Date: Wed, 16 May 2007 00:01:32 -0500
Reply-To: Matt Roberds <mattroberds@COX.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Matt Roberds <mattroberds@COX.NET>
Subject: Re: Newbie battery questions
In-Reply-To: <20070513195212.LXAX20023.eastrmmtai111.cox.net@eastrmimpi04.cox.net>
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
> From: PB <pbrattan@GMAIL.COM>
> Date: Sun, 13 May 2007 12:08:12 -0700
>
> I plan to replace it with a battery that fits in the cavity, but before
> I do, I need to know exactly what the Westy's auxiliary battery is
> supposed to do, and why it is useful (or unnecessary) to have one.
The main battery has two principal jobs and the first one is simple -
store enough energy to spin the starter fast enough to start the engine,
even in Alaska in January. Once the engine is started, the alternator
charges the main battery. The second job of the main battery is a
little more subtle. Most of the time, the alternator alone can put out
enough juice to supply everything in the van. But sometimes, if there
is high demand and low engine RPM, the alternator can't do it all, and
at that time, the battery will "fill in" for the alternator. Once the
demand drops off or the engine RPM comes up, the alternator will
recharge the main battery.
You could, conceivably, connect all the extra Westy stuff (interior
lights, fridge, etc) to the main battery and it would work. There are
two basic problems with this idea, though. The first is that if you
forget and leave something on, the main battery will run down and you
won't be able to start the van. The second is that the main battery
isn't really designed to be run all the way down to zero. It is made
to deliver a short burst of high power (to run the starter), but then
it wants to be recharged pretty soon.
To get around both of these problems, some Westys have an auxiliary
battery. When the engine is not running, the auxiliary battery and the
main battery are not connected, so even if you leave a light on and run
the auxiliary battery all the way down, the main battery is still fully
charged and ready to start the engine. The auxiliary battery is also
designed to be run all the way down to zero - it can deliver a steady
amount of medium power, and doesn't mind waiting a little while before
it gets recharged.
OK, so you've got a main battery that is full and ready to start the
van, and an aux battery that doesn't mind being drained way down by
running the lights. But once you've run the aux battery down, how do
you charge it back up? The answer is to let the alternator do it when
the engine is running - it can charge the aux battery at the same time
it is charging the main battery. What you want is to have the aux
battery and main battery connected in parallel, so the alternator
charges them both, BUT you also want the aux battery and main battery to
disconnect from each other when you shut off the engine.
There are a couple of different ways to do this. One way is to put a
couple of big honkin' diodes (one-way valves for electricity) between
the alternator and the batteries. This lets juice flow from the
alternator to each battery, but not from one battery to the other. The
gadget that does this is usually called an "isolator" and has three stud
terminals on it - one "in" from the alternator, two "outs" to the
batteries. Westys don't normally have one of these, though.
The other way is to use a relay to connect the aux and main batteries
when the engine is running, but to break the connection when the engine
is shut off. This is what Westys have. The coil of the relay is driven
by the wire from the alternator that also runs the alternator warning
light on the dash. This wire only has +12 V on it when the alternator
is turning fast enough to make electricity - if the ignition is on but
the engine isn't running, this wire won't have any voltage on it.
For normal driving around town, you don't need the aux battery at all.
You only really need it if you plan to be parked/camping for a while
and want to use the 12 V accessories in the van (lights, etc) without
danger of running down the main battery.
If you do decide to remove (or disconnect) the aux battery until you
can troubleshoot further, first disconnect the negative cable from
the MAIN battery, then disconnect the negative cable from the AUX
battery, then disconnect the positive cable from the AUX battery. Wrap
the end of the positive cable for the aux battery with electrical tape,
to keep it from accidentally contacting the positive terminal of the aux
battery (if you leave the aux battery in the van) or grounding out on
the sheet metal (if you take the aux battery out). Then reconnect the
negative cable to the MAIN battery and you should be set.
I hope this helps!
Matt Roberds
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