I have a loud screech  coming from the alternator belt. It started doing it just a little when we put in a new alternator. I tightened the belt. It quit for a short time but started again. Then it got worse. 3/4 time. Voltage dropped when slipping.  Tried belt dressing. Replaced both belts today and cleaned the pulleys too. Alignment looks good.  I read somewhere that an old battery can draw so much that the belt slips but I don't believe that. Anybody else have this problem?

If you're seeing a significant voltage drop when the belt screeches, it's very likely to be caused by a sudden increase in the electrical load - I doubt that the battery has anything to do with it, not if it can crank & start the car.  If I were to assume a stock vehicle with unmodified wiring, I would expect that increase to be enough to cause a fuse to blow, and my primary suspect would be that new alternator.

By the way a completely discharged battery (so flat that it won't crank the engine) will not cause the belt to slip & screech, unless that belt is either loose or worn - I'd like to believe that you've personally experienced this scenario at least once, so you know that is an "old battery" tale with no truth to it.

I agree on the dead battery not being a factor. Jumped plenty of dead batteries.  Stock wiring. The pulley  spins fine with the belt off and the belt begins to screech just off Idle without any warm up. Would the resistance to turning happen because of a load within the alternator? It started when the alternator was changed. Just a little at first  but now full time. m The electrical system reads 11.2v when it slips but  12.5v+ when it begins to turn

The "resistance to turning" is directly related to the electrical load, the more electrical load, the harder it will be to turn.

Clip the volt meter to the battery and record the battery voltage before starting the engine, I expect it will be close to 12V or over, if it drops to that 11.2 (or thereabouts) after starting, it's a clear indication of a high current electrical and that would be what's causing the alternator to load up and the belt to slip.  If you have access to a DC clamp meter you should be able to measure the current draw when it happens.

The "resistance to turning" is directly related to the electrical load, the more electrical load, the harder it will be to turn.

Clip the volt meter to the battery and record the battery voltage before starting the engine, I expect it will be close to 12V or over, if it drops to that 11.2 (or thereabouts) after starting, it's a clear indication of a high current electrical and that would be what's causing the alternator to load up and the belt to slip.  If you have access to a DC clamp meter you should be able to measure the current draw when it happens.

Makes sense. Are you suggesting that the draw is to blame not the alternator?  What would draw like that other than the battery? Just now I measured 11.6 at a meter I had  installed in the dash and then 11.7 right after starting. I suspect the battery has not been getting charged much lately.  I can't find the clamp meter PO'd about that lol. I'll look some more. Both belts seem to turn but one must still be slipping.

Generally speaking, batteries don't "draw" current - they supply it - and even when they are being charged, so the current is flowing into the battery, rather than out of it, the charge current is "pushed" from the alternator - it is the alternator that controls how much current is produced and to some extent how much current is going to go to the battery, rather than the battery "drawing' it from the alternator.

The electrical charge control system is not the easiest of things to visualize, but I'll try to explain it - I'm sure you've read the comparisons between electricity & water - battery=storage tank, volts = pressure & amps = gals/min .

Assume you're starting with a half charge battery, the voltage will be around 12V (completely discharged will be just under 11V, fully charged is close to 14V) - when you start the engine the alternator will come on line and start to produce current, this current is going to go to the vehicle's electrical circuits, so, fuel pump, fuel injection system and so on, and some of it will go to charge the battery.  As the battery charges up the voltage will rise very slowly and the alternator will sense the rise in voltage and reduce it's current output to prevent the voltage from going too high (this is the job of the voltage regulator, which is an internal part of the alternator).

Now lets say you turn the headlights on, the alternator has no way to know the headlights are on, so it does not increase output - the current is drawn from the battery, and the voltage starts to drop, which the alternator can sense, so it increases output to prevent the voltage from going too low - it's kind of a balancing act - and the important thing to realize is that the voltage regulator is in charge, voltage drops it increases output, voltage rises, it decreases output.

Now - lets take a look at those voltages - I was asking for the voltmeter to be connected directly to the battery for two reasons - the first is it gives me a clear picture of the battery state of charge, the second is your dash voltmeter indicates the voltage where ever you connected it, which is not directly to the battery, you most likely have the dash meter wired up after the ignition switch, maybe to ignition, maybe to accessory, but there is some wiring, two fuse panels and a switch between where you're measuring and where I want you to measure, any current being drawn when you switch the ignition on, any burned connector or loose connection, they all affect the reading at the dash volt meter.

Measuring at the battery I would expect to see close to 12V with the engine off, and after the engine is started, I'm hoping to see 1~2V over what it was with the engine off - so if it was 11.6, engine off, I'm looking for 12.6~13.6 after start up, that 11.7 is way less than I expect, but can't tell if it's a low battery, a bad alternator, or bad connections.

Thanks Fordem. I learned something and that is good.  Yesterday I charged the battery because it was gradually losing it's charge when the squealing was happening.  Once in a while it would stop and the battery would charge but not often enough. so I charged it overnight. Because of that the voltage at the battery this morning was 12.84 before starting and 12.86 after.   The check would probably be more meaningful after some time or driving.
  I forgot something when I said it was unmodified. There is a charging circuit that as added when I got a Brake Buddy about 10 days ago. It is fed by the towing vehicle but only when it is connected to the RV. I will remove the wire later and see if there is any change. I believe it is just a diode to prevent the car battery from trying to charge the RV.
 I am going to make the trip to pick up an alternator later today. I'll do the voltage check before doing anything else and also disconnect the brake buddy before I go. I'll let you know what happens.

12.84 at the battery before starting and 12.86 after suggests that the charge system is not working, I would have expected to see at least mid to high 13's, let's see what happens when you replace the alternator.

Changed the alternator and the noise is gone. It charges. It was that NAPA rebuilt, less than 4 months old.  Thank you very much for the help.

Glad I could be of assistance - what are the voltages like now?

Voltage was 14 when first started. Since then it runs between 13 and 14.  Since I changed it I did get  a short squeal when turning onto a side street. I will check the tension. Hope it is not like a Freddy Kruger movie 

Since I changed it I did get  a short squeal when turning onto a side street.

Do you have power steering?

I hope this posts. I have been unable to get a reply to post recently.
 Yes power steering, new pump 8 mos ago. Spins easily. Tried turning it lock to lock while at low speeds and idle and no change.
Water pump 8 mos old changed with the timing belt .
The squeal went to constant again over last two days but cut off at about 25mph when it seemed to get a grip.

This was up until this afternoon when I decided to take a close look at the balancer to check for glazing. I removed the belts to check and asked a nearby mechanic if they looked glazed and he said no. I put it back together and made the belts even tighter (1/8" deflection).  As usual it sounds fine right after reassembly.  See what happens tonight.

It seems to be gone for good.  Combination of replacing alternator and over tightening the belt.   Charging is as it should be now. Thanks again.