Date: Thu, 20 Dec 2001 18:57:15 -0500
Reply-To: Julian Burden <julian.burden@SYMPATICO.CA>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Julian Burden <julian.burden@SYMPATICO.CA>
Subject: Re: A twist on dual-battery configurations
Content-Type: text/plain; charset="iso-8859-1"
You make some good points - especially about the voltage sensing of the
alternator.
> Comment: Why would you want your running loads to come from the house
> battery? The house battery is to run things when the engine isn't.
>
When the engine (therefore the alternator) is running it shouldn't matter
which battery the extra loads are connected to. When the engine is not
running I want to keep the starter full (using headlights at the campsite
etc).
> Specious -- the alternator can probably put out 120v into an open
circuit.
>
> >2. energy is lost in the battery isolator - this energy has to come from
the
> >alternator.
>
> True.
Yeah, but 120v open circuit is 0 amps = 0 watts. If you have the same
current, but a higher voltage (caused by pushing through the extra diodes)
the alternator does have to work harder. I actually see these two points as
the one, I was rephrasing points made in the archives.
>
> I don't know for absolute sure, but I'd be astounded if they used two sets
> of coils rather than two sets of diodes...
I agree. (Another case of rephrasing from archives).
>
> Possible concern -- if your diodes Vf don't match Bosch's, you'll have
> different outputs to the two batteries.
>
I don't see this as a problem - Vf's will be within 0.3v - and I think mine
will be the lower of the two (0.3 - 0.4v depending on temp) cable
differences will reduce this gap (I'm not going to be using the same gauge
as the starter cable!). Ideally the batteries should get the voltage they
need, a variation from what they need is probably more important than the
difference between the two batteries.
> >4. Less current will be going through the existing diode pack, meaning
the
> >alternator will run cooler.
>
> Mmmm...but you have to figure out how to cool the external pack. The
> alternator has a bloody great fan sucking air through it...
>
No problem, I've already selected diodes and heatsinks whose rating is
sufficient.
> IMHO, because this does nothing to address the issue of regulation which
is
> more worthy of effort. The actual charging connection is easily addressed
> with a relay or electronic combiner, either of which will allow an
> externally-sensing regulator to charge based on whichever battery needs it
> more -- which the diode isolator does not do. Your extra-diode solution
> does address this, but is still looking inside the alternator, a very poor
> place to find out the state of the battery(s). If you really want to have
> and use a deep-cycle battery, grab the bull by the horns and set things up
> so the sucker will actually charge. I know it's big bucks, but you'll be
> happy later. :) http://www.amplepower.com/
>
I don't like the bigger relay solution ("dumping" from starter to aux), and
I don't see this as much more difficult (apart from the R&D which I enjoy)
than the isolator and voltage sensing method.
> cheers
cheers, and thanks for taking the time to reply.
