Date:         Thu, 13 Dec 2001 15:33:02 -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: Battery Isolators vs. Relays
In-Reply-To:  <5.1.0.14.2.20011213141828.051b8190@pop1.attglobal.net>
Content-Type: text/plain; charset="US-ASCII"

> I believe the amperage control is totally done by the relatively small
> gauge of the wire feeding the aux battery.  Only so much current will flow
> through the wire.

Precisely.  The Hella kit uses a high gauge wire which will only allow a
limited amount of current to pass. Think of electrical flow as water going
through a hose.  A bigger hose lets more water pass; a smaller one restricts
water flow.  If you use a heavier gauge wire than Hella includes with the
kit, you will in fact increase the draw going to the relay (and will pop the
fuse).  This is a very simple and effective way of keeping the current level
under control, when combined with an appropriate relay and fuse.

> I installed exactly like the instructions & when you
> turn on the ignition, the fuse pops immediately. Sean
> had the exact same problem & he feels it's the tiny
> little wires can't handle the load

No, as I explained above, the tiny little wires are tiny precisely to
PREVENT the fuse from blowing due to overload. They are big enough to allow
a moderate rate of charge without being so big as to overload the charging
system. If you install the Hella kit and it pops fuses, there are three
likely possibilities:  1) you used too heavy a wire; try reducing the gauge
(if you didn't use the included wire); 2) you have a short somewhere or a
misconnected wire; or 3) the battery has a dead short and is basically shot.
(As someone suggested, this is unlikely to happen if you use a deep cycle
battery instead of a starting battery for your aux battery. It is also best,
just to be safe, not to run any battery completely dead.)
That said, the Hella kit is one of the most popular items I sell. I have
about 400 of them "out in the field" so far, not to mention that VW used
thousands of virtually identical setups on their campers. The setup
unquestionably works. There is something wrong in the wiring or use if it
keeps blowing fuses.

> In a previous email to the list Ron said that if the relay does fail it is
> likely to fail open, keeping the starter battery circuit functional.
> This is not my understanding of relay failures - I thought that relays
were
> more likely to fail closed because the contacts weld together,
particularly
> if the current rating of the relay is exceeded!

Before that happened the fuse should blow. Therefore the most likely failure
would be simple mechanical failure of the switch, not damage caused by
excessive current. This does not mean that it could not fail in the closed
position, but that it is not as likely to as with an isolator (in my
experience).

> As to being convinced, I am too. I like the 15A limitation in one way (not
> dumping from the starter battery), but not in terms of keeping the second
> battery charged. I run a notebook computer off an inverter and I'd like
the
> second battery to be charged faster than 15A. I'm exploring other options,
> but will probably end up with the relay.

There is certainly a compromise between rapid charging and safety. Hella's
engineers decided that approx. 15 amps was the correct choice. A nice
moderate rate of charge.  If you severely deplete the camping battery, you
will need to drive a little while to completely recharge it; just idling the
van for half an hour will give it some juice but certainly not recharge it
completely. You can estimate if this will be sufficient for you.  Also, if
your aux battery is severely depleted, it is best to minimize the use of
high-drain items that are connected to it while you are driving and
recharging, as this will extend the time needed to recharge the battery.  I
would say that the Hella kit is optimal for using moderate-drain devices
such as interior lights, cigarette-lighter powered fans, laptops, cell phone
chargers, etc.  If you are planning on putting in a monster sound system
with five amps that will kill the battery in 20 minutes, then the Hella kit
is unlikely to recharge the battery as quickly as you might like.

> I'm not sure here if you're suggesting
> that the fridge relay setup is somehow different from some
> other VW dual-battery setups or something else?

The relay used in Westfalias factory equipped with dual batteries (i.e.
Multivans, and '74-79 campers with electric fridges) are similar to the
Hella kit. I believe (and I'm sure that someone can correct me if I'm wrong)
that the one used for the fridge on Westfalias that only came with one
battery (i.e. Vanagon Westfalias with propane fridges) are not exactly the
same. Their function is slightly different, as they are intended to prevent
the fridge from powering up when the key is off, rather than to act as a
relay for a spare battery; however, they may in fact be very similar relays.

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

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