Sound`s like an over-charging problem...regu lator more than likley!
Just my 2 cent`s...hope this help`s...
Rex

Find someone with a good volt ohm meter and hold it on the positive and negative posts of the battery, have someone else fire the car up and let it idle. Should show around battery voltage or a little higher. Tell them to gently start reving the car up, if the voltage goes up past 14.5 volts shut it off. The voltage should never go past 14.5 period. If it does its either the regulator or the wiring. My uncle has a 69 Chevy 4x4 that was having the same problem your having and had replaced EVERYTHING including the wiring and the regulators still kept going bad. I finaly just put a later model one on it and wired it in and eliminated the original regulator since the later ones have them built into the alternator. Dont know if there are any later model ford alternators that would swap as easy but if you cant fix it and arent worried about it being "original" might be something to consider. Hope this helps.
ps. You can put a GM alternator on there by some modifying but dont tell anyone else in here I mentioned using a GM part on a ford!  :lol:

Yup, Had that happen back in the day. Cooked the battery pretty good. Replaced the regulator and the Batt and was back on the road. I was driving from Nashville Tn to home St.Johns Mi at the time, so it had a long time to cook.

Ok, here is the thing, if it's getting down to 11.5, your regulator or alternator isn't working.  Due to it being only below 1000rpm, I'd say the brushes in your alternator are worn down.  A fully charged battery should have 12.75volts on its own, 75% charge is 12.5 volts (thats where most cars are at all the time), 50% charge is about 12.25volts, and anything below 12volts you need a new battery.  This is with the car off and no load btw.  The alternator should be producing 13volts or more at idle and no more than 14.5volts maximum ever.  Newer cars hold it between 13.8-14.5volts all the time, older ones may dip to 13volts.  Hope that helps.  So like I said, you probably need a new alternator and regulator (or just a new style alt with reg built in) along with a battery.

Having had lots of battery/charging system problems over the years, I sympathize.

There is a fairly common alternator problem that can lead to a continuous battery drain condition with the engine shut off.  I've had this same thing happen to me on at least 2 different Ford cars for certain, and know of it happening to other people as well.  In my case, after the first experience, I remembered what to look for. It was a simple part in the alternator that was replaced by the local starter/alternator shop and the problem was solved. I'm thinking that it was the diode array itself, but it's been a long while since the last incident.

If I remember the charging scenario correctly, if ONLY one of 3 diodes in an array burns out, and the other 2 are still working, then the other 2 remaining ones have to work much harder to attempt to keep the battery fully charged.  This may be part of what you are smelling.
The low voltage only being present at 1000 engine RPM or below dovetails nicely with this theory. Especially if you weren't running a heavy electrical load from power hungry accessories, which would make the part time undercharging more obvious and not go away until the engine was at a higher rpm. 

 Passing on a tip from the alternator shop:
To see if this is what is actually draining your battery, try disconnecting the plug to the alternator over night, and check the battery voltage in the morning before trying to start the engine. Then leave the alternator connected over night the next night and compare the 2 readings.

If you have an intermittant problem with the part just starting to fail in the alternator, it may take a while before the problem surfaces again. This is what made it take a while to figure out on one of my cars. It didn't happen every day, but only occasionally at first, and then more and more frequently.

Something simple that gets overlooked in seaching for a charging related problem is poor electrical grounds, like from a broken or deteriorated wire that appears ok from the outside, or from a corroded terminal or an insufficient electrical contact due to dirt or rust.  Check yours, and clean or replace any that may be questionable.

One more thing, an alternators' output is dependent upon the ACTUAL speed at which the alternator itself is revolving, and is NOT completely dependent on the engines' speed, so if for some reason you have an incorrectly sized pulley on the alternator itself OR on the pulley that is driving that one, the alternator can be spinning at a VERY different speed than it should be, and it's output at any given engine RPM may be way out of spec as a result.  A more common reason for having a SLOWLY discharging battery (like over a week, BUT with the car being started and driven each day for example) is simply having too low of an idle speed setting.

It was a long day yesterday.
 I forgot that on many older cars the alternator wires are generally all attached individually and not as a plug, but you still have to disconnect them or the nearest connector (if there is one) that is allowing current to flow "backwards" through the alternator.
Be careful whenever attaching or detatching the individual alternator wires as the battery is always supposed to be disconnected when you do this, to keep from possibly damaging the alternator or voltage regulator (which may be how it happened to begin with).  It's easier to simply disconnect the battery cable itself overnight, but that won't tell you WHERE the drain is coming from.

Update, I may have found a contributing problem. The AC/heater motor may be seized or shorted.