Date:         Thu, 14 Mar 1996 11:41:19 -0500
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         SyncroHead@aol.com
Subject:      Re: Extra battery

In a message dated 96-03-08 09:17:44 EST, you write:

<< 
I've been searching Archives for infor on installing a 2nd battery - for
backup and accessories.
I figured you would probaby know - if you would - let me know your experience
- or where one might locate this in the Archives
Thanks
Jim Hager
87 Syncro >>

Jim,
There were numerous discussions about 6-12 weeks ago on 2nd battery
installations.  There are 4 basic ways to connect a 2nd battery into the
system.  I can't go into all the details, but they should be in the archives.
 Personally, I don't know how to access the archives.  Four basic setups are:

1. Connect both batteries up in parallel.  Use a BIG wire (like the wire from
the battery to the starter. It's BIG!) between the first & second batteries
and creat a good ground for the second battery.  This method makes both
batteries operate like one really BIG battery.  They both get charged &
discharged together.

2. Use a relay to isolate the positive sides of the battery.  The relay
should only provide a connection between the batteries when the engine is
running.  This allows you to discharge the 2nd battery (powering accessories
& such) without having to worry about discharging the primary battery. After
the engine is running, the relay closes the circuit between the batteries
allowing the alternator to charge the second battery in addition to the
first.  This same basic plan can be used with a manually operated switch
between the batteries, BUT you have ro remember to engage the switch at the
proper times (open while using the 2nd battery, closed while alternator is
charging 2nd battery) to avoid discharging both batteries & getting stuck.
 Whether using a switch or a relay you need one that's rated for a lot of
current - like 30 Amps.

3. Use a commercially available (JCWhitney, some FLAPS, boating supply
stores) battery isolator to allow the alternator to charge both batteries,
while keeping their discharges seperate.  This method is similar to the relay
method, but isolators have an inhierent voltage drop across them, I think
about 1/2 volt, preventing either battery from becomming fully charged.  A
solution to this is to use a voltage regulator that allows a higher voltage
to be generated, and then after the isolator's voltage drop the voltage at
the batteries is high enough.  In a Vanagon wiring with this method is a
problem, because of the layout & distance of the circuit between the
alternator/starter & battery.

4. Use a large current diode to isolate just one battery.  This allows the
primary battery to receive full voltage from the alternator (with standard
regulator), while the 2nd battery gets about 1/2 volt less - due to the
diode.  This is the method that I have in my 87 Syncro right now.  I only
have the second battery there as a spare and don't have any loads attached to
it right now.  If the primary battery gets discharged, I can connect a jumper
cable between the two positive posts and allow the 2nd battery to start the
van.


I'd say that the most useful method would be #2 above, relay isolation.  I
didn't choose it for my setup beacuse I did not have loads (like a
refrigerator, etc) that I planned on using the second battery for and
installation & wiring was very simple.  I may convert over to the relay
isolation someday if I get ambitious.

Hope this helps,

Jim Davis
87 GL Syncro
88 GL Wolfsburg