Charging Issue?

A few months back I had the Alternator replaced because the car kept shutting off while driving and I would have to get a jump to get going again.

After replacing the alternator I would get the same problem especially when it was raining. So I connected a voltmeter to the battery and left the voltmeter on the center console so that I can watch the voltage variations as I drive.

I discovered that the reason the battery drains is because I have too many things on and when the car is on idle while in drive the battery is not charging and the voltage begings to drop. I realized that it happend faster while raining because I would have on the lights, wipers, blower and A/C. My idle rpm while in drive and park is around 900 close to but not quite 1000.

To get the system to stop charging while in part I have turn on the lights, high beams, fog lights, blower, A/C and rear defrost. If I turn off any of these the charging system works.

I've also noticed that during the winter I had have the car in high rpms (above 3000) in order to get some heat. But I gues this was due to a bad water pump which I just had replaced after winter was over due to a leak in that area.

I've also evacuated the A/C system and recharged it to system specification. I still have hit high rpm (above 3000) in order to get the hi and lo pressures to where they should be and get cold air out the vents. So in highway driving the A/C is cold but in city stop and go traffic the A/C blows hot.

I checked the belt and it is nice and snug. Not too tight nor too loose.

Basically my question is; why do the rpm have to be above 3000 to get the charging system to work under a heavy load and the A/C to blow cold air?