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Date:         Fri, 16 Jun 2000 17:36:13 -0700
Reply-To:     Bill Davidson <wdavidson@thegrid.net>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Bill Davidson <wdavidson@thegrid.net>
Subject:      Re: Becketts Tome on Dual Batt.s, Relays, Saftey,
              and charging (Very Long)
Comments: To: THX0980@aol.com
Content-Type: text/plain; charset="iso-8859-1"

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 > >


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