Supplementary charging system and method for auxiliary battery of eco-friendly vehicle

A supplementary charging method for an auxiliary battery of an eco-friendly vehicle including a main battery, an auxiliary battery having a voltage lower than a voltage of the main battery, and a low voltage DC-DC converter (LDC) stepping down the voltage of the main battery and providing the stepped-down voltage to the auxiliary battery, the supplementary charging method comprising:

• when a periodic charge time arrives, determining, by a controller, a reference cumulative charge current amount based on a target cumulative charge current amount corresponding to a difference between a state of charge (SOC) of the auxiliary battery and a predetermined target SOC;
• selecting, by the controller, either a constant voltage charging method or a constant current charging method as a charging method based on an aging state of the auxiliary battery;
• when the constant voltage charging method is selected, controlling, by the controller, the LDC to charge the auxiliary battery with a constant voltage for a predetermined reference time;
• when the constant current charging method is selected, controlling, by the controller, the LDC to charge the auxiliary battery with a constant current for the predetermined reference time;
• and
• when a cumulative charge current amount provided to the auxiliary battery is greater than or equal to the reference cumulative charge current amount after operating a charging using the constant-current charging method, terminating the charging by the controller.