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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)
Comments: To: wdavidson@thegrid.net
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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