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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.
Comments: To: mark drillock <drillock@EARTHLINK.NET>
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 > > > >


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