Date:         Sat, 17 Jun 2000 08:45:29 -0700
Reply-To:     Stanley Pitman <stanman@blueneptune.com>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Stanley Pitman <stanman@blueneptune.com>
Subject:      Yet another EE talks batteries
Content-Type: text/plain; charset="us-ascii"

I'm an electrical/electronic engineer with considerable experience and
knowledge of batteries and charging systems.  I want to thank Beckett
Cantrell, especially, for his illuminating posts where he explained the
basics of dual battery relays.   Listen to him, people; he knows whereof he
speaks (and with a sense of humor, no less).  Other good insights were
posted by Harald Nancy, Frank Gruenthaner and a few others.  The "best of"
postings are going into my reference file for technical writing and
teaching.  Thanks guys!

I wanted to respond earlier to some of the misinformation posted by others,
but holding down 1.4 jobs has kept me hopping.  Now that I'm home sick for
a few days, here it comes!  I won't repeat most of the good points made,
except where some clarification and emphasis is in order.

Proper design of a good RV auxiliary battery system is not a simple
process, even for those with some electrical know-how.  Each approach has
to consider costs, performance, reliability, and effects on other parts of
the RV.  Anyone claiming "All you have to do is......." is likely ignorant
of some basics.

Fuses are designed to protect electrical circuits against carrying
electrical current beyond their safe limits. This includes effects on all
equipment, including batteries, wiring, alternators, relays, etc.   In any
circuit, a fuse is sized to protect all the links in that circuit.  Just
deciding on a fuse double your anticipated current drain, without
understanding what you're doing, is short-sighted and can be dangerous.

One thing I haven't yet seen mentioned is that although you can run
electrical devices off your auxiliary battery while charging, the current
they draw will reduce the amount available in that "leg" for charging the
battery.  This can even result in the battery continuing draining, while
you think it's charging.

 The Hella relay kit seems an inexpensive and reasonable solution for many
people (see below).  Yes, you can save a few $ by buying the pieces and
cobbling it together yourself.  Just be sure you really understand what
you're doing.

The kit is probably NOT suitable for someone who often drains the battery
way down, and doesn't drive a long way each day to allow full charging.

Just installing bigger wire and a big honker relay, however, has to be
carefully considered, for all the reasons brought up by Beckett.  If you
make the auxiliary charging current too high, there is strong risk of
damage to battery or alternator, as well as siphoning off high currents
from the starting battery whenever the ignition is turned on; don't tarry
too long before starting!

For those of you serious about understanding what goes on with your
charging systems, I recommend good-quality ammeter and battery-voltage
guages to monitor  "wuzzup?" in your circuits.

I'm thinking of posting a quick tutorial on battery isolators, if there is
any interest.

In any case, it's been most interesting.

P.S.  Just found out from the latest post that the infamous Beckett is a
battery engineer, no less.  Hot Damn !  No wonder he knows his onions, not
to mention anions and cations.