Date: Thu, 30 Mar 2006 22:54:31 -0500
Reply-To: robert shawn feller <feller@CARBONCOW.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: robert shawn feller <feller@CARBONCOW.COM>
Organization: carboncow
Subject: Re: electrical experts: voltage drop and the fridge theory.
In-Reply-To: <442CA410.3020308@earthlink.net>
Content-Type: text/plain; charset="us-ascii"
Thanks Mark, once again great stuff.
I have basically separated my 12v deep cycle system from the traditional
westy relay system. I'm using a simple west marine battery combiner so that
it may share current when the starting motor for charging and the opposite
for the starter to be charged when I'm plugged into "shore power" with my
charging system I've installed.
I think the goal I'm going to look at is to beef up the wiring from the
battery to the 12v punch down strip and keep the wire run to approx. 3 feet
if possible. I think I'm putting my big deep cycle under the sink along with
the charger and the battery combiner (all water proof care of west marine!).
For the type of traveling we hope to do I know we are going to be pulling up
camp every day to drive around town and see sights. I want my battery/fridge
combo to work Ok for 4-6 hours why we shop, sight see or dine. I have faith
that with turning the thermostat down and keeping the wiring run short (and
a bit beefier) I can accomplish this w/o having to constantly get the
propane system running. I'm just wanting better options to fit the need.
I'm a boating guy and I like my deep cycles big and useful, I'm not
convinced I'll like a little deep cycle crammed under the seat!
Shawn
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
mark drillock
Sent: Thursday, March 30, 2006 10:38 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: electrical experts: voltage drop and the fridge theory.
It is refreshing to again see someone getting into the nitty gritty of
fridge electrical operation. We used to do more of this kind of thing on
the list. You are delving into an area where large improvements can
often be made by those willing to do a little work. The Dometic RM182
fridge can work very well on the 12 volt setting but often tends not to
for reasons that aren't its fault. You are discovering one of those
reasons.
First of all, remember that the fridge normally only runs on 12 volts
when the alternator is charging. At that time, a higher system voltage
is available to it than just battery voltage. An extra volt makes a
significant difference but the real problems with weak 12 volt operation
lie elsewhere, like you have uncovered. If the fridge wiring age related
problems can be cleaned up and the source connection modified the
fridge can work great on the 12 volt setting. Both of mine now do.
The largest flaw in the fridge wiring is the decision by Westfalia to
use power from the main fuse/relay panel to run the 12 volt heater. This
results in a very long wiring run and also leaves the fridge susceptible
to a voltage drop whenever the voltage at the main panel is low, as it
often is for various reasons. This is best cured by adding a second
battery under the driver's seat and connecting the fridge to it. Then
make sure the second battery is getting a good charging voltage by using
a real relay or switch, with real wires from the main battery, not the
hokum setup sold by various VW vendors.
The voltage to check first is the voltage directly across the in and out
wires of the 12 volt heating element. Don't just check from the element
plus side to battery neg or chassis ground because that will not tell
you if there is a grounding problem for the element. If the element
ground is weak or missing, the fridge simply won't cool on 12 volts no
matter how high the voltage is at the element input. What counts is the
voltage seen across the element.
If the voltage across the element is very much lower than the battery
voltage, you need to chase down the probably several reasons why and fix
them. Use the straight pin trick to work your way through the wiring and
locate each trouble spot. All connections are suspect and tend to get
cooked over the years when the fridge is left turned to 12 volts even
when not in use for keeping anything cold. This setting causes the
fridge to run whenever the engine is running. Months, years, forever,
until something burns out or someone turns it to another setting. The
wiring and connections can only take so much before need to be freshened up.
Mark
robert shawn feller wrote:
>I am amusing myself since I've been fixing and testing the fridge on the
>bench. It's staying cool to 30 degrees on 110v for me. I just bought a new
>1000amp hour deep cycle for the new wiring I'm doing in my westy related to
>other needs then the fridge.
>
>I figured I would hook it up to the fridge to just see how horrible a job
it
>does (we all know 12v and the fridge are not the best friends). I should
get
>a good day out of it based on calculations.
>
>Here is my question:
>
>The voltage at the battery (under load) reads 12.4 volts but at the
terminal
>it reads 11.5 volts, only 3 feet of wire between the battery and this
>terminal! Is the inefficiency of the battery cooling the system do to
>improper low gauge wiring?
>
>Seems to me that if there is 7% voltage drop in such a short run the wiring
>gauge for the heating element and the wires to the battery could be
>improperly rated. This would then affect amperage and thus the final
wattage
>(which is supposed to be the same as the 110v) is never achieved. I would
>assume those of you keeping the battery under the seat and running more
>length of wire then I would have slightly more drop.
>
> If I recall from my boat wiring days a 3% drop is the recommended goal for
>any length of run from the governing body for boat builders but as high as
>9% for least acceptable.
>
>Any thoughts.or do I not know what I'm talking about!
>
>Shawn
>
>
>
>