A charging system and method that improves utilization of available AC power during onboard charging of energy storage systems of electric vehicles. An onboard charging method for an energy storage system of an electric vehicle, the method using an AC power source, includes a) establishing a maximum DC charging current for the energy storage system responsive to a control signal indicating real-time available current/power from the AC source; and b) controlling a charging system to provide an actual DC charging current, up to the maximum DC charging current, to the energy storage system.