US Patent 10124680 Methods and systems for pre-charging on-board charger of vehicle

An on-board charger for an electric vehicle includes a switch configured to receive an AC voltage, a capacitor connected to the switch and connectable to a traction battery of the electric vehicle, a frequency sensor, and a controller. The frequency sensor is configured to detect during the odd half-cycle of a first cycle of the AC voltage an instantaneous frequency of the AC voltage for the first cycle based on elapsed time between zero-voltage crossings of the odd half-cycle of the first cycle and not based on any other zero-voltage crossings of the AC voltage. The controller is configured to close the switch during the even half-cycle of the first cycle at a first trigger time dependent upon the instantaneous frequency of the AC voltage for the first cycle. The capacitor is enabled to be charged with the AC voltage via the switch while the switch is closed.