US Patent 9610848 Onboard charging system and control method thereof

An onboard charging system in which a main battery (3) is charged with generated power by a solar cell (2) after the generated power is stepped up in a voltage by a step-up converter (9) includes a step-up ratio calculating portion (62) configured to calculate a step-up ratio when stepping up the voltage with the step-up converter (9). The auxiliary battery (4) is set as a charging target battery if it is determined that the calculated step-up ratio is equal to or greater than the determination threshold value, and the main battery (3) is set as the charging target battery if it is not determined that the calculated step-up ratio is equal to or greater than the determination threshold value.