Why cant an alternator be used to power an electric car?

I'm converting a ford probe gt to an all electric vehicle. I plan a 72 volt system with 2 banks of 9 8volt deep cycle batteries. I'm looking at a range of approx 40miles per bank. So what I put on paper looks like this. I have a 24 volt alternator wired to each battery pack. The Alternators are wired to 24v starter batteries. When your main battery pack runs down you switch over to the second battery pack, throw a switch to turn on the alternator and start charging the pack. On a scaled down model I can charge 6 6v batteries with a standard 12 battery and alternator in under 10 minutes. If I get similar results with a 72volt system under actual driving conditions, I should be able to go at least 200 miles b4 the alternator batteries have to be recharged.