2001 Mazda Protege Battery Light/Power Loss w/ A/C On

Air Conditioning problem
2001 Mazda Protege 4 cyl Two Wheel Drive 135000 miles

Mazda 2001 Protege DX 4 cyl l.6L automatic. At random, dashboard battery charge light comes on when driving with the air conditioner on in street and highway driving and sounds/feels as if the car is going to loose its power (a/c clutch or belt?). Light and loss of power sound/feel stops when A/C turned off and runs well. Initially thought it was the charging system and replaced components: the alternator, battery, plugs wires and had ignition coil tested. All tested and amp ranges, etc. Were within operable/satisfactory ranges. A/C cools inside of car well. Refrigerant has not been replaced in 3 years; currently on 3rd hot CA summer. Problem reoccurs randomly: Started up and drove 3 blocks, got on freeway and battery light came on after a few minutes, heard a sound (maybe a/c clutch), turned a/c off and continued to drive. Next day, drove 150 miles on freeway, no problems. Following day, problem occurred after street driving for 15 minutes. Retested charging system at a shop w/ battery and alternator showing operating within specified operable ranges. Had belt alternator and a/c belt tension checked and no problems. Not a knowledgeable person in these matters and cringe at pricey repair costs. Any suggestions which parts may be the root cause with the a/c causing this problem before pricey diagnostics or told I need parts that I do not need and/or whole unit vs. A switch or wire. If there is a short in the compressor, a/c clutch or a/c coil will I need just one of these parts or will most shops not bother and replace all? Could refrigerant not being replaced in 3 years be a cause? Any suggestions appreciated. Shops are in business to make money. Don't know if this is too general a question without readings/diagnostics to expect a reasonable specific solution/answer, so please advise. Thank you most kindly.