Battery vs. Alternator

It's getting cold here in the Northeast and I don't really drive my 88 TC much from November to April. I do start the car and let it idle every few weeks to put a heat cycle through it. Yesterday, the car started fine and idled for 25-30 minutes but once I gave it some gas, the car stalled. I went to turn it back over and it sounded like a dead battery. Meter showed only 11.2 volts across the terminals; not good. I had a similar issue during the summer where the alternator wasn't charging because the sense wire connection came loose. The car would run purely on the battery until it was drained. Before I start troubleshooting the electrical connections to the alternator, can a dead/dying battery cause a car to stall when load is applied? I was always under the impression that the alternator alone could keep the car running but I'm reading mixed things online. Here's some additional information to help:
Alternator is a 3G PA Performance unit: 2 years old
Battery is 3 years old but the car is not driven everyday and it has been drained/recharged 2-3 times during that 3 year period
I always disconnect the negative terminal after running the car like this until the next time I go to run it

Thanks for the help!

Take the battery to the parts store and get it tested, might narrow things down.

What is the batt voltage while the engine is running? Is the engine to body ground strap in good shape and both connections corrosion free?As Pete said, have the batt tested.

Consider getting a float charger from Harbor freight of Ebay to keep the battery charged during winter storage.

Agree with Pete. Sounds like it could be sulfation.

In the summer months of 2017 while I was rebuilding the T5OD transmission I had the same issue. Without a transmission starting the car to charge the battery is not an option. I should have probably removed it and put it on a trickle charge. Anyway, after the job was done the battery went dead a month later. It was a DieHard—I’ve had the most success with them here in this desert heat—and I had Sears test it. One of the cells was almost completely sulfated, so a battery that was about a year and a half old was now useless.

Technically, sulfation happens all the time—it’s a natural part of the chemical process, and is known as “soft” sulfation. Charging the battery usually reverses it. But there will come a time when the rate of sulfation will become so high that even charging doesn’t or can’t remove/restore it. This is known as “hard” sulfation. There a lot of methods and theories to attempt a reversal, but few have been successfully carried out across the full range of available lead-acid batteries.

So I had the battery tested at Autozone and it came back fine. I started the car and checked the voltage. At first, my multimeter was showing 19 volts which is obviously very high and not good but my multimeter is a cheap unit that’s several years old from Harbor Freight. I checked the battery again and it was showing 12.3V, then 12.2 then 12.1 after some time so obviously it’s not getting charged. I have an electrical diagnostics sheet from PA Performance and started with that. The alternator outpost checked out fine; same voltage as the battery. I went on to check the plug but I couldn’t get any readings. I don’t think my multimeter leads are able to contact the connection point in the plug. Does anyone know of any automotive plug specific multimeter leads?

John you need some "back probes". Might be hard to find a local supplier but easy to get online. Here is an example:
https://www.amazon.com/Tool-Aid-23500-Ba...+probe+kit

John you need some "back probes". Might be hard to find a local supplier but easy to get online. Here is an example:
https://www.amazon.com/Tool-Aid-23500-Ba...+probe+kit

That's perfect! Thank you James

FYI, you can do the same thing with a piece of thin ( like 30 to 34 gauge) single strand wire. Stick one end in the female end of the connector, bend it over so the male connector will fit with the thin wire sticking out. Done this many times.