When I drive car with Air conditioner ON battery doesn't get charged?

I am having Ford Ikon 1.8 D NXT of 2003. Now a days it gives trouble in charging battery when I drive car with Air conditoner On upto the destination, in this situation after if we try to crank engine it doesn't starts & indicates symptoms of low batterry. If I switch off AC 1 to 2 Kilometers before my destination then battery gets charged.
Went to authorised service dealer but he could not find out what exacly is the problem. Please help me.