Date: Fri, 16 Jun 2000 13:15:38 EDT
Reply-To: THX0980@aol.com
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Beckett Cantrell <THX0980@aol.com>
Subject: Becketts Tome on Dual Batt.s, Relays, Saftey,
and charging (Very Long)
Content-Type: text/plain; charset="US-ASCII"
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 shunt.(
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 starter, 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
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