A 20-30 feet of 10ga wire acts as a natural "choke" to reduce amperage to a level the trailer battery will be happy with.
Well...that's not exactly correct. A smaller wire does not reduce amperage, it reduces voltage. However, supplying the battery with a lower voltage does cause less amps to flow toward the battery, which is why a smaller wire appears to limit amperage.
But it doesn't. The fuse is what limits the amperage.
It's actually an anomalous condition due to the battery. For a normal load - for instance a 1500w heater - lowering the voltage increases the amperage.
You're not supposed to charge batteries at 50-100A by connecting it direct to another charged battery with fat wire.
Two problems with that statement:
First, how much amperage the battery can absorb depends on the battery. Most AGM batteries can handle more amps than any normal charging system can throw at them. Spiral-wound AGMs, like the Optima Yellowtops, can handle insane amounts of charge amperage as long as you don't overheat them:
"Alternator:
13.65 to 15.0 volts, no amperage limit.
Battery Charger:
13.8 to 15.0 volts, 10 amps maximum, 6-12 hours approximate.
Cyclic Applications:
14.7 volts, no current limit as long as battery temperature remains below 125°F (51.7°C). When current falls below 1 amp, finish with 3 amp constant current for 1 hour.
Rapid Recharge:
Maximum voltage 15.6 volts (regulated), no current limit as long as battery temperature remains below 125°F (51.7°C). Charge until current drops below 1 amp."
http://www.optimabatteries.com/product_support/charging.php
Second, a bigger wire, in and of itself, will not increase the amperage that flows to the depleted battery; that is a function of how depleted the battery is, and how much voltage is present to charge it.
If the depleted battery's resistance only allows it to absorb 30a with a supply voltage of say 13.6v, then it won't absorb more just from having a bigger wire. To make it absorb more amps, you have to supply a higher voltage.
What the bigger wire can do (especially over a long run), is allow a higher supply voltage at the battery, which (depending on the battery's resistance) can allow more amps to flow.
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The key to battery charging is voltage. To charge a battery you have to feed it a voltage that is higher than "whatever the battery is at right now", otherwise current doesn't flow toward the battery. The supply (charging) has to push hard enough to overcome the resistance of the battery. The bigger the difference, the harder the push and the more current (amps) flow (up to a point).
Let's say our truck's alternator is putting out full voltage - the voltage regulator is holding the alternator to an output of say 14.6v. We use a small wire back to the trailer battery. That wire reduces the voltage, because it's small. How much it reduces the voltage is a function of how much load is on it - the greater the load, the more the voltage drop.
Now, depending on how depleted the battery is, the internal resistance is some number. When the battery is very depleted, the number will be high (high resistance), and when it's pretty near full that number will also be high.
So how many amps can possibly flow into the battery depends on:
What is the resistance of the battery at this moment?
How much excess voltage is available to overcome that resistance?
How much will that voltage drop under X load with this size wire?
If the battery is at 12.6v, and the available voltage to charge it is only 13.6v under load, then that's it - only a certain number of amps will flow. Not because the wire limits the amperage, but because the wire limits the voltage. The BATTERY limits the amperage because its internal resistance will only allow X amps of current to flow if the supply voltage is Y volts.
A constant current type charger will crank the voltage up to whatever it has to be in order to get however many amps to flow. If it's a 30a constant current charger, then it will just keep pushing up the supply voltage until 30a is flowing toward the battery.
A constant voltage type charger will hold the voltage at some point, and let the battery absorb however many amps. As the battery's resistance goes down, the amps go up, until the the battery starts getting full and then the amps go down again.
An alternator/voltage regulator setup is a constant voltage charging system.
Okay, great. So now we're using some wire...say #10...to charge our battery and due to the voltage drop, no more than 30a ever flows through the wire (actually, it's probably more like 5a or less), so we're also not blowing the 30a fuse that protects that #10 wire.
Looks good. So what's the problem?
Well, there are two...
The first problem is if the size (and length) of the wire causes a voltage drop under load. If the voltage at the battery is under say 13v - that is going to guarantee that the battery is never going to be fully charged.
The second problem is that by limiting the voltage, we are also limiting the potential amperage and depending on the battery that might not be a smart thing to do. If we've got AGMs in our trailer, then what we really want is to feed those bad boys the max amps our charging system can supply to get them charged up ASAP.
To do that, we need to increase the voltage available at the battery.
To do that, we need wire which doesn't reduce the voltage.
Hence the need for fat wire.
So can we use small wire and more or less trickle charge the battery? Sure can! Voltage drop depends on load, so as the battery voltage rises, the amperage flow (the load) decreases, and the as the load decreases, so does the voltage drop.
So now as the battery voltage rises, maybe instead of 13v at the battery under a 10a charge load, the battery is seeing 13.5v under 1a of charge load. Sweet! So our battery will eventually reach 13.5v! It might take a couple of days, and won't actually be totally charged, but what the hell, better than nuthin'.
So what if we shave some life off our battery by constantly deficit charging to a "less than full" voltage:
http://www.rollsbattery.com/content...d65c08&phpMyAdmin=3jSJ-jdC5E7b53DHgV8TGvpSCF6
http://www.cat.com/cda/layout?m=37407&x=7&f=320846
http://www.hawkeraplus.com/faq.htm
Voltage is the important thing. If a #10 wire will get your battery to it's fully charged voltage (14.7v for the Yellowtop, 14.8v for most Trojan flooded, 14.4v for most AGMs) in a reasonable amount of time, then go for it. If it drops the voltage and never does get your battery to the fully charged voltage, then it doesn't matter what size wire you use - your battery isn't going to be full anyway.
(For the record, I beat the crap out of my aux battery. I plan on it being abused. Which is why I buy a cheap aux battery and replace it every couple of years.
Also, just because something was done that way yesterday, doesn't mean it's valid today. I went and looked at a '72 Winnebago that was being parted out. I noticed it had the converter/charger still in it. I busted out the flashlight, got down on my knees and wiped the dirt off the label and what did I see? 12.6v. Meh. My antique Schumacher battery charger puts out 12.8v and that's not enough to fully charge a modern battery, why would I want to go even lower? Maybe 12.6v was good enough for batteries in the 70's - it sure won't cut it nowadays.)
