Date:         Thu, 20 Dec 2001 19:35:15 -0500
Reply-To:     The Bus Depot <vanagon@BUSDEPOT.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         The Bus Depot <vanagon@BUSDEPOT.COM>
Subject:      Re: A twist on dual-battery configurations
In-Reply-To:  <051301c189b2$0d5f54e0$3b47fea9@grumpy>
Content-Type: text/plain; charset="iso-8859-1"

> I don't like the bigger relay solution ("dumping" from starter to aux)


Me neither, unless you're careful.

First of all, just to be clear, a bigger relay in and of itself won't
increase the rate of charge.  You will also need lower gauge (heavier)
wiring to support it. Otherwise, no matter how much capacity the relay has,
the wiring gauge will limit the rate of charge.

The heavier the wire and bigger the relay, the faster the aux battery will
charge. So why didn't VW and Hella just use massive relays and wiring? (That
way you could recharge your aux battery really fast!!)  Well, because there
is some risk associated with doing so. Say you used a relay/wiring of
infinate capacity (it would pass as much current as the aux battery wanted
it to).  If the aux battery were totally dead or shorted, as soon as you
turned on the ignition, it would try to pull everything from the starting
battery all at once, and the relay would let it. The result of that sudden
massive current transfer could be a fire or explosion.  No, it is not a huge
risk, but it is a real one, just as there is that potential when you try to
jump-start a totally dead or shorted battery with jumper cables.

This is why both Hella and VW opted for a lower capacity relay and wiring.
Yes, it recharges the aux battery gradually (so it takes longer to fully
recharge it).  But it is safe. They picked what they considered to be a
happy medium - fast enough recharging for most uses, but not fast enough to
be potentially dangerous.

Could they have used a higher capacity setup than they did, without any
risk?  Probably. I think both Hella and VW tended to err on the side of
cautiion rather than trying to push the limits - which is a philosophy that
I agree with.  So yes, you could probably use a larger relay and wiring than
Hella did (properly fused) without a large increase in risk.  But be aware
that pushing the envelope too far could be risky.  If you are not
comfortable taking that risk (and don't want to spend the bucks for a
higher-end "inteligent" charging system),  better to stick to Hella's specs
and put up with a bit longer recharge time. At least it's safe, and also
sufficient for most people's uses.

I apologize if some listmembers feel that I am stating the obvious here, but
I am afraid of people taking the "bigger is better" approach, without
understanding the risk, and potentially risking both their van and their
safety.

- Ron Salmon
  The Bus Depot, Inc.
  (215) 234-VWVW
  www.busdepot.com