What the OP is calling a charger is properly called an EVSE. It is just a relay to control AC power along with some electronics to communicate with the electric vehicle. The EVSE advertises to the car the maximum current that is available, and then the car asks the EVSE to close the relay. The charger, as in the circuit element that converts AC to DC and determines the current draw, is located in the EV itself. In the case of the Volt, it is (I infer) 120V/240V at up to 15 amps.

So the proper way to utilize a 30A circuit is to get an EVSE that can be set to advertise only 24A, since the EVSE is considered a continuous load by the NEC. As a practical matter, if you put a 30A EVSE on a 30A circuit, then the worst that should happen is nuisance tripping if you plug in a car with a 30A charger. That will not happen with the Volt and its 15A charger.

Cheers, Wayne