Beckett,
Thanks for the explanation.
Just two thing to add...
1. The Optima is not a gel cell.
2. The Optima specifications list high amperage charging as an approved
method. I have talked to the Optima tech department and they have confirmed
that hi amperage charging.... 90 Amp Bosch Alternator is fine.
Bill

----- Original Message -----
From: <THX0980@aol.com>
To: <wdavidson@thegrid.net>
Cc: <vanagon@gerry.vanagon.com>
Sent: Friday, June 16, 2000 10:15 AM
Subject: Becketts Tome on Dual Batt.s, Relays, Saftey, and charging (Very
Long)

> Right then. Let us start with the safety issue. In theroy, the safest way
to
> charge an automotive or marine or aircraft batt.(24v in the case of the
> latter) or any wet cell storage device( and for those of you running gel
or
> AGM batt.'s, hold on, I'll get to them in a bit...), whilst said batt is
> in-situ, would be with a nice, steady, low amp constant charge. In the
real
> world, this just ain't how it goes down. Given the nature of alternators,
and
> the various demands of an automotive enviornment, what happens is that you
> get a big thumping surge of amps,(say, right after you disengage the
starter)
> followed shortly there after by a much "softer" rate of charge. Now lets
> throw our deeply discharged aux. batt. into the picture. If you've got it
> wired in with a chunk of thumb sized cable, then that sucker is gonna suck
up
> all the amps it can get. This does two very bad things: 1) As the aux.
batt.
> is drained, it is going to, for lack of a better word, act as a
nt.(
> picture jump starting a car with a dead batt. If you connect a good batt.
> directly to the terminals of the dead batt., chances are that the car with
> the dead batt. will crank over slowly. But if you connect the good batt. to
> the ground of the car with the dead batt. and the hot lead of the starter
> solinoid, that puppy will fire right up.)  When you turn the key, current is
> not only going to try to go to a Demand device, like the st
arter, but will
> also try to equalize the charge between the cranker batt., and the aux.
batt.
> (Now remember, folks, we're talking about using a relay here, not an
> isolator, which has its own inharent problems, like voltage drop across
the
> diodes...) This may result in the vehicle not starting. If you do get the
> beastie to start, we come to Very Bad Thing number 2) Bosch and Motorola
> alternators absoultly HATE having to supply anything close to their
"Rated"
> out-put. Let's just say you've got a Bosch 65amp alt. in your Brick. You
> think "...65amps, well thats more than enough to handle the demands of my
> system..." but what you might not know is that 65amp rating is at 100%
duty
> cycle. Your actual usable out-put is 1/2 to 2/3 the total rated out-put.
So,
> now you've down to 32-45amps, usable. But wait, is it hot out? Yes? Then
cut
> another 15-25% off of those numbers. Now you start to sweat(pardon the
pun).
> What it comes down to is that your alt., under normal conditions, has its
> hands full just running the stock systems of your rig. Add in an aux.
batt.
> wired in with a relay and monster cable, and that alt. will let out its
> "smoke" in no time flat. And I do mean smoke. The fire hazzard is real.
That
> is why using a small relay and small ga. wire to charge the aux. batt. is
> much safer. Think of it this way. If some pissy little 18ga wire under
your
> dash shorts out, you'll see some sparks, and think"...Well, rat poop.
There
> goes the connection to my spiffy little motor driven dashboard
hula-girl..."
> If a 2ga. cable shorts, you've got a very large arc welder on the loose.
Not
> pretty at all. To sum it up: A relay in the 15-30amp range, wired in with
> 12ga connections is fine. Hella errs on the side of caution, and who could
> blame them? A big 75amp relay, and 4or 2ga. cable is simply asking for an
> expensive and dangerous problem.
>       Now lets tackle the issue of batt.'s and charging when applied to a
> relay controlled dual set up, where you've got a standard automotive type
> cranker batt. and an aux. deep cycle batt. To wit: Your run-of-the-mill
> cranker batt.(wet cell) is very happy when it is allowed to do one job,
that
> being to give up large ammounts of current for a short period of time, and
to
> be recharged with the same, ie, short recharge period at high amps. This
> happens whenever you start your eng. It then goes about the task of
holding
> on to its charge, while the alt. keeps the other bits going, until you
need a
> little extra bump, like when you turn on the headlights, or fire up that
new
> roof rack mounted Death Ray. Then the batt. takes another hit, the alt.
sees
> this, gives another short burst of high current flow, and things settle
down.
> The deep cycle aux. batt(also wet cell), on the other hand, is loath to do
> this. It is happy with a nice steady rate of discharge, and recharge. So
it
> would seem that we've got a problem, eh? Yup, we do. If the aux. batt. is
> deeply discharged, and you're wired up with massive relays and cable, it's
> gonna die a quick death. And so is your cranker batt., and ditto for the
alt.
> itself. Remember, alt.'s are stupid devices. All they do is sense demand,
and
> keep puting out the amps till said demand is met. So not only is the aux.
> batt. getting way too much juice, so is the cranker, and that alt. of
yours
> is working at its limit(100% Duty Cycle) to supply the needs of the aux.
> batt. Bad, bad, bad.( Okay, here's the part for you Optima, gel-cell and
AGM
> folks) These batt.s, both cranker and deep cycle, handle the above
situation
> pretty darned well. "..Oh, good...Thats what I've got, so no
> problems...Right?" Wrong. Your alt. is still frying itself. So now we come
to
> the solution. It is not a perfect one, but it is an even trade off between
> component and batt. life, safety and re-charge time. Its the use of a
> moderate relay and lighter gauge wiring. Pretty simple, eh? Unfortunatly,
you
> just can't have your cake, and eat it too. If you want to help out the
> situation, get a regulated 120vAC to 12vDC power supply, in an amp range
that
> suits your needs, and wire it in to your system. If your camped in an area
> with hook-ups, the power supply will swich on when it senses 120vAC, and
> by-pass your aux. batt. If your out in the boonies, well then, you've got
to
> make due with what you've got. And the small relay/small wire system is
about
> the best compromise you can get. Please feel free to ask any questions
that
> y'all might still have.
> Beckett
>
>