Belt squeals

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Started doing it today… Issue #1: My alternator belt squeals as the engine revolution goes up. It ONLY happens if the heat is switched on in the 2nd position or up (3rd or 4th). Symptom #2: When I switch the heat on, the voltage gauge shows the voltage dropping quickly down, immediately, from 14-15V to 10V. It gets even lower if the headlights are on, even more if the high beam is on (with the heat). Rising rpm does not stop the squeal nor rises the voltage levels back to normal. If heat is not on, there is no squeal.
In the past I rarely saw my voltage dropping down lower than 12V, and never more than 1-2 Volts at once... and those times it always happened gradually/slowly (for example when I switched the seat heaters on with full blast of heat, radio, headlights+front fog lights+rear fog lights on +radio +etc ). That`s normal, I think, that in that case the voltage drops from 15 to 12-13-14, whatever, and stays there, being lower than the average I see when driving with nothing on (daytime).
Now, as I put the heat on, the voltage immediately jumps down from 15V to 10-11Volts and the belt starts to squeal too as the rpm increases. It does not stop. If the heat is not on, the voltage stays around 14-15V and there is no belt noise at all. (PS.: The digital Voltmeter shows 14.06V in battery after many days of no-use.)
It’s a nasty cold rain here and completely dark now but I`d like to figure this out tomorrow. Any ideas so far?