Charging device, charging system and charging method

The invention relates to a charging device for charging a battery pack, which comprises multiple charging assemblies for charging multiple battery units in electrical series connection inside the charging device. The multiple charging assemblies are configured to charge a first set of battery units in multiple battery units in a first time period and charge a second set of battery units in multiple battery units in a second time period, and any adjacent battery units in the multiple battery units are from different sets in the multiple battery units. Each charging assembly comprises an AC/DC converter for converting inputted AC voltage to first DC voltage and a DC/DC converter for converting the first DC voltage to second DC voltage for charging the battery units. The invention also relates to a corresponding charging system and a charging method.