Date:         Tue, 20 Jun 2000 21:24:44 -0700
Reply-To:     Stanley Pitman <stanman@blueneptune.com>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Stanley Pitman <stanman@blueneptune.com>
Subject:      Battery isolator tutorial
Content-Type: text/plain; charset="us-ascii"

This tutorial seeks to shed a little light on vehicular charging systems,
and the use of battery isolators in particular.  It developed from research
into various issues brought up in recent group postings.  Although mostly
generic in nature, it will also cover specific Vanagon peculiarities.

Alternators exist to power the vehicle's electrical equipment when driving,
as well as recharging energy lost by batteries.  The alternator has three
main parts.  The spinning rotor winding, under control of a regulator,
generates a whirling magnetic field inside the housing.  This moving
magnetic field generates AC voltage in the stator, or stationary windings.
Since AC is not suitable for automotive systems, heavy-duty diodes
"rectify" the AC to turn it into pulsating DC.

A  voltage regulator senses the alternator's output voltage and tries to
keep the voltage at a defined value, by controlling the strength of the
moving magnetic field.  It does this by regulating the current fed into the
rotor winding.  This is an example of a system using negative-feedback for
stability.   Most voltage regulators mount in or are bolted to the outside
of alternators.  They often have auxiliary connections, which we'll cover
later.

A battery's charging voltage and current needs are variable.  After a
battery is fully charged, it is happy with a "float" voltage of about 13.7
volts DC and will draw only a trickle of current.  Charging a full battery
for long periods at higher voltage and current can cause battery damage.

Before an engine startup, a battery will have been depleted to varying
degrees by starter motor, accessories, etc.  If the charging voltage were
set at a fixed 13.7 volts, the recharging process would complete very
slowly.  Since a partly empty battery can easily tolerate higher currents,
the voltage regulator boosts the charging voltage to as high as 14.8 volts
until the battery is near full-charged, then drops back to the float voltage.

In standard single-battery configurations, the alternator's main output
terminal is connected to the positive terminal of the battery.  Because of
the rectifying diodes, no current flows from the battery backward into the
alternator.  Most simple regulator connections (like in the Vanagon) sense
the voltage directly at the alternator's output, which is reasonably close
(electrically speaking) to the battery.  As we'll see, this can cause
problems when any unusual voltage drops are introduced between battery and
alternator.

The simplest battery isolators split the alternator output current into
dual paths leading to dual batteries, and introduce a diode into each path.
 Because current through diodes only flows in the "downhill" direction,
current from either battery can't flow backwards into the other.  This
keeps RV accessories from draining the vehicle's main starting battery.

But......diodes introduce a voltage drop in any circuit; in this case about
0.8 volts.  For simple alternators and voltage regulators (including on our
beloved breadboxes), this is bad news.   Our battery charging voltages and
thus stored energy (for both batteries) will be way too low.  This is why
people who installed a standard isolator in a Vanagon were often blessed
with inadequate electrical performance and early battery failures.

The way around this problem is to let the alternator sense the battery
charging voltage, rather than its own output.  It then can adjust its
output to be one diode-drop above the desired battery voltage.  Some
alternators have special sense wires that connect to the battery.  Some
require special isolators with separate sense outputs.  There are many
isolators out there; it's important to seek good advice and get the right one.

The Vanagons are special cases, requiring a special isolator and a
modification to the alternator's voltage regulator and wiring. In his
posting of 19 June, Harald Rust has very nicely documented installation of
the Sure Power isolators.

The surepower.com website has some interesting and informative info.  Check
it out.

Although the Sure Power units are the best I've seen and have a great
reputation for reliability, I recommended getting a unit conservatively
rated at more than your alternator output.  Above all, avoid brands you've
never heard of, or anyone selling them that claims to have a universal type
to fit all vehicles.

Any caveats ?   There are hazards to breaking the simple standard charge
path and inserting foreign objects.  Isolator failure, though rare on
quality units, can wreak havoc with the system, as can loose or dirty
connections anywhere in the charge loop.  I highly recommend getting good
volt and amp guages to monitor your systems.

This about covers the basics.  It's been fun, and I learned a lot in the
process.

-- Stanley  --