Date:         Thu, 22 Feb 2007 07:31:53 -0700
Reply-To:     David Etter <detter@MAIL.AURACOM.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Etter <detter@MAIL.AURACOM.COM>
Subject:      Re: was fav 12 V acc, now 2nd battery relay system - files posted
In-Reply-To:  <45DD19BE.2020509@cox.net>
Content-type: text/plain; charset=us-ascii; format=flowed

Great explanation Mark,
Thank you.
        My aux. bat. charging system (smaller relay/smaller #10Ga.
wire with a  90 amp. alt) may indeed be too small for just the
reasons you mentioned.  However, this is very dependent on the type
of travel or camping you do. If one travels little and camps a lot in
a limited area, then yes, #10 Ga is too small. However, I tend to
drive 4 - 6 hours day, every day and consequently my aux. bat. is
always charged by the evening.
        My system is also supported by a 120 watt solar panel (when
it's sunny) ,. I apologise here for not giving the whole picture.
Local week-end camping would indeed be better serviced by
implementing the changes you suggest. Different strokes for different
folks.
        Again, and excellent article and certainly worth of archiving
for anyone, Thank you.
                                        David (dsl82westy)

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>I wouldn't change back your relay wiring just yet. The great old posts
>you referenced have a glaring omission in their discussions. The typical
>weakness of the small relay/small wire method of 2nd battery connection
>is not fully explained in them. There is a good reason that devices like
>the 1315 exist.
>
>The fact that some people find the charging rate supplied by the small
>system adequate in no way means every one will. The amount of alternator
>current that flows to the aux battery terminal often needs to be greater
>than what the battery itself needs even for slow charging. People who
>have things connected to the aux battery often need to run those things
>while the vehicle is running as well as stopped. That current, for
>things like stereos, 12 volt fridge operation, devices running on the
>cig lighter socket, inverters and their loads, etc. must also come
>across that one small wire at the same time the charging current does or
>the aux battery will drain rather than charge. Running these things may
>in theory still leave some current left for charging the aux battery but
>in practice the voltage may have dropped low enough to prevent much
>charging from taking place at the aux battery.
>
>A volt meter measurement at the aux battery with the engine running and
>all loads switched on is a good place to start. Compare that measurement
>with what you see at the main battery when you move the meter leads over
>without changing anything else.
>
>The low cost battery relay kits I have seen from various vendors are
>only about half the capacity of the stock dual battery charging systems
>VW fitted on many 85-91 vanagon models. This makes some sense for the
>pre-waterboxer VW vans with their 55 amp alternators of yesteryear but
>waterboxers 90 amp alternators can support a more robust system, as VW
>fitted. VW used metric wire of slightly greater capacity than #10 awg
>and a 40 amp relay connected such that the second battery never supplies
>any starting current. No fuse was used.
>
>Even the VW relay system is too small for some of us. If you need to
>recharge a deeply discharged aux battery in a hurry you may need a
>higher capacity relay system and may find the 1315 a very useful device.
>Most people don't, assuming they or the installer understood the
>limitations and wired things up taking real loads into account, not just
>the simple charging of the batteries.
>
>Mark