pipe that leads eventually to the main
house sewer.

So, now we've got the two batteries in parallel (positive to positive,
negative to negative).  The pair together put out 12 volts, but (if
they're identical batteries) has about twice the capacity of one battery
alone (that is, they would run the same "load", such as a light,
for about twice the time that one battery would).  Simple enough.

We have two problems, though:

(1) They probably aren't really identical batteries.  If they're not,
    they won't charge and discharge identically, and strange bad
    things can happen which will tend to damage the batteries.
    (Imagine a big, fast horse and a small, slow horse yoked
    side by side, pulling the same wagon, and each trying to
    walk at their preferred speed.  You've got unhappy horses.)

(2) We weren't really trying to just build a bigger battery.  We
    really want to generally run some things off of one battery,
    and other things off of the other.  For example, we generally
    want to run the starter and headlights off of the original battery,
    and the interior lights off of the auxilliary battery.

We can take care of both of those by running completely
separate circuits on the positive side - the original battery
leads just to the stuff it's supposed to power, and the aux.
battery leads just to the other stuff.  No connection between
them.  (Except, of course, for the shared-by-all negative wiring.)

Doing that leaves us with one more problem:  How do we
recharge the auxilliary battery?  The original battery's
going to recharge from the alternator when the engine's
running, but (as we have it hooked up now) the aux. battery's
not.  We can give the aux. battery its own separate charging
system - say, a 110V plug-in charger, or a solar panel - and
that would work fine.  But, more typically, we really want to
charge it from the alternator, because that's free, easy, and
usually adequate.

That means we now want our two batteries to be connected
*sometimes* - and typically, that "sometimes" is whenever
the alternator is running.  The simplest way is just a big switch -
turn it on when you want to connect the batteries, and off
when you don't.  That (or some variation of it) is actually a
fairly common solution, especially on boats (particularly old
ones).  The next-simplest way is a relay (which is just an
electrically-operated switch) hooked up so that it automatically
switches "on" when the alternator's running, and "off" when it's
not.  Then there are battery "isolators" and "separators", which
are a bit fancier, and have somewhat different techniques for
connecting the batteries, and deciding when to do so.  (Each
method has advantages and disadvantages, and all can be
used successfully.)

If we want to really get fancy, we can even add in one or
more of those additional charging systems, such as a 110V
charger or solar panel, and use them to charge both the
original battery and the aux. battery.  We'd need to design
the connection between the two batteries accordingly.
Again, a simple switch always works - everything else is
just a way to automate the task, and keep you from having
to decide (and remember) when to turn it on and off.

My own aux. battery system (which uses a relay) is described on
my web site at "http://members.aol.com/ssittservl/Westfalia".
There's also lots of information from various people in the list
archives - the topic comes up fairly regularly.

                                                 -Steven Sittser