Defroster/compressor

The seals in your A/C system will deteriorate at a much quicker rate if you don't use the A/C system. You should run the compressor at least 5-10 minutes per month. However in the winter people don't tend to use their A/C and forget to run it, so auto manufacturers in order to maintain the longevity of a vehicle's A/C system tied the operation of the compressor to the defrost selection on the heater. People use their defrost a lot in the winter. So the A/C system will always be operated and thus maintained.

In order to activate the compressor, an electronically operated clutch is engaged. The reason your charging guage could be dipping is because of the draw on the electrical system when the clutch engages.

Your defroster should not blow cold air unless you have the temperature selector in the cold position.