one wire alternator

LOL - basic electricity - stop over-thinking it! Google and Bing won't help much as there's more misinformation out there than fact due to marketing and the street rodders, etc.
Don't confuse things with guess and conjecture dude. Your third sentence makes no sense at all - none. Zip. The battery is a load - and that load varies with the current charge of the battery, battery type, age, etc. So the current is a factor of the battery as a load and the REGULATED voltage. 
10si - higher voltage rating than WHAT? Come on, you aren't even close. 

>>Current exceeds alternator output, the battery will provide. Until the battery shows weakness in capacity, then the light with turn on<<

Huh? Nope - the light comes on when the voltage at the ign side of the bulb exceeds that of the voltage at the diode trio, or REG terminal. So if the battery or system voltage is higher than the alternator output at that terminal, the light comes on.  Until the battery shows weakness in capacity? Please stop guessing and once again, confusing issues!!

>>You get longer life, better results running around 50% of the rated output.<<
Uh, check again, that's what is happening with the stock systems. You aren't running them hard at all. They don't even get hot. As long as the alternator can stay cool it will last a very long time even running half-capacity or a bit more and check the numbers below - stock is totally safe there (unless the vehicle is modified)
Beware of information on the web-  anyone can create pages that say anything and marketing folks will sell you a 200 amp unit because you need it HAHA. 
What KILLS alternators is this - bad batteries or cars that are NOT maintained and START HARD. Alternators are NOT battery chargers! To crank until the battery is low and then shrug and mumble "that's ok, the alternator will charge it" - that's your killer. And we know there are some reading this NOW who do that very thing. Crank it for a while, let the alternator charge it. Nope. Starter drag? Another killer as it sucks that battery down. Bad or weak battery also presents a LOAD to the alternator and will run it harder and run it hotter. 
Stock in a stock car is just fine, period. Alternator killers are questionable batteries or engines that don't start easily or someone keeps cranking until the battery is low. 

Bottom line, going to a 10 or 12SI alternator make 100% sense just for the fact that you can buy one at any parts store...unless you have a show car that gets trailered around and see's 50 miles per year then stick with a Motorola but if you drive your car daily I find peace of mind knowing if (And when) my alternator fails I can buy one anywhere.

I agree fully with the quote above as a decent reason for changing.
(oh, but can I plug in a fun statement? Let me work on the alternator and you make sure the wiring is decent and you'll have no worries either!)

Anyway, credit where credit is due -  you hit on the reason that does make sense - it's a shame because there's really no other reason to ditch the original. It's a FEAR FACTOR and availability thing. On the practical side, it's "where can i get parts" or "where can I buy a cheap replacement" (and the answer is most salvage yards!)
There was but one reason I agree people make totally logical and financial choices for such a swap and you did well hitting it. Good one, thanks.

I can relate - my Ford pickup battery went over 12 years and the battery in my 70 Javelin was purchased in 2006 as I recall, so is well on the way to a dozen years. (but it just didn't seem to pack the same punch last week when I started it so perhaps it's time to load-test it)
I think the battery I had in my Eagle went close to 10 years before I swapped it out a year or two ago. 

It seems I'm actually recovering from pneumonia - after second trip to urgent care this week and two more heavy-duty meds.............

73hornut wrote: Hi Billd. I currently am running a 140 amp 12si in my Gremlin, to handle the ac dual fans and other accessories. The issue is that it doesn't charge at idle. What would you recommend?

You know about electric fans and their appetite for power. They are nice but as you know, the power to drive them has to come from somewhere. The 12si is a decent choice for "more power" where you need real amps. Electric fans are good as they save HP (and fuel) when the cooling isn't needed and can force air when in heavy city traffic. 

I assume you have it wired correctly if it's a true GM 12si not converted by Summit or Jegs, etc. with a "one wire regulator". ** 
If so then it should get excitation current via the dash light (and/or a resistor) until it starts to charge at higher RPM.
It wired correctly as a 3 wire, check connections - as a drop at a bad connection can cause it to not pt out enough voltage and discharge or not turn out the light until higher RPM. 
A loose belt can cause low or no charge at idle, as can a bad diode (usually the trio)
What about pulley size? Too large and it will spin the alternator too slow at idle.

Just curious, how do you know it doesn't charge at idle and how much do you have to increase the revs to make it charge.

On my 10si's the light only came on when the engine ran really slow and probably an extra 25 RPM might have been enough to put it out.