The problem is simple, no machine is 100% efficient.
I assume you're talking about a hypothetical electric vehicle? (you didn't really say)
Okay, so we have batteries supplying power through wires and a switching system to an electric motor through a system of gears to the wheels. Each item that I just mentioned has a power loss associated with it. Batteries generate heat as they discharge... power lost. Wire has resistance (where did we hear that?) and also generates heat as power passes through it (even if it's not enough that you can feel it)... power lost. Switching systems on DC motors are complex with a lot of heavy-duty transistors that... you guessed it, generate heat... more power lost. They often generate so much heat that they require a cooling fan... more power lost.
Have we gotten to the motor yet? Okay, good. The most efficient electric motors are just a little over 90% efficient... with the waste being lost to heat and friction. More power lost. The driveline? Friction and heat, more power lost.
And finally, we reach the wheels where you want to put an alternator that's going to recharge the batteries to make up for all of this loss AND the power used in actually propelling the vehicle.
The alternator itself is going to consume some power due to friction and... heat, plus the load created by the magnetic field used to create the charging process. An alternator is probable less than 85% efficient.
So... with all of the losses mentioned, it's pretty easy to see that the alternator you put on your electric car is simply not going to be able to keep up with the discharge of the batteries. That's just the way it is. No free lunch in the world of physics.
BUT... it's not a bad idea. Ever heard of "regenerative braking"? An electric vehicle doesn't even need an alternator to do it. The electric motor itself can be reversed and act as a generator! Cars like the Prius use this to recover some energy. When you go from cruising to coasting, the system switches into generator mode and the motor provides charge to the batteries. Used properly (in the same way that we use DCFO in a modern gas-powered vehicle) by coasting as much as possible, you CAN extend the range of an electric vehicle in this manner.
But, you can't recover charge AND use charge at the same time. The net result of that is always going to be negative because you simply can't build a machine that has a greater power output than input.
Hope that helped some.